Mobility and Routing in a Delay-tolerant Network of Unmanned Aerial Vehicles by
Linköping Studies in Science and Technology Thesis No. 1356 Mobility and Routing in a Delay-tolerant Network of Unmanned Aerial Vehicles by Erik Kuiper Submitted to Linköping Institute of Technology at Linköping University in partial fulfilment of the requirements for the degree of Licentiate of Engineering Department of Computer and Information Science Linköpings universitet SE-581 83 Linköping, Sweden Linköping 2008 Mobility and Routing in a Delay-tolerant Network of Unmanned Aerial Vehicles by Erik Kuiper April 2008 ISBN 978-91-7393-937-9 Linköping Studies in Science and Technology Thesis No. 1356 ISSN 0280-7971 LiU-Tek-Lic-2008:14 ABSTRACT Technology has reached a point where it has become feasible to develop unmanned aerial vehicles (UAVs), that is aircraft without a human pilot on board. Given that future UAVs can be autonomous and cheap, applications of swarming UAVs are possible. In this thesis we have studied a reconnaissance application using swarming UAVs and how these UAVs can communicate the reconnaissance data. To guide the UAVs in their reconnaissance mission we have proposed a pheromone based mobility model that in a distributed manner guides the UAVs to areas not recently visited. Each UAV has a local pheromone map that it updates based on its reconnaissance scans. The information in the local map is regularly shared with a UAV’s neighbors. Evaluations have shown that the pheromone logic is very good at guiding the UAVs in their cooperative reconnaissance mission in a distributed manner. Analyzing the connectivity of the UAVs we found that they were heavily partitioned which meant that contemporaneous communication paths generally were not possible to establish. This means that traditional mobile ad hoc network (MANET) routing protocols like AODV, DSR and GPSR will generally fail. By using node mobility and the store-carry-forward principle of delay-tolerant routing the transfer of messages between nodes is still possible. In this thesis we propose location aware routing for delay-tolerant networks (LAROD). LAROD is a beacon-less geographical routing protocol for intermittently connected mobile ad hoc networks. Using static destinations we have shown by a comparative study that LAROD has almost as good delivery rate as an epidemic routing scheme, but at a substantially lower overhead. This work has been supported by LinkLab, a research center for future aviation systems, established by Saab and Linköping University, and the KK foundation through the industrial graduate school SAVE-IT. Department of Computer and Information Science Linköpings universitet SE-581 83 Linköping, Sweden Acknowledgements First I want to thank Saab, and then especially Anders Pettersson and Gunnar Holmberg, for giving me this opportunity to pursue a PhD. With this thesis I should be half way. Without the connection to a practically applicable problem domain I would probably not have chosen to pursue a PhD. I also want to thank my industrial advisor Mats Ekman. I might not have sought your advice that extensively, but the discussions we had gave me some things to think about. To my academic advisor and supervisor Simin Nadjm-Tehrani I would like to extend a thank for guiding me to this point. You especially taught me how to write for an academic audience and not only to report my findings. I might not entirely agree with the anatomy of academic articles, but hopefully I now understand it reasonably well. I also want to thank the other members of RTSlab for your friendship and valuable comments. You helped me to become aware of some of the assumptions I hade made and you pushed me to clarify and further investigate some issues. I hope you will continue to take a coffee break at three even after I am gone. I am grateful to SAVE-IT and the KK foundation for partially funding my research. You might not be in my thoughts every day, but without you this research might never have been done. Finally I want to thank my friend C. Without you I might never have selected to work for Saab, and then this would never have happened. Erik Kuiper v vi Contents 1 Introduction .................................................................................................... 1 1.1 Mobile Ad-hoc Networks ..................................................................... 1 1.2 Intermittently connected MANETs..................................................... 3 1.3 Problem Description.............................................................................. 4 1.4 Contributions ......................................................................................... 6 1.5 Thesis Outline ........................................................................................ 7 2 Background ..................................................................................................... 9 2.1 Mobility Models..................................................................................... 9 2.1.1 Synthetic Mobility Models............................................................. 10 2.1.2 Real-World Mobility Models......................................................... 14 2.2 Routing.................................................................................................. 17 2.2.1 DTN Routing in Opportunistic Networks................................... 18 2.2.2 Beacons-less Routing ...................................................................... 20 3 Reconnaissance Mobility............................................................................ 23 3.1 Scenario................................................................................................. 23 3.2 Random Mobility Model .................................................................... 24 3.3 Distributed Pheromone Repel Mobility Model ............................... 25 3.4 Evaluation............................................................................................. 29 3.4.1 Scan Coverage ................................................................................. 30 3.4.2 Scan Characteristic.......................................................................... 34 3.4.3 Communication............................................................................... 37 4 Routing in DTNs.......................................................................................... 41 4.1 LAROD ................................................................................................. 41 4.2 Broadcast Delay-tolerant Routing (BDTR) ....................................... 46 4.3 Broadcast Routing (BR)....................................................................... 48 4.4 Evaluation............................................................................................. 48 4.4.1 Node density ................................................................................... 50 4.4.2 Node speed...................................................................................... 52 4.4.3 Time to live ...................................................................................... 53 4.4.4 Network load................................................................................... 55 5 Conclusions and Future Work ................................................................... 57 5.1 Conclusions .......................................................................................... 57 5.2 Future Work ......................................................................................... 58 6 Acronyms....................................................................................................... 59 7 References ..................................................................................................... 61 vii viii List of Figures Figure 1. Figure 2. Figure 3. Figure 4. Figure 5. Figure 6. Figure 7. Figure 8. Figure 9. Figure 10. Figure 11. Figure 12. Figure 13. Figure 14. Figure 15. Figure 16. Figure 17. Figure 18. Figure 19. Figure 20. Figure 21. Figure 22. Figure 23. Figure 24. Figure 25. Figure 26. Figure 27. Figure 28. Figure 29. Figure 30. Figure 31. Figure 32. Figure 33. Figure 34. Figure 35. Figure 36. Change of average direction near the edges. ................................. 12 Forwarding areas............................................................................... 21 Local pheromone map after 3600 seconds of simulation. ............ 26 Global pheromone view after 3600 seconds of simulation. ......... 26 Local pheromone map after 7200 seconds of simulation. ............ 27 Global pheromone view after 7200 seconds of simulation. ......... 27 Pheromone search pattern................................................................ 28 Pheromone mobility coverage with global pheromone map. ..... 31 Pheromone mobility coverage with 100% transfer probability. .. 32 Pheromone mobility coverage with 50% transfer probability..... 32 Pheromone mobility coverage with 10% transfer probability..... 32 Pheromone mobility coverage with 0% transfer probability....... 33 Random mobility coverage .............................................................. 33 Random Waypoint mobility coverage............................................ 33 Comparison of average coverage. ................................................... 34 Pheromone mobility with global pheromone map. ...................... 35 Pheromone mobility with 100% transfer probability.................... 35 Pheromone mobility with 50% transfer probability...................... 36 Pheromone mobility with 10% transfer probability...................... 36 Pheromone mobility with 0% transfer probability. ...................... 36 Random mobility............................................................................... 37 Random Waypoint mobility............................................................. 37 Pheromone mobility with global pheromone map. ...................... 38 Pheromone mobility with 100% transfer probability.................... 39 Pheromone mobility with 50% transfer probability...................... 39 Pheromone mobility with 10% transfer probability...................... 39 Pheromone mobility with 0% transfer probability. ...................... 40 Random mobility............................................................................... 40 Random Waypoint mobility............................................................. 40 LAROD forwarding areas. ............................................................... 42 Illustration of vectors used for delay time computations. ........... 44 LAROD pseudo code. ....................................................................... 45 BDTR pseudo code............................................................................ 47 Delivery ratio for different node densities..................................... 51 Overhead for different node densities............................................ 51 Delay for different node densities................................................... 51 ix Figure 37. Figure 38. Figure 39. Figure 40. Figure 41. Figure 42. Figure 43. Figure 44. Figure 45. Delivery ratio for different node speeds. ....................................... 52 Overhead for different node speeds. .............................................. 53 Delay for different node speeds. ..................................................... 53 Delivery ratio for different packet life times.................................. 54 Overhead for different packet life times. ....................................... 54 Delay for different packet life times................................................ 55 Delivery ratio for different network loads. .................................... 56 Overhead for different network loads............................................ 56 Delay for different network loads. .................................................. 56 x List of Tables Table 1. Table 2. Table 3. Table 4. Table 5. Table 6. Table 7. Table 8. Table 9. Table 10. Table 11. Table 12. Table 13. Random waypoint parameters ........................................................ 11 Gauss-Markov parameters. .............................................................. 12 Scenario parameters .......................................................................... 24 Random action table.......................................................................... 24 Pheromone map parameters. ........................................................... 25 Pheromone parameter definition. ................................................... 28 UAV pheromone action table. ......................................................... 29 Never scanned area........................................................................... 35 LAROD parameter definitions. ....................................................... 44 BDTR parameter definitions. ........................................................... 46 Scenario parameters .......................................................................... 49 LAROD parameter values................................................................ 49 BDTR parameter values.................................................................... 49 xi 1 1 Introduction The sharing of information is vital for many tasks and the faster information can be disseminated the sooner or better a task can be completed. With the development of cheap wireless technologies like GSM and Wi-Fi information is often available anytime and anywhere. The limitation of these technologies is that they require an infrastructure of base stations to function. In environments such as disaster areas or during wartime this type of infrastructure is generally not available, but information exchange is still desired. An option to communicate in these environments is to use long range radios that enable point-to-point communication. The problems with these are that they are often expensive, bulky and only provide low bandwidth communication. At the other end of the spectrum there are cheap, small, low power, high bandwidth, but short range radio technologies. If a lot of nodes were equipped with this type of radio then they could automatically form a network and cooperate to forward messages for each other. These types of networks that are cooperatively formed and do not rely on any infrastructure are often called ad-hoc networks. To create local ad-hoc networks there exist technologies like Bluetooth  and ZigBee , but the creation of larger adhoc networks is still in the research domain. 1.1 Mobile AdAd-hoc Networks A mobile ad-hoc network (MANET) is a self-organizing network where nodes with wireless radios cooperate to provide network connectivity. The opposite of an ad-hoc network is a managed network where network connectivity is provided by dedicated wireless access points. These access points are generally non-mobile fixed installations and they are preconfigured to efficiently share the wireless medium. Examples of managed networks are GSM and Wi-Fi hotspots. 2 1 A basic service required from any network is the ability to route messages from source to destination. In wireline networks, such as the internet, routing is performed by dedicated equipment that exchange information with each other to build up a view of the network links between routers and end stations. This works well since links are stable and change infrequently. In MANETs links are less stable due to mobility, interference and fading. This means that the network layout is constantly changing and this has to be handled by the routing protocol. These constant network changes are challenging since it becomes practically impossible to distribute a consistent view of the network to all nodes. In IP based networking the IP address is both a machine unique identifier and a hierarchical location identifier. This means that the address is the mean to locate the node. In a network of mobile nodes hierarchical addressing is not practical since nodes would constantly have to change address due to node mobility. This means that each node will have to be a separate entry in a routing table resulting in large routing tables for large networks. The routing protocols suggested for MANETs can be classified into the two dimensions proactive–reactive and topological–geographic. Proactive protocols constantly maintain routing tables in the nodes while reactive protocols only acquire routing information when it is actually needed. Topological protocols build a view of the network based on how the nodes can communicate with each other while geographical protocols route information based on the geographical location of the nodes. The most widely accepted topology based routing protocols for MANETs are those accepted by the Internet Engineering Task Force (IETF). They have published two proactive routing protocols (Optimized link state routing (OLSR) , Topology dissemination based on reverse-path forwarding (TBRPF) ) and two reactive protocols (Ad hoc on-demand distance vector routing (AODV) , Dynamic source routing (DSR) ). They are currently working on replacing these protocols with one proactive and one reactive protocol. There have been some published reports on the relative performance of these protocols  and though far from conclusive they find that OLSR does 1 In this thesis the words message and packet are generally used interchangeably. If there is a separation then a message is some coherent data that needs to be transmitted and a packet is the physical format of (a part of) a message that is transmitted in the network. 