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A swarm intelligence-based unicast routing protocol for hybrid ad hoc networks
A swarm intelligence-based unicast routing protocol for hybrid ad hoc networks
A swarm intelligence-based unicast routing protocol for hybrid ad hoc networks
A swarm intelligence-based unicast routing protocol for hybrid ad hoc networks
A swarm intelligence-based unicast routing protocol for hybrid ad hoc networks
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A swarm intelligence-based unicast routing protocol for hybrid ad hoc networks


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We present a hybrid routing protocol for both pure and hybrid ad hoc networks which uses the mechanisms of swarm
intelligence to select next hops. Our protocol, Ad hoc Networking with Swarm Intelligence (ANSI), is a congestion-aware
routing protocol, which, owing to the self-organizing mechanisms of swarm intelligence, is able to collect more information
about the local network and make more effective routing decisions than traditional MANET protocols.

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A swarm intelligence-based unicast routing protocol for hybrid ad hoc networks

  1. 1. ARTICLE IN PRESS Journal of Systems Architecture xxx (2006) xxx–xxx ANSI: A swarm intelligence-based unicast routing protocol for hybrid ad hoc networks q,qq Sundaram Rajagopalan *, Chien-Chung Shen DEGAS Networking Group, Department of Computer and Information Sciences, University of Delaware, Newark, DE 19716, USAAbstract We present a hybrid routing protocol for both pure and hybrid ad hoc networks which uses the mechanisms of swarmintelligence to select next hops. Our protocol, Ad hoc Networking with Swarm Intelligence (ANSI), is a congestion-awarerouting protocol, which, owing to the self-organizing mechanisms of swarm intelligence, is able to collect more informationabout the local network and make more effective routing decisions than traditional MANET protocols. Once routes arefound, ANSI maintains routes along a path from source to destination effectively by using swarm intelligence techniques,and is able to gauge the slow deterioration of a link and restore a path along newer links as and when necessary. ANSI isthus more responsive to topological fluctuations. ANSI is designed to work over hybrid ad hoc networks: ad hoc networkswhich consist of both lower-capability, mobile wireless devices and higher-capability, wireless devices which may or maynot be mobile. In addition, ANSI works with multiple interfaces and with both wired and wireless interfaces. Our simulation study compared ANSI with AODV on both hybrid and pure ad hoc network scenarios using both TCPand UDP data flows. The results show that ANSI is able to achieve better results (in terms of packet delivery, number ofpackets sent, end-to-end delay, and jitter) as compared to AODV in most simulation scenarios. In addition, ANSI achievesthis performance with fewer route errors as compared to AODV. Lastly, ANSI is able to perform more consistently, con-sidering the lower variation (measured as the width of the confidence intervals) of the observed values in the results of theexperiments. We show that ANSI’s performance is aided by both its superior handling of routing information and also itscongestion awareness properties, though we see that congestion awareness in ANSI comes at a price.Ó 2006 Elsevier B.V. All rights reserved.Keywords: Swarm intelligence; MANET; Hybrid network; Hybrid routing; Congestion aware routing 1. Introduction q A section of this work was presented at ICAI 2005, June 2005, Hybrid ad hoc networks consist of a mixture ofLas Vegas, NV, USA. mobile, ad hoc network (MANET) nodes and nodesqq This work is supported in part by National Science Foun- which belong to highly capable infrastructure suchdation under grant ANI-0240398. * Corresponding author. Tel.: +1 302 831 1131; fax: +1 302 831 as mesh networks or cellular networks. The problem8458. of hybrid ad hoc networks is to make these networks E-mail address: (S. Rajagopalan). work efficiently without relying on pre-configured1383-7621/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved.doi:10.1016/j.sysarc.2006.02.006
  2. 2. ARTICLE IN PRESS2 S. Rajagopalan, C.-C. Shen / Journal of Systems Architecture xxx (2006) xxx–xxxnetwork topologies or centralized control. Hybrid of lower-level components. The combination/inter-ad hoc networks are useful in many situations where action of lower-level components in SI such asimpromptu communication facilities are required positive/negative feedback and amplification of fluc-such as battlefield communications, and disaster tuations along with multiple interactions are therelief missions. mechanisms which allow a node to change routing Since the problem of hybrid ad hoc networking information quickly and efficiently to adjust to anshares a lot of problems with typical MANET prob- ever-changing local topology and route deteriora-lems, typical routing solutions for hybrid networks tion, thus initiating fewer link breakages.start with a MANET routing solution and then Our protocol, ANSI, uses a highly flexible costapply some optimizations to work for specific sce- function which allows it to use the information col-narios. A number of ad hoc routing protocols have lected from the local ant activity, such as the conges-been proposed, for example [1–5], of which some of tion status of the neighboring nodes, in useful ways.them, like AODV [1] work on hybrid ad hoc net- In addition, the ant-like working of our protocolworks. In proactive protocols such