[IEEE 2012 The 11th Annual Mediterranean Ad Hoc Networking Workshop (Med-Hoc-Net) - Ayia Napa, Cyprus (2012.06.19-2012.06.22)] 2012 The 11th Annual Mediterranean Ad Hoc Networking Workshop (Med-Hoc-Net) - Adaptation delay and its impact on application performance for TDMA ad hoc networks

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<ul><li><p>Adaptation Delay and Its Impact on Application</p><p>Performance for TDMA Ad Hoc Networks</p><p>Jimmi Grnkvist, Jimmy Karlsson, Ulf Sterner, Jan Nilsson, and Anders Hansson</p><p>Division of Information and Aeronautical Systems</p><p>Swedish Defence Research Agency</p><p>Email: {jimmi.gronkvist, jimmy.karlsson, ulf.sterner, jan.nilsson, anders.hansson}@foi.se</p><p>AbstractWith the advances in military ad hoc networking,more capable and adaptive protocol solutions are being proposedand are evolving. Many are based on TDMA. Issues remain,however, concerning the capability of TDMA to adapt to thedynamics of mobile ad hoc networks. To address this issuea general traffic-adaptive TDMA-based ad hoc network usingproactive routing is considered. The focal point is to determinehow the protocol adaptation delay affects the performance ofapplications with different delay requirements. Besides the totaladaption delays, the individual MAC and routing adaptationdelays are also of interest. Results show, among other things, thatit will be unfeasible to adapt the protocols to sessions with delayrequirements that are too strict. Additional backup mechanismshave to be added to deal with such sessions. Moreover, theadaptation delay for the routing is not as crucial as for theTDMA protocol.</p><p>I. INTRODUCTION</p><p>In military ad hoc networking, the main challenge is proto-</p><p>col design. The routing and, perhaps even more, the medium</p><p>access control (MAC) design are crucial for good performance.</p><p>Contention-based MAC, such as Carrier-Sense Multiple Acess</p><p>(CSMA), or reservation-based MAC, such as Time Division</p><p>Multiple Access (TDMA), have different pros and cons. The</p><p>contention-based protocol deals better with dynamic network</p><p>situations whereas the reservation-based protocol has poten-</p><p>tially a larger throughput and better ability to provide QoS</p><p>guarantees. In non-military ad hoc networks, by far the most</p><p>common protocol standard IEEE 802.11 is based on CSMA</p><p>with collision avoidance [1]. However, in military ad hoc</p><p>networks TDMA-based solutions are often selected instead;</p><p>one example is USAP [2]. A number of solutions have been</p><p>proposed for dynamically adapting the TDMA scheme to a</p><p>changing network topology and bandwith requirements [3]</p><p>[5].</p><p>In this paper we consider a general traffic-adaptive TDMA</p><p>ad hoc network with proactive shortest path routing. TDMA</p><p>solutions can be made very efficient in static cases, but</p><p>the challenge lies in making them adaptive to changes. The</p><p>changes we refer to are changes in traffic patterns or network</p><p>topology, e.g. that a link goes down so that a new route has</p><p>to be found and used.</p><p>This work was supported by the FOI research project Communicationnetworks for tactical voice and data, which is funded by the R&amp;D programmeof the Swedish Armed Forces.</p><p>It takes time to adapt; a change has to be detected, informa-</p><p>tion about it has to be spread and the protocols have to react</p><p>to it, e.g. from the point when a link disappears to the point</p><p>when the system has adapted to the new topology. We call the</p><p>time it takes from detecting a change to adapting the protocols</p><p>in all the nodes affected by this change to the new situation</p><p>the adaptation delay. Moreover, we are also interested in the</p><p>individual MAC and routing adaptation delays. Notice that</p><p>the adaptation delay can partly be controlled in the protocol</p><p>design by allowing more or less control information to be</p><p>sent, i.e. overhead. A small adaptation delay is desired but</p><p>can be costly; moreover, there are limitations to how small</p><p>the adaptation delay can be made. The aim of the paper is to</p><p>investigate how services, or traffic sessions with different delay</p><p>requirements, are affected by the adaptation delay. In particular</p><p>we show that the adaptation delay must be considerably</p><p>lower than the application delay requirements for the traffic</p><p>sessions to work satisfactorily. We also consider the MAC</p><p>and routing adaptation delays separately and show that the</p><p>MAC adaptation delay has the greater effect on performance.</p><p>Although TDMA protocols and system are well represented</p><p>in the literature, a similar analysis does not exist.</p><p>The paper is organized as follows: In Section II we more</p><p>precisely describe the adaptation delay and its properties.</p><p>How the adapation delay is modeled and the traffic-adaptive</p><p>TDMA-based ad hoc network we consider are described in</p><p>Section III. Section IV describes the scenario and simulation</p><p>setup. The results are presented in Section V. Finally, conclu-</p><p>sions are presented in Section VI.</p><p>II. PROTOCOL ADAPTATION DELAY</p><p>An ad hoc network can be highly changeable. This is</p><p>mainly due to mobility, but also due to changing traffic</p><p>flows. Different protocol solutions need to deal with these</p><p>changes in different ways and may be more sensitive to some</p><p>changes than others. These changes may lead to rerouting and</p><p>rescheduling of resources, processes that take time and require</p><p>network resources.