3 not perform well in mobile environments compared to AODV and DSR (TBRPF is not evaluated). The main reason is the functioning of the proactive OLSR. With mobile nodes the protocol has to spend a lot of effort trying to update the routing tables, tables that never will be accurate. To remove the need to maintain routing information in routing tables geographical routing protocols have been suggested where Greedy perimeter stateless routing (GPSR)  is the most widely known. Instead of maintaining a route to a destination a geographical routing protocol forwards packets to the geographical location of the destination by locally at each hop selecting a node that will reduce the distance to the destination. This generally means that no global routing information needs to be maintained by the nodes which it is good for highly dynamic environments. The Achilles heel of geographical routing is how to distribute node positions in the network so the sources know where the destinations are. For this there need to be a location service the sources can access. A main problem with location services is how to balance between the overhead of distributing the location information to location servers and the cost of a location query . A related problem that has to be managed is node position errors in packets due to node movement. 1.2 Intermittently connected MANETs The routing protocols described in the previous section all assume that the node density is high enough to guarantee the existence of a contemporaneous path between any sender and receiver. A contemporaneous path is a sequence of wireless links between two communicating nodes such that they can communicate with each other instantaneously if the bandwidth was infinite and there were no transmission delays. If such a path does not exist they will fail to deliver messages. This does not mean that it is impossible to route messages in the absence of contemporaneous paths, only that other principles need to be used. An intermittently connected MANET is a network where the nodes are so sparse or moving in such a way that there exists at least two groups of nodes for which there is no contemporaneous path between the nodes. If the node movement is such that the intercontact times are unknown and unpredictable then the node contacts are called opportunistic. The enabler to route in intermittently connected networks is node mobility. To overcome communication gaps messages are stored in nodes and carried until they can 4 be forwarded. A consequence of this store-carry-forward principle is that 2 delivery times will be longer than if a contemporaneous path existed . Due to the long delays in these networks the connectivity assumptions made in most networks are no longer valid. As a consequence the responsibility for reliable transfers should be moved from the source-destination pair to a system of custodians. The reason to not use the source-destination pair as is normally done is that this generally requires many round-trip exchanges, exchanges that take much time. Instead the responsibility of reliably transferring a message is moved to the network and a system of custodians. In such a system a message is transferred between custodians that take over delivery responsibility of the message. The most straightforward method to send a packet to a node whose location is unknown and where the best path to reach it is unknown is to send it to all nodes in the network. This is done by Epidemic Routing . The problem with this method is that it requires much bandwidth and storage resources. The other extreme is to keep the packet in the source node until the source node meets the destination node due to mobility as is done by Direct Transmission (no relaying) . To be able to better guide a packet to the destination several routing protocols have been proposed that use historical encounter data to decide how to forward a packet. Examples are Utilitybased Routing , MaxProp  and Context-aware routing (CAR) . As for MANETs geographical routing can also be done in intermittently connected networks if the nodes are aware of their geographical position. Two examples of protocols that perform geographical routing in intermittently connected MANETs are Disruption-tolerant geographic routing for wireless ad hoc networks (DTGR)  and Motion vector (MoVe) . 1.3 Problem Description A challenging aspect in the study of routing algorithms is that the node mobility pattern affects the routing performance . This means that a routing protocol should be tested in an environment that as close as possible resembles the environment it will be deployed in. To verify that a 2 This assumes that the transfer rate using wireless transfers (wireless transfer rate * distance) is much higher than the rate using node mobility (speed * message size). 5 routing protocol is suitable for general use it should be tested under several reasonable, but characteristically different, environments. As the main scenario under which to evaluate routing we have chosen a reconnaissance mission in which a group of unmanned aerial vehicles (UAVs) shall detect units on the ground by regularly scanning all parts of a specified area. Since it is probable that the units on the ground do not want to be detected by the UAVs there may not be any apparent pattern to when a UAV scans a particular area. The same type of requirements can be found on the searching behavior in the FOPEN (Foliage Penetration) scenario reported in the work by Parunak et al. . Since most mobility models used in MANET research are either synthetic  or based on human mobility  we have had to develop a mobility model for this scenario. Depending on used node density and communication range the nodes moving under our mobility model will form an opportunistic intermittently connected network. The challenge is then to route messages in a reliable manner while limiting the use of system resources, such as storage, power and wireless bandwidth, in the realistic mobility scenarios envisioned. Since UAVs are location aware due to navigational requirements this could be used to perform geographical routing if the position of the destination is known. To conserve bandwidth and to make it possible to use our routing algorithm in energy-constrained systems we will explore the viability of beacon-less geographical routing in opportunistic intermittently connected MANETs. Beacons are regularly transmitted special messages that are used by many routing algorithms to determine a node’s neighbors. The reason to evaluate beacon-less routing is that beacons as commonly used by routing protocols have the following problems : • They consume a lot of bandwidth • The consume energy from nodes even if they are not performing any routing. • Beacon information may not accurately represent actual communication possibilities due to node mobility since beacon reception and due to fading. 6 1.4 Contributions The main contributions in this thesis are twofold; the study of mobility models and a proposed geographical routing algorithm for intermittently connected MANETs. A distributed pheromone mobility model for reconnaissance applications To evaluate our suggested routing protocol we have used a military reconnaissance scan scenario. The objective in the scenario is that a group of UAVs shall cooperatively scan an area regularly to detect units on the ground. Since it is probable that the units on the ground do not want to be detected by the UAVs they should not move in a deterministic pattern. To coordinate the UAVs we have designed a distributed pheromone mobility model. By using pheromones and localized search the UAVs are guided to areas not recently visited by other UAVs. When a UAV moves around it places pheromones on the areas it has scanned. Since it is not possible to place these pheromones in the environment as would have be done in a natural system the UAV places them in a local pheromone map. To share this pheromone information with the other UAVs each UAV regularly broadcasts a local area pheromone map. All UAVs that receive the broadcast merge this information into their pheromone map. The distributed pheromone mobility results presented in this thesis extends the results presented in the following paper: Erik Kuiper, Simin Nadjm-Tehrani. Mobility Models for UAV Group Reconnaissance Applications. Proceedings of International Conference on Wireless and Mobile Communications. July 2006. IEEE A beacon-less geographical routing protocol for intermittently connected networks Most routing protocols use beacons to know who their neighbors are. While it is a quite simple and effective method it has the problem of creating a lot of overhead, and the information gathered by beacons is always to some extent out of date. In this thesis we present location aware routing for delay-tolerant networks (LAROD). LAROD forwards messages using greedy geographical routing without the use of beacons and employs the store-carry-forward principle when a message cannot be forwarded due to network partitions. 7 LAROD is previously presented in: Erik Kuiper, Simin Nadjm-Tehrani, Geographical Routing in Intermittently Connected Ad Hoc Networks. The First IEEE International Workshop on Opportunistic Networking. March 2008. IEEE 1.5 Thesis Outline This thesis is organized as follows. In Chapter 2 other work relating to mobility models and routing in intermittently connected MANETs are presented. In Chapter 3 our distributed pheromone model is described and evaluated. In Chapter 4 LAROD is described and evaluated. Finally Chapter 5 presents our conclusions and ideas for future work. 8 9 2 Background This chapter gives an overview of the current state regarding mobility models for MANET research and routing in intermittently connected MANETs. 2.1 Mobility Models A natural way to simulate wireless networks is to place all the nodes in a Euclidian space and simulate the radio communication based on the nodes’ placement using a radio model. To control the placement and mobility of the nodes a model or real-world mobility trace is used. The mobility models used in ad hoc network research are usually synthetic constructs , but recently some mobility models have been suggested that are based on data from mobility traces . The choice of mobility description is important in MANET routing research since it has been shown that the mobility affect the performance of routing protocols . Even if a real-world trace has the actual movement of the nodes under study the problem is that a trace only represents one possible movement pattern. To be able to statistically verify a routing protocol more data is generally needed than traces can give. To provide this a mobility model is needed that can generate traces with the same properties as real collected traces would have. Such a model can then generate as many traces as needed providing statistical diversity while still maintaining realistic properties. Any mobility model claiming to model real-world mobility should be verified against realworld traces as done by Kim et al.  and Zhang et al. . Even though the mobility models used by Marfia et al.  have not been validated against real traces the routing results reported illustrate the impact small differences in mobility can have on routing results. Another aspect of mobility models is if they are intended to be descriptive or prescriptive. A descriptive mobility model tries to describe how nodes move in some kind of environment. A prescriptive mobility model on the other hand describes how a node should move. The main difference between the two types is how they are verified. A descriptive model needs to be verified 10 against the real mobility it tries to describe. A prescriptive model on the other hand needs to be verified against what the nodes try to achieve with their mobility. A good example of descriptive mobility models are the vehicle models used by Marfia et al. in . The pheromone models used by Sauter et al.  are on the other hand prescriptive since they are used to control how the nodes move to achieve a mission objective. 2.1.1 Synthetic Mobility Models Since it is difficult, costly and not always possible to obtain real-world mobility traces from which mobility models can be created researchers have developed synthetic mobility models. These models range from the very abstract random waypoint mobility model  to more realistic node movement like the obstacle mobility model  and vehicular mobility . Many of the used mobility models are entity mobility models which mean that the nodes move independently. In reality the decision by a node of how to move is often influenced by other nodes. This fact has made people design mobility models like the reference point group mobility model  and pheromone based models like the ones suggested by Sauter et al. . A survey of different mobility models used in MANET research can be found in . In the following subsection some synthetic mobility models are described that have been used in the evaluations or influenced the mobility models used in this thesis. 22.214.171.124 Random waypoint The most widely used mobility model is MANET research is the random waypoint mobility model . In random waypoint a node randomly selects a destination and speed and then moves in a straight line to the selected destination. When the destination is reached the node optionally pauses for some random time until the process is restarted. The random values used are normally drawn from a rectangular distribution. The simplicity of the model and its widespread use means that it has maintained its popularity, but there are problems with the model. Without reflecting too much on the properties of the mobility model many researchers initialize a simulation by distributing nodes randomly over the simulation area using a rectangular distribution. The problem with this is that the stationary distribution is not rectangular. In  Navidi and Camp showed that the stationary distribution is actually according to equation (1) assuming 11 a rectangular simulation area of unit size, no pause times and rectangular distributions in selecting speed and destination. [ k (x 2 − x1 ) + ( y 2 − y1 ) g ( x) = 2∫ ∫ ∫ ∫ x 2 − x1 0 x 0 0 k ≈ 1.9179 x 1 1 1 ] 2 1/ 2 2 dy1 dy 2 dx1dx2 (1) The node speeds are not uniform either, but instead distributed according to equation (2) with the same assumptions. The parameters of the equations are defined in Table 1. 1 f ( s ) = s log(v1 / v0 ) 0 v0 < s < v1 otherwise Table 1. Parameter x1, x2 ,y1 ,y2 k s v0 v1 (2) Random waypoint parameters Description Path end points. Constant to get a total density of 1. Node speed. Node minimum speed. Node maximum speed. These results mean that if a simulation is initialized by placing the nodes using a rectangular distribution the statistical mobility properties will continuously change until the steady state distribution is reached. This means that simulation results obtained from the beginning of the simulation will be different compared to results obtained late in a simulation. In  Navidi and Camp also derived expressions for the speed and x- and y-coordinates with pausing. 126.96.36.199 Gauss-Markov Instead of determining the destination of a node you could regularly update the direction in which the node is moving. The Gauss-Markov mobility model  does this by updating the speed and direction of a node at fixed intervals. Equations (3) and (4) describe the updates and the parameters are defined in Table 2. 12 sn = αs n −1 + (1 − α )s + d n = αd n −1 + (1 − α )d + (1 − α )s 2 (3) xn −1 (1 − α )d 2 Table 2. (4) x n−1 Gauss-Markov parameters. Parameter sn Description Speed at step n. s Average speed. sx Random variable from a Gaussian distribution. dn Direction at step n. d dx Average direction. α Randomness factor. 0 ≤ α ≤ 1 Random variable from a Gaussian distribution. To ensure that a node does not move outside the simulation area the average direction is changed when the node approaches the edge according to Figure 1. 315º 270º 0º 45º Figure 1. 225º 180º 90º 135º Change of average direction near the edges. 