as [5], nodes in allows for the maintenance of multiple routes to athe network maintain routing information to all destination. In nodes which use proactive routingother nodes in the network by periodically exchang- in ANSI, this fact is used to perform stochastic rout-ing routing information. Nodes using reactive pro- ing, and in nodes that use perform reactive routingtocols, such as [1,2], delay the route acquisition (pure MANET nodes), when one route fails, othersuntil a demand for a route is made. Hybrid proto- may be used. Our motivation comes from the factcols, like [4,6], use a combination of both proactive that different networks face different conditions,and reactive activities to gather routes to the desti- and thus a protocol suite should allow for variousnations in a network—nodes using ZRP, for exam- configurations as the network conditions dictate.ple, proactively collect routes in their zone, and Furthermore, supporting multiple routes simulta-other routes are collected reactively. In [6], on the neously is essential to ensure survivability of the net-other hand, the level of proactive activity and reac- work [9]. ANSI facilitates ad hoc unicast routing bytive activity are chosen autonomously by the nodes exploiting route finding behaviors that are emergentin the network, and proactive activity is only seen from ant packets working collectively, rather thanaround favorite destination nodes. In most tradi- explicitly coding them to cope with the problem.tional reactive protocols, like [1,2], only when a We formulate the routing problem at node i as aroute breaks irreparably does the protocol mecha- set of ‘‘food foraging’’ problems from nest i, wherenisms repair the damage. In reality, route deteriora- each ‘‘food source’’ is a destination d in the net-tion in mobile networks is most often not sudden work. In this formulation, next hops are evaluatedbut gradual,1 and most often available routes get on the basis of the strength of the pheromone trail2better/deteriorate gradually and not suddenly. So on the link connecting a node and a next hop.the routing protocol should continuously maintain The remainder of this paper is organized as fol-information about the nodes in the local area to per- lows: In the next section, we discuss a number ofform effectively and avoid too may link breakages. approaches and protocols which are related to our In this paper, we present a hybrid routing suite research. In Section 3, we describe in detail the com-(with both proactive and reactive components) for ponents of ANSI unicast routing protocol, and fol-hybrid ad hoc networks which uses the mechanisms low it with Section 4 where we discuss the results ofof swarm intelligence [7] to select good routes to des- the comparison of simulated models of ANSI withtinations. We use Swarm Intelligence (SI) because SI a popular routing protocol, AODV [1]. We concludemechanisms allow for self-organizing systems [8] and in Section 5 with a brief note on future research effort.maintain state information about the neighboringnetwork better than traditional MANET routing 2. Related workmechanisms. Self-organizing systems are robustenvironments where erroneous system behavior is The main ingredients of SI, positive/negativecorrected autonomously by the coordinated working reinforcement, and amplification of fluctuations 1 Some routes, such as routes to neighbors, break suddenly, 2when the neighbors go out of range. We are commenting on the The computational equivalent of the chemical deposited ongeneral case here. the forest floor by ants.
  3. 3. ARTICLE IN PRESS S. Rajagopalan, C.-C. Shen / Journal of Systems Architecture xxx (2006) xxx–xxx 3are achieved in an environment with multiple inter- DBF—ants are not used as feedback agents to rein-actions among nodes [7]. Because the above compo- force routes positively (in the case when a route isnents of SI are the lower-level components required still good), negatively (when a route is no longerfor self-organizing behavior, the benefits of using good) or explore new routes randomly—ants in thisSI-based algorithms are not fully accrued if the approach are unicast to specified direction, notany of the above lower-level components are not allowing for amplification of fluctuations, andpresent in a swarm intelligence-based protocol. This depending on known metrics such as timestamp ofis because in any SI-based system, these aspects a route in the routing together in learning about the network. The approach used in [13] by Heissenbttel and In [10] Baras and Mehta describe a swarm intel- Braun also relies on location information, and is aligence-based reactive ad hoc routing protocol purely proactive routing approach based on divid-called PERA. PERA uses broadcast forward ants ing the network into logical zones and assigningas exploratory agents sent out on-demand to find logical routers to each. Ants—forward ants andnew routes to destinations. Each ant holds a list of backward ants—are used by logical routers in thisnodes that were visited while exploring the network, approach to periodically check if the logical linksand since these ants are broadcast at each node, a connecting it to a randomly chosen destination areforward ant can result in several backward ants— functional and reflect on the current state of the net-ants sent by destination nodes in response to for- work surrounding the logical router. Positive andward ants. This uncovers several routes for each for- negative reinforcement are achieved by means ofward ant sent, and at each node these multiple multiple interactions and pheromone additions (byroutes