</p><p>In principle, we can describe this process in three steps:</p><p>detection, dissemination and decision. The first step is to detect</p><p>the change. To detect a link failure, one has to transmit a</p><p>packet over the link; more specifically, a receiver of a link can</p><p>estimate its quality only by receiving (or attempting to do so)</p><p>what is actually transmitted on the link. As this does not occur</p><p>2012 The 11th Annual Mediterranean Ad Hoc Networking Workshop (Med-Hoc-Net)</p><p>978-1-4673-2039-9/12/$31.00 2012 IEEE 55</p></li><li><p>all the time, there will be a certain delay in detecting events</p><p>on links. On a highly loaded link this delay may be short, but</p><p>unless traffic flows in both directions we may still see a certain</p><p>delay. For a link with low traffic flow (or none), the only</p><p>available information may be the administrative data regularly</p><p>sent by the routing and MAC protocols that are available (if</p><p>any).</p><p>In a similar way there may be delays in detecting traffic</p><p>changes that may, for example, require more time slots to be</p><p>given to the nodes on the path used for a new traffic flow,</p><p>i.e. how many packets that need to arrive before a node even</p><p>realizes it needs more resources.</p><p>The second step is to disseminate the information to the</p><p>nodes that need it. Depending on how many nodes that need</p><p>this information, the time this will take is very protocol-</p><p>dependent. Some information may only be needed locally. In</p><p>such cases, the result is very fast dissemination. In others,</p><p>information is needed all over the network.</p><p>The third step can be called decision (or negotiation) and is</p><p>the reaction to the information. In most of the existing proto-</p><p>cols the nodes can simply react to the information (reroute, or</p><p>accept a scheduling decision), but in some cases there may be</p><p>other responses (for example if the update needs to be accepted</p><p>before the protocol change is finished).</p><p>Some additional considerations to the detection of changes</p><p>need to be discussed in addition to the above. First, that a</p><p>change has occured may not even be relevant until the link is</p><p>considered for use. Proactive algorithms may detect changes</p><p>and in some cases also update protocols, but if there is no</p><p>useful traffic, it will have limited relevance for the performance</p><p>of the network. This complicates how this should be properly</p><p>measured because links without useful traffic may have longer</p><p>detection time than very busy links. For these reasons, we will</p><p>exclude the actual detection time from the adaptation delay we</p><p>investigate in this paper. Therefore, the time it takes from a</p><p>detected change until all protocols have adapted to the new</p><p>situation is defined as the adaptation delay A.It should be noted that the proactive type of algorithms</p><p>we study in the paper tends to detect changes on regularly</p><p>transmitted packets, e.g., HELLO messages, containing sta-</p><p>tus information. Similarly, the dissemination and negotiation,</p><p>which times are part of A is also done by transmittingpackets at regularly intervals. The detection time, although not</p><p>directly part of the definition, should therefore be proportional</p><p>to A.A proactive routing protocol (e.g. OLSR [6]) will have</p><p>the following properties regarding adaptation delay: Normally,</p><p>updates are done without any consideration of traffic load.</p><p>Hence, only link changes will have any impact on the protocol.</p><p>Detection of link changes is based on administrative data such</p><p>as HELLO messages that are transmitted regularly. A node</p><p>can adapt to such a detected change immediately without</p><p>any information spreading (i.e. if a link fails, the node can</p><p>make a local rerouting to decide the next hop based on</p><p>information it already has in its data base) or decisions in</p><p>other nodes. The information about the change is spread</p><p>to the rest of the network through administrative messages</p><p>such as HELLO messages (locally) and Topology Control</p><p>(TC) messages (globally). Nodes receiving such messages</p><p>can update their routing table immediately without further</p><p>considering of other nodes.</p><p>A traffic-adaptive TDMA protocol will have the following</p><p>properties regarding adaptation delay: The scheduling process</p><p>will normally support reuse of time slots if nodes are suffi-</p><p>ciently far apart. To reach high efficiency, time slots need to</p><p>be assigned based on the actual traffic load on the nodes. This</p><p>means that both link changes as well as traffic changes will</p><p>have an impact on scheduling. All changes to the schedule</p><p>need to be negotiated in some way between the nodes that are</p><p>affected by the change. The number of nodes affected by a</p><p>change depends on the requirement for reuse of time slots. A</p><p>2-hop distance is common, but both more and less can be used</p><p>in some cases. Nevertheless, the number of nodes affected by a</p><p>change to the MAC schedule is normally less than the number</p><p>of nodes affected by a routing change.</p><p>It should be noted that there are correlations between the</p><p>routing and MAC adaptation delays that are not considered.