13 188.8.131.52 Pheromone Based Mobility Models Animals like ants use pheromones to guide them to the locations they need to go to like food resources. The distributed nature of pheromone based systems and the observation that complex behavior can emerge from the simple control logic of the agents have made pheromone controlled mobility interesting to study. The viability of the principle has been shown by simulation and practical tests . Sauter et al.  show by simulation that pheromone logic can be used for several types of surveillance and target acquisition and tracking scenarios. They have also shown by practical demonstration that the technique works in practice. To guide the vehicles several types of pheromones are used, both repulsive and attractive. Repulsive pheromones will make a vehicle avoid an area where attractive pheromones will encourage vehicles to come to an area. For the basic surveillance scenario two types of pheromones are used, one repulsive and one attractive. In their scenario the area to be surveyed generates attractive pheromones. When an area is visited the attractive pheromones are removed and no new pheromones are generated for some set time. To avoid that two vehicles try to survey the same area a vehicle places repulsive pheromones in the next place it plans to move to. The pheromones placed diffuse, that is slowly spread in the local environment. This creates pheromone gradients that the vehicles use to guide their movement. There are two main issues with their model. The first is that there seems to be a global pheromone map that all agents can access. This might closely simulate the real-life insect pheromone systems, but in a mechanical system where pheromones need to be placed in a virtual map this means that there is a central node managing the map. This design makes the system sensitive to the failure of that node and all vehicles require good communication to this node. Another issue is that they do not discuss how a vehicle determines where to go. That it is based on the pheromone map is clear, but the areas evaluated in order to select where to go are not described. Parunak et al.  propose two approaches to perform target localization and imaging. In the entity (individualistic) approach the UAVs use offline determined paths to guide their movement. In the group (team) approach visitation pheromones are used to deter UAVs from visiting areas recently visited. To produce a distributed and robust solution each vehicle maintains its own pheromone map. When a UAV passes through an area it updates its internal map and broadcasts its position, which makes it possible for all 14 UAVs within communication range to update their maps. When a UAV shall decide on its movement it randomly selects a location, where the probability is inversely proportional to the distance to the location and the pheromone concentration in the location. Unfortunately the paper does not provide any evidence of the performance of the localization and imaging approaches, which makes them difficult to evaluate. Gaudiano et al.  test several control principles for an area-coverage mission. From the tested approaches the pheromone one was the best. The problem with their pheromone strategy is that is seems to rely on a global pheromone map, giving the same problem as with the Sauter et al. solution. Additionally, the pheromones do not fade with time (dissipate) in the simple reconnaissance scenario, a property that they do use in a suppression mission scenario also presented in the paper. In the suppression mission the UAVs search for mobile targets and when found they try to destroy them. 2.1.2 Real-World Mobility Models To better resemble real-world behavior, mobility models can be designed based on observed real-world movement. Researchers have essentially used three types of mobility information to create mobility models based on realworld data. • Actual node movement • Node connections • Node connections to a base station To be able to create a mobility model based on observed behavior the system to be described must actually exist. If that is not the case then synthetic modeling is the only option. Due to the cost and difficulty of collecting traces the trace data will generally not be enough to run the amount of simulations needed to get enough confidence in the performance of tested routing protocols. For this reason it is generally good to build a synthetic model based on trace data. The model can then generate an infinite number of traces with the same statistical mobility properties as the real-world nodes have. 15 184.108.40.206 Position Based Traces The best traces are those that collect the actual node mobility. Unfortunately these are very difficult and expensive to collect since a large number of nodes need to be equipped with position tracing equipment. For this reason large scale position based traces have not been collected by the research community. In  Rhee et al. collected traces from in total 44 individuals at five different sites. While this is a relatively small population they found that all walks had statistical features similar to those of Levy walks . Instead of actually collecting node movement Hsu et al.  have performed surveys on the USC campus and have constructed a model mobility where nodes move between popular locations. They found that people tend to aggregate at popular locations and that ad hoc network connectivity between these locations is poor due to the low concentration of nodes between the popular locations. 220.127.116.11 Node Connection Based Traces Instead of collecting node positions it is generally easier to collect node inter contact times (ICTs). As with collecting position based traces all nodes need to be equipped with measuring devices, but instead of measuring where a node is the times at which the nodes are in communication range are measured. From these traces it is generally not possible to create a mobility model in the Euclidian space, but instead an inter contact model is created. The limit of such a model is that radio interference is generally difficult to represent in simulations using inter contact models. Examples of collected ICT traces are the studies by Su et al.  and Hui et al. . Both groups recorded the ICTs of humans carrying specially prepared Bluetooth devices. The traces were collected from ten to fifty persons. A consequence of the small populations in the traces is that the routing options are much more limited than if a larger population would have been used. Su et al. performed some routing experiments on the collected traces. They got a median latency of just under three days with a delivery ratio of 86% with epidemic routing. For most practical applications these results are not good enough, but the authors expect that the results would be vastly improved with a larger population, that is a denser network. Hui et al. found that the inter contact times exhibited a power law distribution with a coefficient of less than one. Analyzing what a power law distribution meant for routing they found that with a coefficient of less than 16 one all naive routing algorithms (including epidemic routing) had an infinite expected delay. Since normally used mobility models like random waypoint do not exhibit this type of ICTs there is a need to investigate routing performance under this type of mobility. At the University of Massachusetts, Amherst, a testbed composed of 30 busses has been constructed . Each bus is equipped with a computer with two IEEE 802.11b interfaces and a GPS receiver. From this testbed they have collected actual contacts and transfer possibilities. This means that the traces contain the actual amount of data that can be transferred at each contact. The GPS traces collected contained several gaps where contact with the satellites had been lost due to placement constraint of the GPS receiver. To be able to use the GPS data the missing parts had to be reconstructed . 18.104.22.168 Base Station Based Traces In networks with base stations the connection and disconnections of nodes to the base stations can be measured. The advantage compared to the two previously presented methods is that the nodes do not need any instrumentation and the connection information is normally collected anyway by the base stations. The disadvantage is that only approximate node location and node inter contact information is available. Two publicly available data sets from Wi-Fi base stations are from UCSD  and Dartmouth College . In  Song et al. used the Dartmouth traces to create a connection trace with the assumption that all nodes connected to a base station can communicate with each other, but with no other nodes. This is a very rough model since it does not take interference into account and it makes very simplifying assumption regarding the nodes that can communicate with each other. Two nodes connected to the same base station might not be able to communicate if they are located at opposite ends and two nodes connected to different base stations should be able to communicate if the nodes are located close to each other. If the locations of the base stations are known and if some node movement properties are known then a mobility model from the base station traces can be created. In  Kim et al. have used traces from Wi-Fi phone users at the Dartmouth College to create a mobility model for these users. Combining syslog data from the base stations and with the knowledge of the locations of the base stations they created a mobility model for Wi-Fi phone users. The 17 model was validated against walks from users holding both a phone and a GPS receiver. 2.2 Routing Routing in connected mobile ad hoc networks has been studied extensively and routing protocols like AODV , DSR  and GPSR  have been suggested. All these protocols assume the existence of a contemporaneous path between sender and receiver. In networks without contemporaneous paths, but where node mobility can overcome partitions, a different type of routing algorithm is required. In RFC 4838  Cerf et al. describe an architecture for delay-tolerant and intermittently connected networks (DTNs). Their architecture is designed for heterogeneous networks that are subject to long delays and/or discontinuous end-to-end connectivity. The architecture is based on asynchronous messaging and uses postal mail as a model of service classes and delivery semantics. The architecture makes the following three assumptions: • That storage is available and well distributed throughout the network. • That storage is sufficiently persistent and robust to store data until forwarding can occur. • That the “store-and-forward” model is a better choice than attempting to effect continuous connectivity or other alternatives. Due to the long delays and disconnections in DTNs end-to-end reliability methods like acknowledgements and timed out retransmissions are not suitable for DTNs. To be able to offer reliable transfers in DTNs RFC 4838 provide custody transfer. In custody transfer a message is moved between custodians that take responsibility for reliable delivery of the message. In essence the network guarantees that a message is not lost. The mobility of the nodes does mean that the network topology will constantly change and that nodes constantly come in contact with new nodes and leave the communication range of others. In RFC 4838 Cerf et al. classify the contacts based on their predictability into scheduled, predicted and opportunistic contacts. With scheduled contacts the nodes know when they will be able to communicate with a specific peer. If nodes can estimate likely meeting times or meeting frequencies you have a network with predicted contacts. If no information is available on node contacts then the contacts are 18 opportunistic. In this thesis we will study routing in DTNs with opportunistic contacts. An overview of different routing strategies in delay-tolerant networks can be found in Zhang’s survey . 2.2.1 DTN Routing in Opportunistic Networks Routing in DTNs with opportunistic contacts is challenging since contact times and durations are not known in advance. The challenge for the routing protocol is to determine if a packet shall be handed over to a peer or not when they meet. Factors that influence this decision are probability that the peer can move the packet closer to the destination, available buffer spaces in the two nodes and relative priority to forward this packet compared to other packets the node holds. If nodes are location aware then the relative position and direction of the nodes can be used to influence the forwarding decision. Three examples of location unaware routing protocols for this environment are Randomized Routing , Epidemic Routing  and Spray and Wait . In Randomized Routing only a single copy of a packet is present in the network. When two nodes meet a packet is handed over to the other node at some set probability. This means that a packet randomly walks around in the network until it reaches the destination. This routing principle is better than keeping a packet at the source node until it comes in contact with the destination provided that the transmission speed is faster than the mode movement or if node movements are local. In Epidemic Routing (ER) all packets are distributed to all nodes in the network (or at least a considerably large subset of nodes) giving a high cost in both transfer and storage overhead. When two nodes meet they exchange information on the messages stored in the nodes. Each node then decides on the messages it wants to receive and request these from the other node. If a node’s buffer space becomes full it drops the oldest messages first to make place for new messages. A major problem with ER is that some messages might not be transmitted in an overload situation since there is no prioritization regarding transmission or drop order. Due to the epidemic spread of messages in ER the network will be overloaded even at relatively low transmission rates. To better handle transmission in an overload situation Ramanathan et al. have proposed prioritized epidemic routing (PREP) . The addition PREP does to ER is that it prioritizes packets when it comes to 19 transmission and deletion. By this it ensures that the packet that has the most to gain on being transferred gets transmitted first and when a packet needs to be dropped the packet expected to suffer the least from being removed is dropped first. By these simple mechanisms PREP manages a good delivery ratio even when the network is overloaded. In Spray and Wait a packet is distributed to a limited number of nodes who hold on to the packet until they meet the destination. The recommended initial distribution method is to use binary spraying. When a node with more than one copy of the packet meets a node that has not seen the packet then half of the packets are handed over to the new node. With Spray and Wait a destination close to the source will probably receive the packet during the spraying phase. Destinations further away will have to wait until node mobility moves a node that stores the packet within communication distance to the destination. A strength of Spray and Wait is that the transmission overhead of each generated packet is bounded. Spray and Wait can be an efficient protocol if the nodes that carry the packet cover a large part of the network with their mobility. To improve Spray and Wait Spyropoulos et al.  have suggested to only spray to nodes that are more likely to encounter the destination. Each node has then a utility value and only nodes with a good enough utility value will be selected for spraying. If the nodes are location aware and the (approximate) location of the destination is known then the packets can be forwarded by geographic routing. Li et al.  have modified GPSR  to better handle temporary disruptions in relatively sparse networks (55 nodes/km² compared to our even sparser scenario that has 10-30 nodes/km²). By using temporary storage (up to 2 seconds) and having a set of possibly reachable neighbors they substantially increased the delivery ratio compared to GPSR. Their approach is geared towards handling short temporary disruptions due to obstructions, node mobility or interference, and not intended to handle substantial disconnections. LeBrun et al.  have performed geographical routing in a very sparse (0.34.4 nodes/km²) delay-tolerant network with a stationary destination. In their motion vector (MoVe) routing algorithm a message is handed over to a peer if, given their current directions, the peer is expected to come closer to the destination than the holder of the packet. To limit the overhead MoVe uses a request-response mechanism. This means that only nodes holding a message transmit HELLO messages. When another node hears a HELLO message it 20 responds with a RESPONSE message. When a link is established using this exchange the nodes start to exchange information to determine if the message shall be handed over or not. 2.2.2 Beacons-less Routing Most routing protocols require knowledge of a node’s neighbors to make their routing decisions. This information is generally gathered by the use of beacons, messages broadcasted regularly that will be heard by all nodes within communication distance. Knowledge of your neighbors makes more informed routing decisions possible, but beacons have their drawbacks. In  Heissenbüttel et al. describe the problems with beacons and present some remedies. The main problems with beacons are: • Energy is consumed to transmit, receive and process the beacons. • The beacons interfere with data transmissions. • Neighbor information can be inaccurate due to node mobility. The main problem with inaccurate neighbor information is that transmissions are attempted to nodes that have moved out of range. These transmissions will cost a lot of energy and bandwidth. Given these problems with beacons alternatives to beaconing should be evaluated to see if better solutions can be found. There have been several suggestions for beacon-less routing protocols: • Beacon-less routing (BLR)  • Implicit geographical forwarding (IGF)  • Geographic random forwarding (GeRaF)  • Contention-based forwarding (CBF)  • Priority-based stateless geographical routing (PSGR)  • Guaranteed delivery beacon-less forwarding (GDBF) . They all do geographical routing which means that the nodes have to be location aware and to select the forwarding node they use broadcasts and timers. Instead of selecting the forwarder from a list of neighbors and sending the data packet to the selected node, as done by most geographical routing protocols, they broadcast the data packet or a ready/request to send (RTS) to all neighbor nodes. All nodes in a defined forwarding area are eligible forwarders and set timers based on how good they are as forwarders where the best forwarder gets the shortest time. When a timer expires a node either 21 broadcasts the data packet or a clear to send (CTS). The eligible forwarders that overhear such a transmission generally abort their timer. The protocols use one of two transfer principles. They either use a RTS/CTS exchange followed by an acknowledged point-to-point data message transfer (IGF, PSGR, GDBF) or the data message is broadcasted and successful transfer is acknowledged upon previous holder overhearing the forwarder rebroadcast the message (BLR, CBF). GeRaF does not model on the transmission layer and can use both principles. The rationale for choosing either method is not discussed in any of the papers. An argument for using the RTS/CTS is that the RTS and CTS messages are relatively short and it makes it possible to transmit the (long) data message using a point-to-point transfer with reliability mechanisms such as acknowledgements and resending. An argument for directly sending the data message is that the RTS/CTS sequence consumes relatively much bandwidth compared to the data packet due to message spacing times of the MAC protocol. We hope to be able to investigate this tradeoff in the future under realistic transmission models where interference and transmission errors are well modeled. The forwarding area used is often of limited size so that all nodes in the area can hear the transmissions of all other nodes in the area assuming a constant radio range. Commonly used shapes of the forwarding area are a 60º circle sector, a Reuleaux triangle or a circle (see Figure 2a-c). The longest distance for all the shapes is normally the assumed communication distance. BLR, IGF and CBF use these types of forwarding areas. Holder Holder Sector (a) Holder Reuleaux (b) Figure 2. Holder Circle (c) Progress (d) Forwarding areas. In GeRaF, PSGR and GDBF all nodes that provide progress are eligible forwarders (see Figure 2d). The fact that overhearing between all nodes is not possible is treated differently by the protocols. GeRaF has not dealt with the issue since it is only simulated using a high level simulator. GDBF assumes that overhearing the CTS sent by the forwarder and data packet sent by the holder is enough. PSGR treats collisions between CTS packets in great detail and tries to ensure that no collisions between CTS packets occur. 