found to the destinations are maintained as forward and backward ants) and pheromone aging,probability values. As with AntNet [11], the routing respectively. Random amplification of a new goodtable Ri at node i is a probability matrix with a route in the face of topological fluctuations isprobability entry Pijd as the probability that a data possible by random dissemination of ants to desti-packet at i’s FIFO queue will take the next hop j be routed to d. Positive reinforcement is managed In [14], Gunes et al. outline ARA, a multipath,in PERA using forward/backward ants and nega- purely reactive scheme. ARA uses forward antstive reinforcement is implicit—no explicit aging of and backward ants to create fresh routes from athe pheromone trails is done. After a route has been node to a destination. When routes to a destinationestablished, PERA regularly uses forward ants to D are not known at S, a forward ant is broadcast,find newer routes to destinations. This is wasteful, taking care to avoid loops and duplicate ants. Whenconsidering the fact that forward ants cause a lot a forward ant is received at an intermediate node Xof network resources to be consumed and should via node Y, the ant reinforces the link XY in X tonot be sent when not necessary. route to all the nodes covered so far by the forward In [12], Camara and Loureiro outline a source ant. When a forward ant is received at D, a back-routing scheme in which the network relies on loca- ward ant is created which backtracks the path oftion information and support from fixed infrastruc- the corresponding forward ant. At each node theture. Owing to a source routing approach, the backward ant is received, the link via which thealgorithm relies heavily on a source M destination backward ant is received is reinforced, like the for-route which is available at the time of message cre- ward ant does, for all nodes which have been visitedation. New nodes in the network start with using by the backward ant. In ARA, data packets per-their neighbor’s routing table. The routing table, form the necessary (positive) reinforcement requiredgenerated using shortest path algorithms, on the to maintain routes. When a path is not taken, it sub-other hand, may contain information which is out- sequently evaporates (negative reinforcement) anddated. Ants are unicast from a source to specific des- cannot be taken by subsequent data packets. Undertinations, for example, the destination node may be the described scheme, amplification of topologicalthe node with the oldest information in the routing and network fluctuations is not possible excepttable. This mechanism is used to make sure that the under extreme conditions when routes breakrouting information in the source is updated and often.recent. Thereby, ants are used in [12] with the In [15,16], Di Caro et al. describe AntHocNet, asemantics of routing information updates, like clas- hybrid, stochastic approach to the routing problemsical distance vector protocols such as DSDV or in MANET. AntHocNet is a congestion-aware pro-
  4. 4. ARTICLE IN PRESS4 S. Rajagopalan, C.-C. Shen / Journal of Systems Architecture xxx (2006) xxx–xxxtocol which only finds routes on-demand, but once a ing in immobile, highly capable infrastructure androute is established, the route is proactively main- applies it only in those nodes, rather than lettingtained. This approach, argued by the authors to pure MANET nodes incur the costs due to the samebe more ant-like [16] than other competing ant- under high mobility conditions. Lastly, the flexiblebased protocols, will fail to reduce overheads in very cost function (specifically, the congestion-awarenesshigh traffic/mobility scenarios, owing to the rate at property) in ANSI leverages the inherent nature ofwhich proactive ants are potentially unicast when swarm intelligence by collecting multiple routesthe mobility increases. This is because in high mobil- and using them to perform load balancing in allity/traffic scenarios, routes get invalidated often and sections of the network. This, as we will see, alsoproactive activity has to increase appropriately to alleviates the tendency to create hotspots in thekeep a valid view of the network for routing, thus network.increasing the load placed on the network. Indeed,we do agree with the comment that the authors of 3. ANSI unicast routing protocolAntHocNet make regarding the repeated path sam-pling, and ANSI manages to steer clear from 3.1. Protocol overviewrepeated path sampling by carefully choosing whento engage in route discovery activity. ANSI is a hybrid routing protocol for hybrid ad In [17], Wedde et al. present a new routing hoc networks comprising of both proactive and reac-algorithm for energy efficient routing in mobile tive routing components. Pure MANET (mobile)ad hoc networks. In their approach, they show nodes in ANSI use only reactive routing, and choosethat BeeAdHoc, a reactive source-routing protocol routes deterministically, while nodes belonging toinspired by the foraging principles of honey bees, is more capable, infrastructured (immobile) networksable to achieve energy consumption characteristics use a combination of both proactive and reactiveas compared to DSR, AODV and DSDV without routing and perform stochastic routing when multi-compromising on traditional performance metrics ple paths are available. The outline of the processsuch as packet delivery and throughput. of ANSI routing is as follows: Our protocol, ANSI, is a hybrid protocol pro-posed for hybrid ad hoc networks. Some character- 1. When a route to a destination D is required, butistics seen