</p><p>For example, a short routing adaption delay, accomplished</p><p>by frequent routing updates, would also stress the MAC</p><p>layer. Nevertheless, in the paper the effects on the application</p><p>performance of the adaption delays on the routing and MAC</p><p>layer are treated independently.</p><p>Notice that adaptation delay can partly be controlled in the</p><p>protocol design by allowing more or less control information</p><p>to be sent, i.e. overhead. A small adaptation delay is desired</p><p>but can be costly; moreover, there are limitations to how small</p><p>the adaptation delay can be made.</p><p>III. MODELING ADAPTATION DELAY</p><p>To investigate the effects of MAC and routing adaptation</p><p>delays on delay sensitive traffic, we use idealized protocols that</p><p>allow us to vary the adaptation delay in a controlled manner. In</p><p>this section we describe how the adapation delay is modeled in</p><p>our evaluation. We begin with a description of a basic model</p><p>of the detection, dissemination and decision process that we</p><p>use both at the MAC layer and the routing layer. We continue</p><p>with the physical layer after which we proceed through the</p><p>stack to the application layer.</p><p>A. Detection, Dissemination and Decision Process Model</p><p>The detection, dissemination and decision processes, de-</p><p>scribed in II, is modeled individually for each protocol layer.</p><p>Each protocol layer collect information about changes for</p><p>a time C , before the dissemination and decision processbegins, see Figure 1. In average it will take a time C/2 froma change actually occurred until the dissemination phase starts</p><p>becouse of a fixed TDMA frame and slot structure. Moreover,</p><p>we assume lower layer information is available at all the layers</p><p>and the same collection time for all layers.</p><p>The dissemination and decision are assumed to have a</p><p>duration D after which the protocol has adapted to thechanges. Thus the average time between the detection of a</p><p>56</p></li><li><p>Figure 1. Illustration of the information detection, dissemination, anddecision process.</p><p>change and the subsequent adaption of the protocol will be</p><p>A = C/2 + D. A change detected between time t1 andtime t2, in Figure 1, will take effect at time t3 with a adaptationdelay of A.</p><p>B. Physical Layer</p><p>An essential part of modeling an on-ground or near-ground</p><p>radio network is the electromagnetic propagation characteris-</p><p>tics due to the terrain variation. A common approach is to</p><p>use the basic path-loss, Lb, between two nodes. To estimatethe basic path-loss between the nodes, we use a uniform</p><p>geometrical theory of diffraction (UTD) model by Holm [7].</p><p>To model the terrain profile, we use a digital terrain database.</p><p>All our calculations of the basic path-loss are carried out using</p><p>the wave propagation library DetVag-90 R [8].</p><p>We define the signal-to-noise ratio (SNR), here defined as</p><p>Eb/N0, in the receiver node, , as follows</p><p> =P GT GRNR LbR</p><p>,</p><p>where P denotes the power of the transmitting node (equalfor all nodes), GT the antenna gain of the transmitter, GR theantenna gain of the receiver, NR is the receiver noise power,R is the data rate, and Lb is the basic path-loss between thetransmitter and receiver. We assume isotropic antennas in this</p><p>study.</p><p>The SNR for the links in a mobile ad hoc network will</p><p>often change quickly as the nodes move around in the terrain.</p><p>The quality of the link estimates will thus degenerate fairly</p><p>fast with time. To reduce the risk of using links with too low</p><p>SNR, a hysteresis functionality is used.</p><p>In Figure 2 we illustrate the hysteresis functionality. We</p><p>assume here that two nodes can communicate over a link if</p><p> &gt; low, i.e. between time t1 and time t5. However, thetransmitter will not start using the link before &gt; highat time t2. When the higher SNR level high is reached thetransmitter will also start announcing the link to its neighbors.</p><p>The node will continue announcing the link to its neighbors</p><p>until &lt; high at time t3. If the transmitter has announced alink to its neighbors it will continue to use the link as a normal</p><p>link when high &gt; low until its neighbors are notifiedthat the link do not exist anymore at time t4. The transmitterwill then stop using the link.</p><p>Figure 2. Illustration of the link hysteresis model.</p><p>The choice of hysteresis, i.e., high, will affect how dynamicthe topology will be for a given mobility, i.e., the number</p><p>of changes that occurs within a given time frame. A large</p><p>hysteresis will make the topology less dynamic which is</p><p>an advantage, but also reducing the number of available</p><p>links which is a disadvantage. Hence, it is a tradeoff which</p><p>hysteresis to select, in this paper we have set it to 6 dB.</p><p>C. Medium Access Control</p><p>To divide the radio channel between the nodes, we use an</p><p>idealized traffic-adaptive TDMA protocol. The time is divided</p><p>into time slots of duration Ts. The time slots are grouped intorepeating frames consisting of NF time slots.Each node will try to allocate enough time slots in each</p><p>frame so that it can manage its resource demands. Nodes with</p><p>more resources than needed will release unused time slots.</p><p>All traffic flows are assumed to have equal priority, i.e. if</p><p>resource demands exceed available resources, new demands</p><p>will be rejected in favor of existing allocations. If two nodes</p><p>simultaneously try to a...</p></li></ul>


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