22 The different protocols use different criteria for the characteristics of a good forwarding node. CBF, GeRaF, PSGR and GDBF all prioritize long steps, that is the forwarder should be as close to the destination as possible. BLR on the other hand prioritizes short steps. The reason for this is that BLR alternates between finding a path (and transmitting the first packet) using a geographic beacon-less strategy and sending packets through the found path using pointto-point transfers. If the nodes can adjust their transmission power to the minimum required to make a reliable transfer then short hops consume less system bandwidth than long hops. IGF considers both the power available in the nodes and the progress made. With equal energy nodes closer to the destination are selected, but as energy is depleted the timer is increased which means that nodes with low energy are less likely to be selected as forwarders. 23 3 Reconnaissance Mobility Since the performance of routing algorithms will change depending on the scenario and scenario parameters  it is important to carefully select a mobility model that represents the environment a routing protocol is intended to work in. We have chosen to create a mobility model that could represent how UAVs move when performing reconnaissance of an area. 3.1 Scenario The main scenario used to evaluate the suggested routing protocols is a military reconnaissance scan scenario. The objective is to scan an area regularly using multiple UAVs. Since it is probable that the units on the ground do not want to be detected by the UAVs there may not be any apparent pattern to when a UAV will scan a particular area. The same type of requirements can be found on the searching behavior in the FOPEN (Foliage Penetration) scenario reported in the work by Parunak et al. . To collect the reconnaissance data the UAVs have a camera directed downwards covering a rectangular image centered at the UAV. The images captured are then processed by the UAV. If something of interest is detected then this information needs to be sent to a unit that can act on the information. For the routing simulations in chapter 4 we have used four stationary receivers, but other configurations are conceivable including mobile receivers. A fixed wing aircraft is limited in its movement in that it has a minimum and maximum air speed and that an instantaneous change of direction is not possible. As we are mainly interested in the behavior of the system of UAVs a coarse description of the movements of the individual UAVs (as opposed to a detailed kinematic model) has been used. The UAVs’ movements are described using a 2D model with fixed speed, constant radius turns, and no collisions. The reason to use a 2D model is that all UAVs are flying at about 24 the same altitude and there is no need to model start and landing. A fixed speed is relatively realistic in a reconnaissance scenario. There should be a speed drop during turns, but the benefit of modeling that is expected to be minor. The reason to use constant radius turns is that it is much easier to model, and a more realistic progressive turn model is not expected to add any major value to the simulation. The reason that collisions do not have to be modeled is that it is assumed that the UAVs can make altitude adjustments to avoid collisions. For the scenario we envision the real-world parameters according to Table 3. The parameters are based on reasonable assumptions made by domain experts. Table 3. Radio range UAV flight speed UAV flight altitude UAV turn radius Camera coverage area Scenario parameters 8000 m 150 km/h (81.0 knots) 3500 m (11 000 feet) 500 m 2000x1000 m 3.2 Random Mobility Model As a comparative baseline for the main mobility model described in the next section we have created a simple random mobility model. The random model is a Markov process  where the UAVs regularly decide whether to go straight ahead, turn right or turn left. For our simulations this decision is taken every other second with the probabilities given in Table 4. If a UAV moves closer than the turn radius to an edge then it turns towards the centre of the search area until it has reached a randomly chosen direction -45° to 45° related to the normal of the edge of the search area. Compared to the GaussMarkov model in  this model has no mean direction and the directional change is given as three discrete values, not a continuous distribution. Table 4. Last action Straight ahead Turn left Turn right Random action table. Probability of action Turn left Straight Turn right ahead 10% 80% 10% 70% 30% 0% 0% 30% 70% 25 3.3 Distributed Pheromone Repel Mobility Model To produce a mobility control algorithm that is robust and random we have designed a distributed pheromone repel model. By using pheromones and localized search the UAVs are guided to areas not recently visited by other UAVs. When a UAV moves around it places pheromones on the areas it has scanned with its camera. Since it is not possible to place these pheromones in the environment as would be done in a natural system the UAV places them in a local pheromone map. The pheromone map is a grid where each element contains a timestamp representing the last time the element was scanned. Since timestamps are used pheromones placed will slowly fade away. To share this pheromone information with the other UAVs each UAV regularly broadcasts a local area pheromone map. All UAVs that receive the broadcast merge this information into their pheromone map. The broadcast frequency and size of the map broadcasted needs to be adjusted to limit the bandwidth required for the transfer of the pheromone information. The actual parameters used can be found in Table 5. Table 5. Pheromone map parameters. Grid element size Transfer map size Transfer interval 100 m 5000x5000 m 10 seconds Figure 3 to Figure 6 illustrate the difference between the local pheromone map held in a UAV and the total pheromone data present in the system. In the figures black represents fresh pheromones and white represents no pheromone information or pheromones older than the time out of one hour. Also drawn on the maps is the path of the UAVs whose local pheromone map is shown. The pheromone maps are taken from simulations where the probability of a successful transfer of pheromone data was set to 50%. As we will show in the evaluations in section 3.4 there is no real benefit for a UAV to have access to the global pheromone map. The important thing is to have a reasonably accurate pheromone map in the UAVs vicinity. 26 Figure 3. Figure 4. Local pheromone map after 3600 seconds of simulation. Global pheromone view after 3600 seconds of simulation. 27 Figure 5. Figure 6. Local pheromone map after 7200 seconds of simulation. Global pheromone view after 7200 seconds of simulation. 28 As with the random model a UAV decides to turn left or right or go straight ahead every other second. But instead of making this decision with fixed probabilities, the probabilities are based on the pheromone smell in three evaluation areas. Each area is a circle and they are placed as shown in Figure 7. To get the pheromone smell from the time stamps stored in the pheromone map the times are scaled so that the current time gives maximum intensity and the smell is then linearly reduced to zero at the fade away time. The computation is done according to equation (5) with parameters as defined in Table 6. The smell zero at time zero is needed to handle elements that have never been scanned since the elements in the pheromone map are initialized to zero. 2000 Left Center Right 1500 1000 500 Scan area UAV 0 −500 −1000 Figure 7. Table 6. Parameter sarea stotal selement telement t dtime_out −500 0 500 1000 Pheromone search pattern. Pheromone parameter definition. Description Total smell in an area (left, center, right) sleft + scenter + sright Smell in a pheromone map element Timestamp in a pheromone map element Current time Pheromone fade away time sarea = ∑ selement area selement 0 = 0 t element − t + d time _ out t element = 0 t element ≤ t − d time _ out otherwise (5) 29 Since a UAV should go to places not recently visited it should prefer areas with a low pheromone smell. For that reason the probability of action is defined as specified in Table 7. If no pheromone smell is reported for any direction then a random direction is chosen as in the random model. If the center and either the left or right has no smell then a random direction is chosen between these two. The area outside the search area is given a high pheromone smell (higher than the ordinary full intensity) for the UAVs to avoid it. A special rule has been added to handle the case when a UAV flies directly into a corner of the search area. If only guided by the pheromones then a UAV flying into a corner would get very high smells in the left and right areas and a low smell in the center area. This would mean that the UAV would be guided straight into the corner. To counter this problem a UAV turns right if both the right and left areas have a smell intensity that indicate that parts of the evaluation area is outside the search area. Table 7. Turn left (stotal – sleft) / (2 * stotal) UAV pheromone action table. Probability of action Straight ahead (stotal – scenter) / (2 * stotal) Turn right (stotal – sright) / (2 * stotal) 3.4 Evaluation As described in section 3.1 the main objective of the reconnaissance scenario is to scan all parts of an area regularly. If the area is large and the requirement of how often each part of the area shall be scanned is low the several UAVs have to cooperate to perform the scanning. For the evaluations we have set the requirement that each part of the reconnaissance area should be scanned at least once every hour. To reflect this the pheromone fade time was set to one hour for the pheromone mobility model. The area over which reconnaissance should be performed was set to a 90x90 km square and 90 UAVs were used. Initially all UAVs started from the center of the south edge moving north. This start will challenge the mobility model to get the UAVs to spread out over the entire reconnaissance area. In addition to the two mobility models presented in sections 3.2 and 3.3 the scenario was also run with the random waypoint mobility model with no wait times. The reason to include random waypoint is that it is the most commonly used mobility model in ad-hoc networking research. The mobility 30 models were tested by performing 50 independent runs per model where each run simulates 3 hours. To evaluate the robustness of the pheromone logic the distributed pheromone mobility model was tested with several data transfer probabilities and compared to an ideal case where a global pheromone map was accessible to all UAVs. The transfer probabilities used were 100%, 50%, 10% and 0%. This simulates a range of cases from perfect transmission capability (100%) to the absence of radio communication (0%). A transfer probability of 0% will mean that a UAV is only guided by its own pheromones. To evaluate the main scanning objective we have looked at the scan coverage, that is the percentage of the area scanned the last hour. A secondary requirement was to scan in an unpredictable pattern. To evaluate this property we have studied the probability distribution of the time between scans. Finally we also looked at the wireless connectivity to determine the viability for ad hoc routing in the evaluated setting. 3.4.1 Scan Coverage Initially the UAVs shall scan the area as fast as possible. When the initial scan is completed the UAVs need to continuously monitor the area by rescanning every part at least once per hour. The coverage data from the different runs are presented in Figure 8 to Figure 14. The figures show the average coverage from the 50 runs and the 95% confidence interval. In Figure 15 the averages from some of the runs are collected for easier comparison. The absolute maximum scan speed is 0.083 km²/second/UAV according to equation (6) and the data from section 3.1. Given the area of 8100 km² and 90 UAVs the fastest time to cover the whole area (which is in practice impossible) is 18 minutes (1056 seconds). Adding the overhead of turning and additional requirements like randomness a coverage time of 40 minutes (2400 seconds) should be feasible. Extrapolating the steepest part of the coverage curve of the pheromone and random waypoint mobility models the coverage time is a little more than 40 minutes. Scan speed = UAV speed * scan area width (6) Comparing Figure 8 and Figure 9 we see that using a global pheromone map does not give any better results than using a distributed pheromone map. This means that the UAVs manage to distribute the local information needed for the mobility decisions. Looking at Figure 10 we see that if only 50% of the 31 transmitted pheromone maps are received then the UAVs still have enough information to efficiently scan the area. When the transfer probability drops to 10%, Figure 11, then it takes substantially more time to reach a steady state and a slightly lower steady state coverage is achieved. But considering the large loss of information the result is still good. In Figure 13 we see the problem with not having pheromones that can guide the nodes to previously uncovered areas. With the random mobility model it takes much time for the UAVs to become evenly spread out over the area. In simulations with a smaller area, but with the same node density the random mobility model achieved a steady state coverage of about 90±8% with a 95% confidence interval. Adding only local pheromone information as reported in Figure 12 the coverage improves, but not enough to provide acceptable results. In Figure 14 the properties of the random waypoint mobility model becomes relatively clear. The initial coverage rate is good since the nodes move out from the start position to different points in the area. The reason that the coverage stabilizes around 84% is due to the steady state distribution of the nodes. Since more nodes are present in the central parts of the reconnaissance area than at the edges the areas close to the edges do not get scanned regularly enough. 1 0.9 0.8 Coverage 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000 Time (seconds) Figure 8. Pheromone mobility coverage with global pheromone map. 32 1 0.9 0.8 Coverage 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000 Time (seconds) Figure 9. Pheromone mobility coverage with 100% transfer probability. 1 0.9 0.8 Coverage 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000 Time (seconds) Figure 10. Pheromone mobility coverage with 50% transfer probability. 1 0.9 0.8 Coverage 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000 Time (seconds) Figure 11. Pheromone mobility coverage with 10% transfer probability. 33 1 0.9 0.8 Coverage 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000 Time (seconds) Figure 12. Pheromone mobility coverage with 0% transfer probability. 1 0.9 0.8 Coverage 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000 Time (seconds) Figure 13. Random mobility coverage 1 0.9 0.8 Coverage 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000 Time (seconds) Figure 14. Random Waypoint mobility coverage 34 1 0.9 0.8 Coverage 0.7 0.6 0.5 0.4 0.3 0.2 Pheromone 100% Pheromone 10% Pheromone 0% Random Waypoint 0.1 0 0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000 Time (seconds) Figure 15. Comparison of average coverage. 3.4.2 Scan Characteristic The random and pheromone mobility models have been designed to produce unpredictable scanning patterns. The randomness of the scanning can be investigated by looking at the probability distribution of the time between scans. In Figure 16 to Figure 22 the average probability distributions and the 95% confidence interval of the three models are shown. The area under the curve between two time points is the probability that the next scan will occur during this time period after the current scan. The question is then; what is the desired distribution? To this question there is no definite answer, but a uniform distribution (the horizontal dashed line from 0 to 2160 seconds) should be an attractive result. This would mean that the probability of the next scan is evenly distributed over some time period. The only firm requirement on the distribution function is that it shall be zero after one hour to meet the one hour rescan requirement. What is very obvious from these graphs is that as long as the nodes can exchange some pheromone information the pheromone model manages quite well to avoid rescanning a recently scanned area (does not peak for low time between scans). As we saw in the scan coverage graph no model manages to achieve 100% coverage. This is here seen by that the function is not zero after one hour. The limitation of the probability distribution graphs is that they do not include the areas never scanned or only scanned once. To see the capability of the models to scan the complete area at least once during the three hours the maximum, median and minimum uncovered area for the 50 runs are shown in Table 8. These numbers clearly show the ability of the pheromone model to 35 cover the complete area. The concentration of the nodes to the central parts of the area by the random waypoint mobility model is exhibited by the relatively large values for the never scanned area. Table 8. Never scanned area Max 0.04% 0.07% 0.09% 3.66% 13.52% 29.76% 6.79% Pheromone global Pheromone 100% Pheromone 50% Pheromone 10% Pheromone 0% Random Random Waypoint Median 0.01% 0.01% 0.01% 0.18% 4.73% 20.50% 5.20% Min 0.00% 0.00% 0.00% 0.02% 1.93% 13.21% 3.80% −4 Probability density 15 x 10 10 5 0 500 0 1000 1500 2000 2500 3000 3500 4000 4500 5000 Time until next scan (seconds) Figure 16. Pheromone mobility with global pheromone map. −4 Probability density 15 x 10 10 5 0 0 500 1000 1500 2000 2500 3000 3500 4000 4500 5000 Time until next scan (seconds) Figure 17. Pheromone mobility with 100% transfer probability. 36 −4 Probability density 15 x 10 10 5 0 500 0 1000 1500 2000 2500 3000 3500 4000 4500 5000 Time until next scan (seconds) Figure 18. Pheromone mobility with 50% transfer probability. −4 Probability density 15 x 10 10 5 0 500 0 1000 1500 2000 2500 3000 3500 4000 4500 5000 Time until next scan (seconds) Figure 19. Pheromone mobility with 10% transfer probability. −4 Probability density 15 x 10 10 5 0 0 500 1000 1500 2000 2500 3000 3500 4000 4500 5000 Time until next scan (seconds) Figure 20. Pheromone mobility with 0% transfer probability. 37 −4 Probability density 15 x 10 10 5 0 500 0 1000 1500 2000 2500 3000 3500 4000 4500 5000 4500 5000 Time until next scan (seconds) Figure 21. Random mobility. −4 Probability density 15 x 10 10 5 0 0 500 1000 1500 2000 2500 3000 3500 4000 Time until next scan (seconds) Figure 22. Random Waypoint mobility. 3.4.3 Communication Reconnaissance data has no value if it cannot be transmitted to where it is needed and nodes can only transmit data between each other if they are within radio range. With the used node density of 0.011 nodes/km² communication would be impossible if the nodes were evenly distributed in a regular grid since they then would have a distance of about 9.5 km to their closest neighbor which is larger than the radio range of 8 km. In Figure 23 to Figure 29 the average number of partitions and 95% confidence interval for the three mobility models are illustrated. A partition consists of the UAVs that can pair-wise establish a contemporaneous path with each other, either directly or via peers. 38 In Figure 23 to Figure 26 we see that the pheromone mobility manages to spread the nodes well since we have a high number of partitions. With the low (10%) transfer probability it just takes some more time to reach the spread. In Figure 28 we again observe that the UAVs moving under the random mobility model have not yet reached a steady state. The same conclusion is also valid for nodes moving under the pheromone mobility model but where no pheromone data is exchanged between the nodes (see Figure 27). For the random waypoint mobility model the increased density of nodes in the central parts of the simulation area leads to a lower number of partitions compared to the pheromone mobility model as shown in Figure 29. With an average of about 30 partitions for configurations with good coverage the existence of a contemporaneous path between two random nodes is not likely. To be able to send the information to any selected peer delay-tolerant routing principles need to be used. This is the topic of the next chapter where delay-tolerant routing on nodes that move under the pheromone mobility model is further studied. 50 45 40 Partitions 35 30 25 20 15 10 5 0 0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000 Time (seconds) Figure 23. Pheromone mobility with global pheromone map. 39 50 45 40 Partitions 35 30 25 20 15 10 5 0 0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000 Time (seconds) Figure 24. Pheromone mobility with 100% transfer probability. 50 45 40 Partitions 35 30 25 20 15 10 5 0 0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000 Time (seconds) Figure 25. Pheromone mobility with 50% transfer probability. 50 45 40 Partitions 35 30 25 20 15 10 5 0 0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000 Time (seconds) Figure 26. Pheromone mobility with 10% transfer probability. 40 50 45 40 Partitions 35 30 25 20 15 10 5 0 0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000 Time (seconds) Figure 27. Pheromone mobility with 0% transfer probability. 50 45 40 Partitions 35 30 25 20 15 10 5 0 0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000 9000 10000 Time (seconds) Figure 28. Random mobility. 50 45 40 Partitions 35 30 25 20 15 10 5 0 0 1000 2000 3000 4000 5000 6000 7000 8000 Time (seconds) Figure 29. Random Waypoint mobility. 41 4 Routing in DTNs Routing in opportunistic DTNs is challenging since it is generally not possible to establish a contemporaneous path between source and destination. Using the store-carry-forward principle it is possible to transfer messages between nodes by exploiting node mobility. The main difficulty is then to decide when and to whom to forward a message that a node is carrying. In this chapter we describe and evaluate our proposed routing algorithm LAROD (location aware routing for opportunistic delay-tolerant networks). LAROD is evaluated against two other routing protocols described in sections 4.2 and 4.3. 4.1 LAROD LAROD is a geographical routing protocol for DTNs that combines the geographical beacon-less routing with the store-carry-forward principle. In its essence LAROD uses greedy packet forwarding when possible. When greedy forwarding is not possible the node holding the packet (the custodian) waits until node mobility makes it possible to resume greedy forwarding. To reduce system wide overhead LAROD uses the same basic beacon-less routing strategy as BLR  and CBF  (see section 2.2.2). Instead of forwarding a packet to a neighbor based on location data received from beacons a custodian simply broadcasts a message when it wants to transmit it. The “best” forwarding node receiving the message then forwards the message in the same manner. The custodian that sent the message will overhear this transmission and conclude that the forwarder has taken over custody of the packet. If no such transmission is heard the node regularly broadcasts the message until a forwarder becomes available due to node mobility. 42 Forwarding area Circle Custodian Destination (a) Figure 30. Forwarding area Progress Custodian Destination (b) LAROD forwarding areas. To select the “best” node to forward a message among all nodes receiving it a forwarding area and timer delay function is used. The nodes that are eligible to forward a packet are determined by a forwarding area related to the node performing the broadcast. In  Heissenbüttel et al. recommended a circle as the best forwarding area (see Figure 30a). Our simulations have shown that this area is too small due to the low density of nodes we use. Instead all nodes that provide some minimum progress (dprogress) towards the destination are eligible as forwarders (see Figure 30b). An implication of using this larger forwarding area is that multiple copies of a packet can be active since all nodes in the forwarding area might not hear the broadcast made by the best forwarder. This should not be seen as a problem, since it enables exploration of multiple paths to the destination. All nodes receiving the packet in the forwarding area are tentative forwarders and to select the best forwarder they all set a delay timer when to rebroadcast the packet. The node with the timer that expires first will rebroadcast the packet. This broadcast will be heard by the old custodian and most of the other tentative forwarders. Overhearing the transmission they will relinquish custody of the packet. Since the forwarding area chosen is larger than the transmission range it is possible that several nodes take over custody of the packet. The delay function that computes the actual delay (tdelay) is defined in equation (7) with the equation parameters defined in 0. The vectors and distances used in the equation are illustrated in Figure 31. Depending on the relative position of the receiver, custodian and destination the delay is computed based on one of three cases. The delay for all three cases has a linear component and a random component. When the custodian is so close to the destination that the destination should have received the packet all tentative forwarders calculate the linear delay based on the distance between the tentative forwarder and the destination (first row in equation (7)). In all other cases the delay is based on the distance from the custodian along the line connecting the custodian 43 and destination. Nodes not on the line get their distance by perpendicularly projecting their position on to the line. Nodes with a computed distance of less than dzero have a linear component with a maximum delay (tlmax) at the custodian and zero delay at dzero (last row in equation (7)). The dzero distance should be set close to the expected radio range. For nodes with a distance of more than dzero the linear component is zero (line two in equation (7)). The reason to have this case is that due to radio propagation factors nodes further away than the expected radio range can receive a packet. To prevent simultaneous transmission from nodes whose linear delay is zero all cases add a small random delay (0 to trmax) to the computed linear delay. trmax should be set much smaller than tlmax to make the impact of the random factor small on the linear factor. To be able to calculate the delays all LAROD data packets contain the position of the source, destination and last forwarder (custodian). For BLR Heissenbüttel et al. recommended the inverse function, that is short steps. This was done to be able to transmit at low power in the unicast phase to increase total system bandwidth. Since there is no unicast phase in LAROD it exploits the benefits of long steps. In the case that the custodian does not hear a rebroadcast it will perform a new broadcast after some time (trebroadcast). To stop transmission when a packet has reached its destination the destination broadcasts an acknowledgement packet. This packet is rebroadcasted by all custodians and tentative custodians hearing it will stop their retransmissions. All nodes hearing an acknowledgement packet will store the packet until the packet times out (tTTL). If a node receives a packet for which it previously has received an acknowledgement then it broadcasts an acknowledgement to stop the transmission of the packet. To prevent that a packet indefinitely tries to find a path to its destination all packets have a time to live (TTL) expressed as a duration (tTTL). When the TTL expires a packet is deleted by its custodian. For a full description of the routing protocol see Figure 32. LAROD is based on the principle that the custody of a packet is moved to the most recent node that accepted the packet. This means that the protocol is not robust to node loss. If a node fails for some reason then all packets that the node holds will be lost unless they happened to be duplicated due to diverging paths. 44 Table 9. LAROD parameter definitions. Parameter tdelay Description Delay until node shall broadcast received message. tlmax Maximum linear delay. trmax Maximum random delay. tTTL Delay until node shall rebroadcast a message for which it has not overheard a transmission by a forwarder. Maximum time a packet is permitted to live. dzero Distance from custodian to have zero linear delay. dprogress r n r d r r Minimum progress required to be an eligible forwarder. X Uniform random variable in range [0, 1]. trebroadcast t delay Vector from sender to potential custodian. Vector from sender to destination. Vector from potential custodian to destination. r r ⋅t + Χ ⋅ t rmax d zero lmax = Χ ⋅ t rmax r r d zero − nprojd ⋅ t lmax + Χ ⋅ t rmax d zero r d ≤ d zero r r n projd ≥ d zero (7) otherwise Receiver → n → r Custodian Destination → → n proj d → d dprogress dzero Figure 31. Illustration of vectors used for delay time computations. 45 Source node at data packet generation Broadcast data packet Set up timer for rebroadcasting packet to trebroadcast Destination node at data packet reception The packet is received for the first time Deliver data packet to application Broadcast ack packet A duplicate data packet is received Broadcast ack packet All intermediate (non-destination) nodes at data packet reception If an ack has been received for the packet Broadcast ack packet Else if the node is in the forwarding area If the node does not have an active3 copy of the packet Set up timer for rebroadcast to tdelay If the node has an active copy of the packet Do nothing Else Remove active copy if it has one At ack packet reception If the node has an active copy of the packet Remove active copy Broadcast ack packet Else Do nothing When a data packet rebroadcasting timer expires If the packet’s TTL has expired (tTTL) Remove packet Else Broadcast data packet Set up timer for rebroadcasting the packet to trebroadcast Figure 32. 3 LAROD pseudo code. A node has an active copy of a packet if it has a copy of the packet waiting on a timer to be rebroadcasted. 46 4.2 Broadcast DelayDelay-tolerant Routing (BDTR) To be able to estimate the delivery rate of a delay-tolerant routing protocol which searches all possible paths to a destination we propose BDTR. BDTR is a flooding based routing protocol that limits the flooding to an area centered round the source and destination. In contrast to epidemic routing  it does not use beacons to find its neighbors. Instead it uses the same principle as LAROD does, when it has something to send is simply broadcasts the message. All nodes within the flooding area that receive the message will rebroadcast it after a short random delay. By this mechanism all nodes in the partition of the source node should quickly receive a copy of the message and if there is a contemporaneous path to the destination in the broadcast area the packet will soon reach the destination. To overcome partitions all nodes in the flooding area regularly rebroadcast the data packet. When the packet reaches the destination an acknowledgment packet is sent and it is in turn rebroadcasted by all nodes that have received the data packet. As for LAROD a node will store the acknowledgement packet until the data packet times out (tTTL) and if a node receives a data packet for which it has received an acknowledgement packet it will broadcast an acknowledgement packet. For a full description of the routing protocol and the parameters see Figure 33 and Table 10. Table 10. Parameter tdelay trebroadcast tTTL BDTR parameter definitions. Description Delay until node shall broadcast received message. Delay until node shall rebroadcast a message for which it has not overheard a transmission by a forwarder. Maximum time a packet is permitted to live. For the simulations the flooding area used was a circle with a diameter slightly larger than the distance between source and destination. This area means that not all possible paths are searched, but it gives a good balance between search space size and limiting the overhead. Tests with the complete simulation area as broadcast area have shown that the overhead was prohibitive and that it affected the ability to search all paths even at the low loads used in our simulations. 47 BDTR is a simple protocol and robust in the sense that several nodes have an individual packet and try to forward it to the destination. This means that failure of a single node does not impact the routing to a great degree. The rebroadcast and resend strategies used by BDTR means that it will quickly saturate the wireless medium in dense networks or networks that transmit large amounts of data. This means that BDTR by design in not suited for these environments. Source node at data packet generation Broadcast data packet Set up timer for rebroadcasting packet to trebroadcast Destination node at data packet reception The packet is received for the first time Deliver data packet to application Broadcast ack packet A duplicate data packet is received Broadcast ack packet All intermediate (non-destination) nodes at data packet reception If an ack has been received for the packet Broadcast ack packet Else if the node is in the flooding area If the node does not have an active4 copy Set up timer for initial rebroadcast to tdelay If the node has an active copy Do nothing Else Do nothing At ack packet reception If the node has an active copy of the packet Remove active copy Broadcast ack packet Else Do nothing When a data packet rebroadcasting timer expires If the packet’s TTL has expired (tTTL) Remove packet Else Broadcast data packet Set up timer for rebroadcasting the packet to trebroadcast Figure 33. 4 BDTR pseudo code. A node has an active copy of a packet if it has a copy of the packet waiting on a timer to be rebroadcasted. 48 4.3 Broadcast Routing (BR) To evaluate how often a contemporaneous path exists between a sourcedestination pair a simple broadcast routing scheme was used. BR is identical to BDTR except that the nodes do not continuously resend packets, they are only re-broadcasted once. For BR we have used the entire simulation area as broadcast area. 4.4 Evaluation The above routing protocols have been implemented in a modified version of ns2 2.30 . The main changes to the simulator are: (1) addition of the main algorithm LAROD, and the new baselines (BDTR and its instance BR), (2) addition of the pheromone repel mobility model, (3) support for application level broadcasts, (4) an application to generate and consume events. As wireless communication technology we use IEEE 802.11 as implemented in ns2 2.30 with default parameters using the two-ray ground radio propagation model. For the evaluation we use the scenario described in section 3.1 but we have scaled the parameters to match the default radio range in ns-2. In Table 11 the parameters are given both as the values used in the simulations and what they correspond to in the scale used in section 3. The scale change makes comparison with other studies easier since they generally use ns-2 with default parameters. When scanning the area a UAV flies over it may detect something that needs to be communicated to a ground unit that can act on it. In the current simulations we have had four stationary ground units. The generation of detect events and selection of receiver are both random functions. For the routing experiments we are interested in the performance when the nodes are at the mobility model’s steady state. For this reason the UAVs are initially randomly distributed over the reconnaissance area. The simulation is then run for 1 hour without detect-event packets being generated to populate the pheromone repel model with data. The detect events are then enabled for 1 hour and the simulation ends after all detect-event packets have timed out. In the evaluations each data point is the average result of ten simulations. 49 Table 11. Parameter Reconnaissance area Placement of ground units Node density UAV speed Packet time to live (tTTL) Detect data generation rate Radio range Scenario parameters ns-2 2000x2000 meters Center of each quadrant 2 10-30 (20) nodes/km 0.8-2.0 (1.4) m/s 200-600 (400) seconds 12-120 (36) pkt/hour/UAV 250 meters Real-world 64x64 kilometers 2 0.010-0.029 (0.020) nodes/km 92-230 (161) km/h 8000 meters The node speed used in our simulations might seem low compared to the node speeds in other MANET papers , but it is actually close to the average speed used in most simulations. The speed presented in most papers is the top speed for the random waypoint mobility model. According to the equations in  for a top speed of 20 m/s and minimum speed of 0.1 m/s with no pause time 49.8% of the nodes would on average move slower than 1.4 m/s. Adding the fact that most simulations use pause times this means that they have large number of relatively stable nodes that can be used to create stable routes. In Table 12 and Table 13 the parameter values used for LAROD and BDTR are given. BR uses the same initial rebroadcast delay as BDTR. For values where a range is given the actual value used is randomized in the range using a rectangular distribution. Table 12. LAROD parameter values. Parameter tlmax trmax trebroadcast dzero dprogress Table 13. Parameter tdelay trebroadcast Flooding area diameter Value 1.0 seconds 0.2 seconds 8.0-12.0 seconds 240 meters 10 meters BDTR parameter values. Value 0.8-1.2 seconds 8.0-12.0 seconds Source-destination distance + 100 meters 50 The goal of the evaluations is to measure the ability of LAROD to efficiently and reliably perform routing in a mobile and intermittently connected network. We have measured the delivery ratio (proportion of packets delivered to the destination) and overhead (average number of packet transmissions per generated data packet) for the different protocols while changing the node density, node speed, packet life time and network load. To gain further insight in the delivery properties we also study the accumulated delivery ratio against delivery delay for some selected data points. 4.4.1 Node density The higher the node density the more connected the network will be and the easier it is to find a path between two nodes. As can be seen from the delivery ratio of BR in Figure 34 the node density has a dramatic impact on the existence of contemporaneous paths between source and destination. By using store-carry-forward both LAROD and BDTR can overcome communication gaps and they significantly improve the delivery ratio compared to BR. Even if LAROD and BDTR give about the same delivery ratio the overhead of BDTR is much higher as can be seen in Figure 35. At low node densities a BDTR packet is only receive by a few nodes since fewer nodes are in the broadcast area. Additionally the clusters of connected nodes (partitions) will contain fewer nodes. Both these factors result in fewer transmissions generated at low node densities. As the node density increases more nodes will receive a BDTR packet and the number of transmissions increases. The reason the transmissions fall at higher node densities is that packets will be delivered faster to the destination due to increased connectivity. This means that the packet will live a shorter time in the network, reducing the number of transmissions. For LAROD increased density means that fewer rebroadcasts are required to overcome partitions and thus the overhead is lower for higher node densities. Studying the delay curves in Figure 36 we see that even with similar eventual delivery ratio LAROD and BDTR have different delay properties. For higher node densities the wider search of BDTR results in lower average delivery delay since BDTR finds paths that LAROD does not. At low node densities, on the other hand, there are so few paths available that both BDTR and LAROD seem to find similar paths. 51 1 0.9 0.8 Delivery ratio 0.7 0.6 0.5 0.4 0.3 0.2 LAROD progress BDTR circle BR all 0.1 0 18 16 14 12 10 20 24 22 26 28 30 Node density (nodes/km2) Figure 34. Delivery ratio for different node densities. 300 LAROD progress BDTR circle BR all Transmissions per data packet 250 200 150 100 50 0 10 12 18 16 14 20 24 22 26 28 30 Node density (nodes/km2) Figure 35. Overhead for different node densities. 1 0.9 0.8 Delivery ratio 0.7 LAROD 30 nodes/km2 LAROD 20 nodes/km2 0.6 LAROD 10 nodes/km2 0.5 BDTR 30 nodes/km2 0.4 BDTR 10 nodes/km2 BDTR 20 nodes/km2 0.3 0.2 0.1 0 0 50 100 150 200 250 300 350 Delay (s) Figure 36. Delay for different node densities. 400 52 4.4.2 Node speed A higher node speed should give a higher delivery ratio since the increased mobility will change the network topology at a higher pace enabling messages to bridge communication gaps faster. In Figure 37 we see that this is the case for both LAROD and BDTR. It is interesting to note that the delivery ratio for BR increases somewhat with higher mobility. This probably comes from the fact that BR has a short random delay between message reception and the resending of a packet. During this time the nodes will move making it possible to reach nodes that were not reachable at packet reception. In Figure 38 we see that increased node speed decreases the overhead for BDTR and LAROD. This is a natural consequence of the improved delivery ratio at higher node speeds. Since most packets will be delivered before their expiry time fewer packets will be active in the network at higher delivery ratios and thereby reducing the number of rebroadcasts. Looking at the delay curves for LAROD and BDTR in Figure 39 we see that LAROD essentially has the same delivery ratio independent of speed after 10 seconds. This is essentially the delivery of packets to nodes with a contemporaneous path from the source. After that node mobility is required to close partition gaps which is done faster at higher speeds. For BDTR the initial delivery ratio (first 10 seconds) is significantly affected by the node speed. The reason is probably that with higher node speeds nodes receiving a packet do scatter a bit more before a packet is rebroadcasted than with lower speeds resulting in less interference. As for the node density DBTR has a slightly better delivery ratio due to that more paths are searched. 1 0.9 0.8 Delivery ratio 0.7 0.6 0.5 0.4 0.3 0.2 LAROD progress BDTR circle BR all 0.1 0 0.8 1 1.2 1.4 1.6 1.8 2 Node speed (m/s) Figure 37. Delivery ratio for different node speeds. 53 300 LAROD progress BDTR circle BR all Transmissions per data packet 250 200 150 100 50 0 2 1.8 1.6 1.4 1.2 1 0.8 Node speed (m/s) Figure 38. Overhead for different node speeds. 1 0.9 0.8 Delivery ratio 0.7 0.6 0.5 0.4 0.3 LAROD 2.0 m/s LAROD 1.4 m/s LAROD 0.8 m/s BDTR 2.0 m/s BDTR 1.4 m/s BDTR 0.8 m/s 0.2 0.1 0 0 50 100 150 200 250 300 350 400 Delay (s) Figure 39. Delay for different node speeds. 4.4.3 Time to live Increasing packet life time should increase the delivery ratio since more time is allowed for node mobility to bridge network voids. A longer life time will on the other hand also mean that the overhead will increase since more rebroadcasts will occur due to that packets will try to reach the destination for a longer time. In Figure 40 we see the expected delivery rate improvement for both LAROD and BDTR. Since BR is not affected by packet life time the delivery rate is basically constant. In Figure 41 we see a significant increase in the overhead for BDTR as the packet life time is increased. The overhead using LAROD also increases, but at a much lower rate. 54 Studying the accumulated delivery ratio related to delivery delay LAROD have essentially identical curved independent of time to live (TTL) value. For BDTR the impact of the higher network load associated with a high TTL value is obvious in Figure 42. This means that even if the eventual delivery ratio is higher for a TTL of 600 seconds than 200 seconds the time to deliver a packet will generally be longer. As TTL is a tunable protocol parameter the question is which value to choose. For data with a clear freshness requirement the TTL should match the requirement if it does not result in an unacceptably high network load. For other types of data the main limitation on a high TTL is acceptable network load and resource consumption to deliver a packet. 1 0.9 0.8 Delivery ratio 0.7 0.6 0.5 0.4 0.3 0.2 LAROD progress BDTR circle BR all 0.1 0 200 250 300 350 400 450 500 550 600 Packet life time (s) Figure 40. Delivery ratio for different packet life times. 350 Transmissions per data packet 300 250 200 150 LAROD progress BDTR circle BR all 100 50 0 200 250 300 350 400 450 500 550 600 Packet life time (s) Figure 41. Overhead for different packet life times. 55 1 0.9 0.8 Delivery ratio 0.7 0.6 0.5 0.4 0.3 0.2 BDTR 200 s BDTR 400 s BDTR 600 s 0.1 0 0 100 200 300 400 500 600 Delay (s) Figure 42. Delay for different packet life times. 4.4.4 Network load To investigate the scalability of the routing protocols when it comes to network load different event generation rates were tested. Ideally a fully scalable protocol will give the same result independent of network load. In Figure 43 we see that the delivery ratio of BDTR drops as the load increases while it stays constant for LAROD. The problem for BDTR is that the network becomes congested and that several transmissions fail due to collisions. This clearly shows that the broadcast strategy of BDTR has scalability issues. For LAROD the critical load level is not yet reached and as a result the delivery ratio is stable. In Figure 44 we see the consequence of the lower delivery ratio on the load for BDTR. As the delivery ratio decreases and delivery times increases the packets live longer in the network and as a result the per packet overhead increases. As LAROD has not reach a point where congestion becomes a problem all LAROD delay curves are essentially identical to the 20 nodes/km² curve in Figure 36. For BDRT on the other hand the effect of congestion is very obvious in the delay curves in Figure 45. As the load increases more collisions will occur and delivery takes more time. 56 1 0.9 0.8 Delivery ratio 0.7 0.6 0.5 0.4 0.3 0.2 LAROD progress BDTR circle BR all 0.1 0 120 100 80 60 40 20 Load (pkt/hour/UAV) Figure 43. Delivery ratio for different network loads. 350 Transmissions per data packet 300 LAROD progress BDTR circle BR all 250 200 150 100 50 0 120 100 80 60 40 20 Load (pkt/hour/UAV) Figure 44. Overhead for different network loads. 1 0.9 0.8 Delivery ratio 0.7 0.6 0.5 0.4 0.3 0.2 BDTR 12 packets/hour/UAV BDTR 72 packets/hour/UAV BDTR 120 packets/hour/UAV 0.1 0 0 50 100 150 200 250 300 350 Packet delivery latency (s) Figure 45. Delay for different network loads. 400 57 5 Conclusions Conclusions and Future Work 5.1 Conclusions Routing in mobile ad hoc networks is challenging since the network layout is constantly changing and constant distribution of the current network layout to all nodes in the network is generally too costly. For this reason geographical routing protocols are generally well suited for MANETS since they are essentially stateless. The ability to use geographical routing does rely on the nodes being location aware and that the (approximate) location of a destination is known. Depending on node density and node movement groups of nodes may be separated from each other so that communication between nodes in different partitions using contemporaneous paths is not possible. Due to node mobility communication is still feasible by utilizing the store-carry-forward principle. When message progress is not possible by wireless transfer the message is stored in a suitable node until node mobility provides opportunities for further wireless progress. In this thesis we have studied communication in a reconnaissance scenario in which a group of UAVs cooperatively shall detect units on the ground. To guide them in a distributed and robust way we have designed a distributed pheromone model where the UAVs are guided by virtual pheromones. This model has been shown to provide good search coverage even when communication between UAVs is poor. What the model also showed was that with the assumed system parameters the connectivity between the UAVs was low. This means that regular MANET routing protocols do not work since they assume the existence of contemporaneous paths. Since the UAVs are location aware we have explored the feasibility of combining greedy geographical routing with the store-carry-forward principle. To avoid position inaccuracies introduced by beacon based 58 geographical protocols we used the ideas from beacon-less geographical routing protocols from MANET research and designed a routing protocol for intermittently connected networks, LAROD. Our simulations have shown that the delivery rate of LAROD is close to the delivery rate of an epidemic routing protocol under the overload limit, that is the best possible result achievable. Moreover, we have shown that the overheads measured in terms of transmissions are considerably lower even under high load scenarios. 5.2 Future Work A main constraint in our work is that we have assumed that the destinations are static and that their positions are known to all sources. We wish to remove this limitation by introducing a distributed location service in the network. By this service the nodes can acquire a position or position estimate of a node. Due to propagation delays, especially noticeable in intermittently connected networks, the geographical routing needs to be able to handle incorrect position information. One option could be to dynamically update the positions in the packets as more accurate information of node location is available at forwarders closer to the destination. Another aspect we want to explore is how a UAV determines the nodes that are interested in the intelligence it gathers. By some kind of publish subscribe system the destinations should be able to inform UAVs of the data they are interested in. On the radio level we wish to explore the difference between broadcast transmissions as used by LAROD and CTS/RTS with reliable data transfer. What we are interested in exploring is how the reliable data transfer in the CTS/RTS scheme affects the transfer probability. To properly explore the tradeoffs the currently used two ray-ground radio model probably needs to be replaced by a more realistic model like Nakagami  or log-normal shadowing . To become widely accepted a routing protocol should work well in a wide range of environments. For this reason we intend to evaluate our suggested routing algorithms under several different mobility models. At the same time we will also continue to develop our scenario to become as realistic as reasonably possible. 59 6 Acronyms Acronym AODV BDTR BLR BR CAR CBF CTS DSR DTGR DTN ER FOPEN GDBF GeRaF GPS GPSR GSM ICT IEEE IETF IGF LAROD OLSR MANET MoVe PREP PSGR RFC RTS TBRPF TTL UAV Description Ad hoc on-demand distance vector routing Broadcast delay-tolerant routing Beacon-less routing Broadcast routing Context-aware routing Contention-based forwarding Clear to send Dynamic source routing Disruption-tolerant geographic routing for wireless ad hoc networks Delay-tolerant network, Disruption-tolerant network Epidemic routing Foliage penetration Guaranteed delivery beacon-less forwarding Geographic random forwarding Global positioning system Greedy perimeter stateless routing Global system for mobile communications Inter contact time Institute of electrical and electronics engineers Internet Engineering Task Force Implicit geographical forwarding Location aware routing for delay-tolerant networks Optimized link state routing Mobile ad-hoc network Motion vector Prioritized epidemic routing Priority-based geographical forwarding Request for comment Ready/request to send Topology dissemination based on reverse-path forwarding Time to live Unmanned aerial vehicle 60 61 7 References          B. 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IEEE Datum Date Avdelning, institution Division, department Institutionen för datavetenskap LINKÖPINGS UNIVERSITET Språk Language Department of Computer and Information Science Rapporttyp Report category Svenska/Swedish X Engelska/English X Licentiatavhandling Examensarbete C-uppsats D-uppsats ISBN ISRN 2008-04-14 978-91-7393-937-9 LiU-Tek-Lic- 2008:14 Serietitel och serienummer Title of series, numbering ISSN 0280-7971 Övrig rapport Linköping Studies in Science and Technology URL för elektronisk version Thesis No. 1356 Titel Title Mobility and Routing in a Delay-tolerant Network of Unmanned Aerial Vehicles Författare Author Erik Kuiper Sammanfattning Abstract Technology has reached a point where it has become feasible to develop unmanned aerial vehicles (UAVs), that is aircraft without a human pilot on board. Given that future UAVs can be autonomous and cheap, applications of swarming UAVs are possible. In this thesis we have studied a reconnaissance application using swarming UAVs and how these UAVs can communicate the reconnaissance data. To guide the UAVs in their reconnaissance mission we have proposed a pheromone based mobility model that in a distributed manner guides the UAVs to areas not recently visited. Each UAV has a local pheromone map that it updates based on its reconnaissance scans. The information in the local map is regularly shared with a UAV’s neighbors. Evaluations have shown that the pheromone logic is very good at guiding the UAVs in their cooperative reconnaissance mission in a distributed manner. Analyzing the connectivity of the UAVs we found that they were heavily partitioned which meant that contemporaneous communication paths generally were not possible to establish. This means that traditional mobile ad hoc network (MANET) routing protocols like AODV, DSR and GPSR will generally fail. By using node mobility and the store-carry-forward principle of delay-tolerant routing the transfer of messages between nodes is still possible. In this thesis we propose location aware routing for delay-tolerant networks (LAROD). LAROD is a beacon-less geographical routing protocol for intermittently connected mobile ad hoc networks. Using static destinations we have shown by a comparative study that LAROD has almost as good delivery rate as an epidemic routing scheme, but at a substantially lower overhead. Nyckelord Keywords mobility models, routing, ad hoc networks, delay-tolerant networks Department of Computer and Information Science Linköpings universitet Linköping Studies in Science and Technology Faculty of Arts and Sciences - Licentiate Theses No 17 No 28 No 29 No 48 No 52 No 60 No 71 No 72 No 73 No 74 No 104 No 108 No 111 No 113 No 118 No 126 No 127 No 139 No 140 No 146 No 150 No 165 No 166 No 174 No 177 No 181 No 184 No 187 No 189 No 196 No 197 No 203 No 212 No 230 No 237 No 250 No 253 No 260 No 283 No 298 No 318 No 319 No 326 No 328 No 333 No 335 No 348 No 352 No 371 No 378 No 380 No 381 No 383 No 386 No 398 Vojin Plavsic: Interleaved Processing of Non-Numerical Data Stored on a Cyclic Memory. (Available at: FOA, Box 1165, S-581 11 Linköping, Sweden. FOA Report B30062E) Arne Jönsson, Mikael Patel: An Interactive Flowcharting Technique for Communicating and Realizing Algorithms, 1984. Johnny Eckerland: Retargeting of an Incremental Code Generator, 1984. Henrik Nordin: On the Use of Typical Cases for Knowledge-Based Consultation and Teaching, 1985. Zebo Peng: Steps Towards the Formalization of Designing VLSI Systems, 1985. Johan Fagerström: Simulation and Evaluation of Architecture based on Asynchronous Processes, 1985. Jalal Maleki: ICONStraint, A Dependency Directed Constraint Maintenance System, 1987. Tony Larsson: On the Specification and Verification of VLSI Systems, 1986. Ola Strömfors: A Structure Editor for Documents and Programs, 1986. Christos Levcopoulos: New Results about the Approximation Behavior of the Greedy Triangulation, 1986. Shamsul I. Chowdhury: Statistical Expert Systems - a Special Application Area for Knowledge-Based Computer Methodology, 1987. Rober Bilos: Incremental Scanning and Token-Based Editing, 1987. Hans Block: SPORT-SORT Sorting Algorithms and Sport Tournaments, 1987. Ralph Rönnquist: Network and Lattice Based Approaches to the Representation of Knowledge, 1987. Mariam Kamkar, Nahid Shahmehri: Affect-Chaining in Program Flow Analysis Applied to Queries of Programs, 1987. Dan Strömberg: Transfer and Distribution of Application Programs, 1987. Kristian Sandahl: Case Studies in Knowledge Acquisition, Migration and User Acceptance of Expert Systems, 1987. Christer Bäckström: Reasoning about Interdependent Actions, 1988. Mats Wirén: On Control Strategies and Incrementality in Unification-Based Chart Parsing, 1988. Johan Hultman: A Software System for Defining and Controlling Actions in a Mechanical System, 1988. Tim Hansen: Diagnosing Faults using Knowledge about Malfunctioning Behavior, 1988. Jonas Löwgren: Supporting Design and Management of Expert System User Interfaces, 1989. Ola Petersson: On Adaptive Sorting in Sequential and Parallel Models, 1989. Yngve Larsson: Dynamic Configuration in a Distributed Environment, 1989. Peter Åberg: Design of a Multiple View Presentation and Interaction Manager, 1989. Henrik Eriksson: A Study in Domain-Oriented Tool Support for Knowledge Acquisition, 1989. Ivan Rankin: The Deep Generation of Text in Expert Critiquing Systems, 1989. Simin Nadjm-Tehrani: Contributions to the Declarative Approach to Debugging Prolog Programs, 1989. Magnus Merkel: Temporal Information in Natural Language, 1989. Ulf Nilsson: A Systematic Approach to Abstract Interpretation of Logic Programs, 1989. Staffan Bonnier: Horn Clause Logic with External Procedures: Towards a Theoretical Framework, 1989. Christer Hansson: A Prototype System for Logical Reasoning about Time and Action, 1990. Björn Fjellborg: An Approach to Extraction of Pipeline Structures for VLSI High-Level Synthesis, 1990. Patrick Doherty: A Three-Valued Approach to Non-Monotonic Reasoning, 1990. Tomas Sokolnicki: Coaching Partial Plans: An Approach to Knowledge-Based Tutoring, 1990. Lars Strömberg: Postmortem Debugging of Distributed Systems, 1990. Torbjörn Näslund: SLDFA-Resolution - Computing Answers for Negative Queries, 1990. Peter D. Holmes: Using Connectivity Graphs to Support Map-Related Reasoning, 1991. Olof Johansson: Improving Implementation of Graphical User Interfaces for Object-Oriented KnowledgeBases, 1991. Rolf G Larsson: Aktivitetsbaserad kalkylering i ett nytt ekonomisystem, 1991. Lena Srömbäck: Studies in Extended Unification-Based Formalism for Linguistic Description: An Algorithm for Feature Structures with Disjunction and a Proposal for Flexible Systems, 1992. Mikael Pettersson: DML-A Language and System for the Generation of Efficient Compilers from Denotational Specification, 1992. Andreas Kågedal: Logic Programming with External Procedures: an Implementation, 1992. Patrick Lambrix: Aspects of Version Management of Composite Objects, 1992. Xinli Gu: Testability Analysis and Improvement in High-Level Synthesis Systems, 1992. Torbjörn Näslund: On the Role of Evaluations in Iterative Development of Managerial Support Sytems, 1992. Ulf Cederling: Industrial Software Development - a Case Study, 1992. Magnus Morin: Predictable Cyclic Computations in Autonomous Systems: A Computational Model and Implementation, 1992. Mehran Noghabai: Evaluation of Strategic Investments in Information Technology, 1993. Mats Larsson: A Transformational Approach to Formal Digital System Design, 1993. Johan Ringström: Compiler Generation for Parallel Languages from Denotational Specifications, 1993. Michael Jansson: Propagation of Change in an Intelligent Information System, 1993. Jonni Harrius: An Architecture and a Knowledge Representation Model for Expert Critiquing Systems, 1993. Per Österling: Symbolic Modelling of the Dynamic Environments of Autonomous Agents, 1993. Johan Boye: Dependency-based Groudness Analysis of Functional Logic Programs, 1993. No 402 No 406 No 414 No 417 No 436 No 437 No 440 FHS 3/94 FHS 4/94 No 441 No 446 No 450 No 451 No 452 No 455 FHS 5/94 No 462 No 463 No 464 No 469 No 473 No 475 No 476 No 478 FHS 7/95 No 482 No 488 No 489 No 497 No 498 No 503 FHS 8/95 FHS 9/95 No 513 No 517 No 518 No 522 No 538 No 545 No 546 FiF-a 1/96 No 549 No 550 No 557 No 558 No 561 No 563 No 567 No 575 No 576 No 587 No 589 No 591 No 595 No 597 Lars Degerstedt: Tabulated Resolution for Well Founded Semantics, 1993. Anna Moberg: Satellitkontor - en studie av kommunikationsmönster vid arbete på distans, 1993. Peter Carlsson: Separation av företagsledning och finansiering - fallstudier av företagsledarutköp ur ett agentteoretiskt perspektiv, 1994. Camilla Sjöström: Revision och lagreglering - ett historiskt perspektiv, 1994. Cecilia Sjöberg: Voices in Design: Argumentation in Participatory Development, 1994. Lars Viklund: Contributions to a High-level Programming Environment for a Scientific Computing, 1994. Peter Loborg: Error Recovery Support in Manufacturing Control Systems, 1994. Owen Eriksson: Informationssystem med verksamhetskvalitet - utvärdering baserat på ett verksamhetsinriktat och samskapande perspektiv, 1994. Karin Pettersson: Informationssystemstrukturering, ansvarsfördelning och användarinflytande - En komparativ studie med utgångspunkt i två informationssystemstrategier, 1994. Lars Poignant: Informationsteknologi och företagsetablering - Effekter på produktivitet och region, 1994. Gustav Fahl: Object Views of Relational Data in Multidatabase Systems, 1994. Henrik Nilsson: A Declarative Approach to Debugging for Lazy Functional Languages, 1994. Jonas Lind: Creditor - Firm Relations: an Interdisciplinary Analysis, 1994. Martin Sköld: Active Rules based on Object Relational Queries - Efficient Change Monitoring Techniques, 1994. Pär Carlshamre: A Collaborative Approach to Usability Engineering: Technical Communicators and System Developers in Usability-Oriented Systems Development, 1994. Stefan Cronholm: Varför CASE-verktyg i systemutveckling? - En motiv- och konsekvensstudie avseende arbetssätt och arbetsformer, 1994. Mikael Lindvall: A Study of Traceability in Object-Oriented Systems Development, 1994. Fredrik Nilsson: Strategi och ekonomisk styrning - En studie av Sandviks förvärv av Bahco Verktyg, 1994. Hans Olsén: Collage Induction: Proving Properties of Logic Programs by Program Synthesis, 1994. Lars Karlsson: Specification and Synthesis of Plans Using the Features and Fluents Framework, 1995. Ulf Söderman: On Conceptual Modelling of Mode Switching Systems, 1995. Choong-ho Yi: Reasoning about Concurrent Actions in the Trajectory Semantics, 1995. Bo Lagerström: Successiv resultatavräkning av pågående arbeten. - Fallstudier i tre byggföretag, 1995. Peter Jonsson: Complexity of State-Variable Planning under Structural Restrictions, 1995. Anders Avdic: Arbetsintegrerad systemutveckling med kalkylkprogram, 1995. Eva L Ragnemalm: Towards Student Modelling through Collaborative Dialogue with a Learning Companion, 1995. Eva Toller: Contributions to Parallel Multiparadigm Languages: Combining Object-Oriented and Rule-Based Programming, 1995. Erik Stoy: A Petri Net Based Unified Representation for Hardware/Software Co-Design, 1995. Johan Herber: Environment Support for Building Structured Mathematical Models, 1995. Stefan Svenberg: Structure-Driven Derivation of Inter-Lingual Functor-Argument Trees for Multi-Lingual Generation, 1995. Hee-Cheol Kim: Prediction and Postdiction under Uncertainty, 1995. Dan Fristedt: Metoder i användning - mot förbättring av systemutveckling genom situationell metodkunskap och metodanalys, 1995. Malin Bergvall: Systemförvaltning i praktiken - en kvalitativ studie avseende centrala begrepp, aktiviteter och ansvarsroller, 1995. Joachim Karlsson: Towards a Strategy for Software Requirements Selection, 1995. Jakob Axelsson: Schedulability-Driven Partitioning of Heterogeneous Real-Time Systems, 1995. Göran Forslund: Toward Cooperative Advice-Giving Systems: The Expert Systems Experience, 1995. Jörgen Andersson: Bilder av småföretagares ekonomistyrning, 1995. Staffan Flodin: Efficient Management of Object-Oriented Queries with Late Binding, 1996. Vadim Engelson: An Approach to Automatic Construction of Graphical User Interfaces for Applications in Scientific Computing, 1996. Magnus Werner : Multidatabase Integration using Polymorphic Queries and Views, 1996. Mikael Lind: Affärsprocessinriktad förändringsanalys - utveckling och tillämpning av synsätt och metod, 1996. Jonas Hallberg: High-Level Synthesis under Local Timing Constraints, 1996. Kristina Larsen: Förutsättningar och begränsningar för arbete på distans - erfarenheter från fyra svenska företag. 1996. Mikael Johansson: Quality Functions for Requirements Engineering Methods, 1996. Patrik Nordling: The Simulation of Rolling Bearing Dynamics on Parallel Computers, 1996. Anders Ekman: Exploration of Polygonal Environments, 1996. Niclas Andersson: Compilation of Mathematical Models to Parallel Code, 1996. Johan Jenvald: Simulation and Data Collection in Battle Training, 1996. Niclas Ohlsson: Software Quality Engineering by Early Identification of Fault-Prone Modules, 1996. Mikael Ericsson: Commenting Systems as Design Support—A Wizard-of-Oz Study, 1996. Jörgen Lindström: Chefers användning av kommunikationsteknik, 1996. Esa Falkenroth: Data Management in Control Applications - A Proposal Based on Active Database Systems, 1996. Niclas Wahllöf: A Default Extension to Description Logics and its Applications, 1996. Annika Larsson: Ekonomisk Styrning och Organisatorisk Passion - ett interaktivt perspektiv, 1997. Ling Lin: A Value-based Indexing Technique for Time Sequences, 1997. No 598 No 599 No 607 No 609 FiF-a 4 FiF-a 6 No 615 No 623 No 626 No 627 No 629 No 631 No 639 No 640 No 643 No 653 FiF-a 13 No 674 No 676 No 668 No 675 FiF-a 14 No 695 No 700 FiF-a 16 No 712 No 719 No 723 No 725 No 730 No 731 No 733 No 734 FiF-a 21 FiF-a 22 No 737 No 738 FiF-a 25 No 742 No 748 No 751 No 752 No 753 No 754 No 766 No 769 No 775 FiF-a 30 No 787 No 788 No 790 No 791 No 800 No 807 Rego Granlund: C3Fire - A Microworld Supporting Emergency Management Training, 1997. Peter Ingels: A Robust Text Processing Technique Applied to Lexical Error Recovery, 1997. Per-Arne Persson: Toward a Grounded Theory for Support of Command and Control in Military Coalitions, 1997. Jonas S Karlsson: A Scalable Data Structure for a Parallel Data Server, 1997. Carita Åbom: Videomötesteknik i olika affärssituationer - möjligheter och hinder, 1997. Tommy Wedlund: Att skapa en företagsanpassad systemutvecklingsmodell - genom rekonstruktion, värdering och vidareutveckling i T50-bolag inom ABB, 1997. Silvia Coradeschi: A Decision-Mechanism for Reactive and Coordinated Agents, 1997. Jan Ollinen: Det flexibla kontorets utveckling på Digital - Ett stöd för multiflex? 1997. David Byers: Towards Estimating Software Testability Using Static Analysis, 1997. Fredrik Eklund: Declarative Error Diagnosis of GAPLog Programs, 1997. Gunilla Ivefors: Krigsspel coh Informationsteknik inför en oförutsägbar framtid, 1997. Jens-Olof Lindh: Analysing Traffic Safety from a Case-Based Reasoning Perspective, 1997 Jukka Mäki-Turja:. Smalltalk - a suitable Real-Time Language, 1997. Juha Takkinen: CAFE: Towards a Conceptual Model for Information Management in Electronic Mail, 1997. Man Lin: Formal Analysis of Reactive Rule-based Programs, 1997. Mats Gustafsson: Bringing Role-Based Access Control to Distributed Systems, 1997. Boris Karlsson: Metodanalys för förståelse och utveckling av systemutvecklingsverksamhet. Analys och värdering av systemutvecklingsmodeller och dess användning, 1997. Marcus Bjäreland: Two Aspects of Automating Logics of Action and Change - Regression and Tractability, 1998. Jan Håkegård: Hiera rchical Test Architecture and Board-Level Test Controller Synthesis, 1998. Per-Ove Zetterlund: Normering av svensk redovisning - En studie av tillkomsten av Redovisningsrådets rekommendation om koncernredovisning (RR01:91), 1998. Jimmy Tjäder: Projektledaren & planen - en studie av projektledning i tre installations- och systemutvecklingsprojekt, 1998. Ulf Melin: Informationssystem vid ökad affärs- och processorientering - egenskaper, strategier och utveckling, 1998. Tim Heyer: COMPASS: Introduction of Formal Methods in Code Development and Inspection, 1998. Patrik Hägglund: Programming Languages for Computer Algebra, 1998. Marie-Therese Christiansson: Inter-organistorisk verksamhetsutveckling - metoder som stöd vid utveckling av partnerskap och informationssystem, 1998. Christina Wennestam: Information om immateriella resurser. Investeringar i forskning och utveckling samt i personal inom skogsindustrin, 1998. Joakim Gustafsson: Extending Temporal Action Logic for Ramification and Concurrency, 1998. Henrik André-Jönsson: Indexing time-series data using text indexing methods, 1999. Erik Larsson: High-Level Testability Analysis and Enhancement Techniques, 1998. Carl-Johan Westin: Informationsförsörjning: en fråga om ansvar - aktiviteter och uppdrag i fem stora svenska organisationers operativa informationsförsörjning, 1998. Åse Jansson: Miljöhänsyn - en del i företags styrning, 1998. Thomas Padron-McCarthy: Performance-Polymorphic Declarative Queries, 1998. Anders Bäckström: Värdeskapande kreditgivning - Kreditriskhantering ur ett agentteoretiskt perspektiv, 1998. Ulf Seigerroth: Integration av förändringsmetoder - en modell för välgrundad metodintegration, 1999. Fredrik Öberg: Object-Oriented Frameworks - A New Strategy for Case Tool Development, 1998. Jonas Mellin: Predictable Event Monitoring, 1998. Joakim Eriksson: Specifying and Managing Rules in an Active Real-Time Database System, 1998. Bengt E W Andersson: Samverkande informationssystem mellan aktörer i offentliga åtaganden - En teori om aktörsarenor i samverkan om utbyte av information, 1998. Pawel Pietrzak: Static Incorrectness Diagnosis of CLP (FD), 1999. Tobias Ritzau: Real-Time Reference Counting in RT-Java, 1999. Anders Ferntoft: Elektronisk affärskommunikation - kontaktkostnader och kontaktprocesser mellan kunder och leverantörer på producentmarknader,1999. Jo Skåmedal: Arbete på distans och arbetsformens påverkan på resor och resmönster, 1999. Johan Alvehus: Mötets metaforer. En studie av berättelser om möten, 1999. Magnus Lindahl: Bankens villkor i låneavtal vid kreditgivning till högt belånade företagsförvärv: En studie ur ett agentteoretiskt perspektiv, 2000. Martin V. Howard: Designing dynamic visualizations of temporal data, 1999. Jesper Andersson: Towards Reactive Software Architectures, 1999. Anders Henriksson: Unique kernel diagnosis, 1999. Pär J. Ågerfalk: Pragmatization of Information Systems - A Theoretical and Methodological Outline, 1999. Charlotte Björkegren: Learning for the next project - Bearers and barriers in knowledge transfer within an organisation, 1999. Håkan Nilsson: Informationsteknik som drivkraft i granskningsprocessen - En studie av fyra revisionsbyråer, 2000. Erik Berglund: Use-Oriented Documentation in Software Development, 1999. Klas Gäre: Verksamhetsförändringar i samband med IS-införande, 1999. Anders Subotic: Software Quality Inspection, 1999. Svein Bergum: Managerial communication in telework, 2000. No 809 FiF-a 32 No 808 No 820 No 823 No 832 FiF-a 34 No 842 No 844 FiF-a 37 FiF-a 40 FiF-a 41 No. 854 No 863 No 881 No 882 No 890 FiF-a 47 No 894 No 906 No 917 No 916 FiF-a-49 FiF-a-51 No 919 No 915 No 931 No 933 No 938 No 942 No 956 FiF-a 58 No 964 No 973 No 958 FiF-a 61 No 985 No 982 No 989 No 990 No 991 No 999 No 1000 No 1001 No 988 FiF-a 62 No 1003 No 1005 No 1008 No 1010 No 1015 No 1018 No 1022 FiF-a 65 Flavius Gruian: Energy-Aware Design of Digital Systems, 2000. Karin Hedström: Kunskapsanvändning och kunskapsutveckling hos verksamhetskonsulter - Erfarenheter från ett FOU-samarbete, 2000. Linda Askenäs: Affärssystemet - En studie om teknikens aktiva och passiva roll i en organisation, 2000. Jean Paul Meynard: Control of industrial robots through high-level task programming, 2000. Lars Hult: Publika Gränsytor - ett designexempel, 2000. Paul Pop: Scheduling and Communication Synthesis for Distributed Real-Time Systems, 2000. Göran Hultgren: Nätverksinriktad Förändringsanalys - perspektiv och metoder som stöd för förståelse och utveckling av affärsrelationer och informationssystem, 2000. Magnus Kald: The role of management control systems in strategic business units, 2000. Mikael Cäker: Vad kostar kunden? Modeller för intern redovisning, 2000. Ewa Braf: Organisationers kunskapsverksamheter - en kritisk studie av ”knowledge management”, 2000. Henrik Lindberg: Webbaserade affärsprocesser - Möjligheter och begränsningar, 2000. Benneth Christiansson: Att komponentbasera informationssystem - Vad säger teori och praktik?, 2000. Ola Pettersson: Deliberation in a Mobile Robot, 2000. Dan Lawesson: Towards Behavioral Model Fault Isolation for Object Oriented Control Systems, 2000. Johan Moe: Execution Tracing of Large Distributed Systems, 2001. Yuxiao Zhao: XML-based Frameworks for Internet Commerce and an Implementation of B2B e-procurement, 2001. Annika Flycht-Eriksson: Domain Knowledge Management inInformation-providing Dialogue systems, 2001. Per-Arne Segerkvist: Webbaserade imaginära organisationers samverkansformer: Informationssystemarkitektur och aktörssamverkan som förutsättningar för affärsprocesser, 2001. Stefan Svarén: Styrning av investeringar i divisionaliserade företag - Ett koncernperspektiv, 2001. Lin Han: Secure and Scalable E-Service Software Delivery, 2001. Emma Hansson: Optionsprogram för anställda - en studie av svenska börsföretag, 2001. Susanne Odar: IT som stöd för strategiska beslut, en studie av datorimplementerade modeller av verksamhet som stöd för beslut om anskaffning av JAS 1982, 2002. Stefan Holgersson: IT-system och filtrering av verksamhetskunskap - kvalitetsproblem vid analyser och beslutsfattande som bygger på uppgifter hämtade från polisens IT-system, 2001. Per Oscarsson:Informationssäkerhet i verksamheter - begrepp och modeller som stöd för förståelse av informationssäkerhet och dess hantering, 2001. Luis Alejandro Cortes: A Petri Net Based Modeling and Verification Technique for Real-Time Embedded Systems, 2001. Niklas Sandell: Redovisning i skuggan av en bankkris - Värdering av fastigheter. 2001. Fredrik Elg: Ett dynamiskt perspektiv på individuella skillnader av heuristisk kompetens, intelligens, mentala modeller, mål och konfidens i kontroll av mikrovärlden Moro, 2002. Peter Aronsson: Automatic Parallelization of Simulation Code from Equation Based Simulation Languages, 2002. Bourhane Kadmiry: Fuzzy Control of Unmanned Helicopter, 2002. Patrik Haslum: Prediction as a Knowledge Representation Problem: A Case Study in Model Design, 2002. Robert Sevenius: On the instruments of governance - A law & economics study of capital instruments in limited liability companies, 2002. Johan Petersson: Lokala elektroniska marknadsplatser - informationssystem för platsbundna affärer, 2002. Peter Bunus: Debugging and Structural Analysis of Declarative Equation-Based Languages, 2002. Gert Jervan: High-Level Test Generation and Built-In Self-Test Techniques for Digital Systems, 2002. Fredrika Berglund: Management Control and Strategy - a Case Study of Pharmaceutical Drug Development, 2002. Fredrik Karlsson: Meta-Method for Method Configuration - A Rational Unified Process Case, 2002. Sorin Manolache: Schedulability Analysis of Real-Time Systems with Stochastic Task Execution Times, 2002. Diana Szentiványi: Performance and Availability Trade-offs in Fault-Tolerant Middleware, 2002. Iakov Nakhimovski: Modeling and Simulation of Contacting Flexible Bodies in Multibody Systems, 2002. Levon Saldamli: PDEModelica - Towards a High-Level Language for Modeling with Partial Differential Equations, 2002. Almut Herzog: Secure Execution Environment for Java Electronic Services, 2002. Jon Edvardsson: Contributions to Program- and Specification-based Test Data Generation, 2002 Anders Arpteg: Adaptive Semi-structured Information Extraction, 2002. Andrzej Bednarski: A Dynamic Programming Approach to Optimal Retargetable Code Generation for Irregular Architectures, 2002. Mattias Arvola: Good to use! : Use quality of multi-user applications in the home, 2003. Lennart Ljung: Utveckling av en projektivitetsmodell - om organisationers förmåga att tillämpa projektarbetsformen, 2003. Pernilla Qvarfordt: User experience of spoken feedback in multimodal interaction, 2003. Alexander Siemers: Visualization of Dynamic Multibody Simulation With Special Reference to Contacts, 2003. Jens Gustavsson: Towards Unanticipated Runtime Software Evolution, 2003. Calin Curescu: Adaptive QoS-aware Resource Allocation for Wireless Networks, 2003. Anna Andersson: Management Information Systems in Process-oriented Healthcare Organisations, 2003. Björn Johansson: Feedforward Control in Dynamic Situations, 2003. Traian Pop: Scheduling and Optimisation of Heterogeneous Time/Event-Triggered Distributed Embedded Systems, 2003. Britt-Marie Johansson: Kundkommunikation på distans - en studie om kommunikationsmediets betydelse i affärstransaktioner, 2003. No 1024 No 1034 No 1033 FiF-a 69 No 1049 No 1052 No 1054 FiF-a 71 No 1055 No 1058 FiF-a 73 No 1079 No 1084 FiF-a 74 No 1094 No 1095 No 1099 No 1110 No 1116 FiF-a 77 No 1126 No 1127 No 1132 No 1130 No 1138 No 1149 No 1156 No 1162 No 1165 FiF-a 84 No 1166 No 1167 No 1168 FiF-a 85 No 1171 FiF-a 86 No 1172 No 1183 No 1184 No 1185 No 1190 No 1191 No 1192 No 1194 No 1204 No 1206 No 1207 No 1209 No 1225 No 1228 No 1229 No 1231 No 1233 No 1244 No 1248 No 1263 FiF-a 90 No 1272 Aleksandra Tešanovic: Towards Aspectual Component-Based Real-Time System Development, 2003. Arja Vainio-Larsson: Designing for Use in a Future Context - Five Case Studies in Retrospect, 2003. Peter Nilsson: Svenska bankers redovisningsval vid reservering för befarade kreditförluster - En studie vid införandet av nya redovisningsregler, 2003. Fredrik Ericsson: Information Technology for Learning and Acquiring of Work Knowledge, 2003. Marcus Comstedt: Towards Fine-Grained Binary Composition through Link Time Weaving, 2003. Åsa Hedenskog: Increasing the Automation of Radio Network Control, 2003. Claudiu Duma: Security and Efficiency Tradeoffs in Multicast Group Key Management, 2003. Emma Eliason: Effektanalys av IT-systems handlingsutrymme, 2003. Carl Cederberg: Experiments in Indirect Fault Injection with Open Source and Industrial Software, 2003. Daniel Karlsson: Towards Formal Verification in a Component-based Reuse Methodology, 2003. Anders Hjalmarsson: Att etablera och vidmakthålla förbättringsverksamhet - behovet av koordination och interaktion vid förändring av systemutvecklingsverksamheter, 2004. Pontus Johansson: Design and Development of Recommender Dialogue Systems, 2004. Charlotte Stoltz: Calling for Call Centres - A Study of Call Centre Locations in a Swedish Rural Region, 2004. Björn Johansson: Deciding on Using Application Service Provision in SMEs, 2004. Genevieve Gorrell: Language Modelling and Error Handling in Spoken Dialogue Systems, 2004. Ulf Johansson: Rule Extraction - the Key to Accurate and Comprehensible Data Mining Models, 2004. Sonia Sangari: Computational Models of Some Communicative Head Movements, 2004. Hans Nässla: Intra-Family Information Flow and Prospects for Communication Systems, 2004. Henrik Sällberg: On the value of customer loyalty programs - A study of point programs and switching costs, 2004. Ulf Larsson: Designarbete i dialog - karaktärisering av interaktionen mellan användare och utvecklare i en systemutvecklingsprocess, 2004. Andreas Borg: Contribution to Management and Validation of Non-Functional Requirements, 2004. Per-Ola Kristensson: Large Vocabulary Shorthand Writing on Stylus Keyboard, 2004. Pär-Anders Albinsson: Interacting with Command and Control Systems: Tools for Operators and Designers, 2004. Ioan Chisalita: Safety-Oriented Communication in Mobile Networks for Vehicles, 2004. Thomas Gustafsson: Maintaining Data Consistency im Embedded Databases for Vehicular Systems, 2004. Vaida Jakoniené: A Study in Integrating Multiple Biological Data Sources, 2005. Abdil Rashid Mohamed: High-Level Techniques for Built-In Self-Test Resources Optimization, 2005. Adrian Pop: Contributions to Meta-Modeling Tools and Methods, 2005. Fidel Vascós Palacios: On the information exchange between physicians and social insurance officers in the sick leave process: an Activity Theoretical perspective, 2005. Jenny Lagsten: Verksamhetsutvecklande utvärdering i informationssystemprojekt, 2005. Emma Larsdotter Nilsson: Modeling, Simulation, and Visualization of Metabolic Pathways Using Modelica, 2005. Christina Keller: Virtual Learning Environments in higher education. A study of students’ acceptance of educational technology, 2005. Cécile Åberg: Integration of organizational workflows and the Semantic Web, 2005. Anders Forsman: Standardisering som grund för informationssamverkan och IT-tjänster - En fallstudie baserad på trafikinformationstjänsten RDS-TMC, 2005. Yu-Hsing Huang: A systemic traffic accident model, 2005. Jan Olausson: Att modellera uppdrag - grunder för förståelse av processinriktade informationssystem i transaktionsintensiva verksamheter, 2005. Petter Ahlström: Affärsstrategier för seniorbostadsmarknaden, 2005. Mathias Cöster: Beyond IT and Productivity - How Digitization Transformed the Graphic Industry, 2005. Åsa Horzella: Beyond IT and Productivity - Effects of Digitized Information Flows in Grocery Distribution, 2005. Maria Kollberg: Beyond IT and Productivity - Effects of Digitized Information Flows in the Logging Industry, 2005. David Dinka: Role and Identity - Experience of technology in professional settings, 2005. Andreas Hansson: Increasing the Storage Capacity of Recursive Auto-associative Memory by Segmenting Data, 2005. Nicklas Bergfeldt: Towards Detached Communication for Robot Cooperation, 2005. Dennis Maciuszek: Towards Dependable Virtual Companions for Later Life, 2005. Beatrice Alenljung: Decision-making in the Requirements Engineering Process: A Human-centered Approach, 2005 Anders Larsson: System-on-Chip Test Scheduling and Test Infrastructure Design, 2005. John Wilander: Policy and Implementation Assurance for Software Security, 2005. Andreas Käll: Översättningar av en managementmodell - En studie av införandet av Balanced Scorecard i ett landsting, 2005. He Tan: Aligning and Merging Biomedical Ontologies, 2006. Artur Wilk: Descriptive Types for XML Query Language Xcerpt, 2006. Per Olof Pettersson: Sampling-based Path Planning for an Autonomous Helicopter, 2006. Kalle Burbeck: Adaptive Real-time Anomaly Detection for Safeguarding Critical Networks, 2006. Daniela Mihailescu: Implementation Methodology in Action: A Study of an Enterprise Systems Implementation Methodology, 2006. Jörgen Skågeby: Public and Non-public gifting on the Internet, 2006. Karolina Eliasson: The Use of Case-Based Reasoning in a Human-Robot Dialog System, 2006. Misook Park-Westman: Managing Competence Development Programs in a Cross-Cultural OrganisationWhat are the Barriers and Enablers, 2006. Amra Halilovic: Ett praktikperspektiv på hantering av mjukvarukomponenter, 2006. Raquel Flodström: A Framework for the Strategic Management of Information Technology, 2006. No 1277 No 1283 FiF-a 91 No 1286 No 1293 No 1302 No 1303 No 1305 No 1306 No 1307 No 1309 No 1312 No 1313 No 1317 No 1320 No 1323 No 1329 No 1331 No 1332 No 1333 No 1337 No 1339 No 1351 No 1353 No 1356 Viacheslav Izosimov: Scheduling and Optimization of Fault-Tolerant Embedded Systems, 2006. Håkan Hasewinkel: A Blueprint for Using Commercial Games off the Shelf in Defence Training, Education and Research Simulations, 2006. Hanna Broberg: Verksamhetsanpassade IT-stöd - Designteori och metod, 2006. Robert Kaminski: Towards an XML Document Restructuring Framework, 2006 Jiri Trnka: Prerequisites for data sharing in emergency management, 2007. Björn Hägglund: A Framework for Designing Constraint Stores, 2007. Daniel Andreasson: Slack-Time Aware Dynamic Routing Schemes for On-Chip Networks, 2007. Magnus Ingmarsson: Modelling User Tasks and Intentions for Service Discovery in Ubiquitous Computing, 2007. Gustaf Svedjemo: Ontology as Conceptual Schema when Modelling Historical Maps for Database Storage, 2007. Gianpaolo Conte: Navigation Functionalities for an Autonomous UAV Helicopter, 2007. Ola Leifler: User-Centric Critiquing in Command and Control: The DKExpert and ComPlan Approaches, 2007. Henrik Svensson: Embodied simulation as off-line representation, 2007. Zhiyuan He: System-on-Chip Test Scheduling with Defect-Probability and Temperature Considerations, 2007. Jonas Elmqvist: Components, Safety Interfaces and Compositional Analysis, 2007. Håkan Sundblad: Question Classification in Question Answering Systems, 2007. Magnus Lundqvist: Information Demand and Use: Improving Information Flow within Small-scale Business Contexts, 2007. Martin Magnusson: Deductive Planning and Composite Actions in Temporal Action Logic, 2007. Mikael Asplund: Restoring Consistency after Network Partitions, 2007. Martin Fransson: Towards Individualized Drug Dosage - General Methods and Case Studies, 2007. Karin Camara: A Visual Query Language Served by a Multi-sensor Environment, 2007. David Broman: Safety, Security, and Semantic Aspects of Equation-Based Object-Oriented Languages and Environments, 2007. Mikhail Chalabine: Invasive Interactive Parallelization, 2007. Susanna Nilsson: A Holistic Approach to Usability Evaluations of Mixed Reality Systems, 2008. Shanai Ardi: A Model and Implementation of a Security Plug-in for the Software Life Cycle, 2008. Erik Kuiper: Mobility and Routing in a Delay-tolerant Network of Unmanned Aerial Vehicles, 2008.