in traditional on-demand routing not known at a node S, S broadcasts a forwardprotocols can be seen in ANSI. For example, an reactive ant to discover a route to D.optimization used in AODV, expanding ring search, 2. When D receives the forward reactive ant from S,is also used in ANSI, albeit more efficiently, owing it source-routes a backward reactive ant to theto the use of history information. Unlike traditional source S. The backward reactive ant updatesMANET protocols which engage in route mainte- the routing table of all the nodes in the path fromnance/discovery activity only when links break, S to D, allowing for data transfer from S to D.ANSI continuously updates a node’s neighborhood 3. When a route fails at an intermediate node X, Xinformation using data packets and control packets first checks if there are other routes which can beto alleviate the negative effects due to flooding the used to route the packet to D. If not, then ANSInetwork with route discovery/maintenance. In addi- buffers the packets which could not be routedtion, unlike traditional MANET protocols, ANSI and initiates a route discovery to find D by usinghas a flexible cost function which allows it to per- a forward reactive ant to perform local routeform metric-centered routing. In our implementa- repair. Additionally, X sends a route error mes-tion, we have performed congestion-aware routing, sage back to the source node S.but it is easy to see how this cost function can be 4. Nodes belonging to more capable, infrastructuredmodified to perform, say, energy efficient routing. networks maintain routes to their connected com- When compared to other ant algorithms for ponents proactively, by periodic routing updatesMANET routing, we note that to the best of our using proactive ants. Nodes belonging to moreknowledge, there exists no other ant algorithm for capable, infrastructured networks also use sto-hybrid ad hoc networks, but ANSI is able to per- chastic routing when multiple paths are available.form well in both pure MANET and hybrid ad In addition, each node in the infrastructurehoc networks. In addition, the ANSI design under- collects information about which mobile nodesstands the advantages of proactive/stochastic rout- are connected to which infrastructure node.
  5. 5. ARTICLE IN PRESS S. Rajagopalan, C.-C. Shen / Journal of Systems Architecture xxx (2006) xxx–xxx 55. When a route at D is known at a MANET node maintains a row for the destination-next hop S, ANSI deterministically chooses the best next pair (d, j) along with the sijd (t), gijd, wijd, and aijd hop to reach the destination. If S is part of a values described below: highly capable infrastructure, then S may choose (a) sijd(t) is the pheromone trail concentration to perform stochastic routing to the destination left on a trail ij used as a first hop to desti- D, depending on the availability of multipath nation d at current time t due to all the ants routes. that have traversed the trail, taking into consideration the pheromone evaporation We claim ANSI will perform better than typical (see Eq. (6)). s is thus a weighted measureMANET protocols because of the working of the of how many times the trail ij was traversedSI mechanisms at each node, which maintain rout- by packets intended to d and is thereby aing information and local information more effec- measure of the goodness of trail ij.tively than traditional MANET routing protocols. (b) gijd is the heuristic value of going from j to i.In addition, the congestion-awareness of ANSI also In our mapping, g is a measure of the dis-helps in controlling the extent of congestion in high tance to the destination, distijd, going fromtraffic scenarios. Lastly, in hybrid networks, ANSI i to d, when using next hop j. We set 1is able to leverage the power of nodes belonging gijd ¼ 1 þ distijd .to more capable networks to assist in routing activ- (c) wijd 2 [0, 1] is the value of the congestionities of the network. In Sections 3.3.1–3.3.4, we status at node j. If wijd = 1, then, node j isexplain the details of the above actions, and show considered not congested, and if wijd = 0,how the SI mechanisms work at each node in main- then the node j is considered congested.taining routing information. The value of w at a node j is measured as the ratio of empty space in packets in the3.2. Protocol model IP queue size to the number of packets already in the IP queue at j.3.2.1. Data structures (d) We see that the goodness of a next hop j is Data structure (1) below is the ant structure, car- directly proportional to sijd(t), inverselyried by all ants, and data structures (2) and (3) proportional to distijd and directly propor-below are maintained at each node, and are updated tional to wijd. Thus, we write:every time an ant arrives at the node. a aijd ¼ ðcs Á sijd ðtÞ Þ Á ðcg Á gb Þ Á ðcw Á wc Þ ijd ijd(1) Ant structure: The following information is car- ð1Þ ried by an ant p: where cs > 0, cg > 0, and cw > 0 are arbi- (a) The ant ID of the ant, which is the (node trary constants, and a, b, c are integers such ID, sequence number) pair. that a, b, c > 0. (b) The number of nodes, m, which p visits, For our use, we need to normalize the including the node p originated from. above value of aijd so that we may gauge (c) The nodes-visited-stack (adapted from the relative effectiveness of each next hop. [11]), Sp , containing information about We normalize it such that aijd 2 [0, 1]: nodes V = {v1, v2, . . . , vm}, that can be a reached by backtracking the ant p’s move- ðcs Á sijd ðtÞ Þ Á ðcg Á gb Þ Á ðcw Á wc Þ ijd ijd aijd ¼P a b c ment (using the nodes-visited-stack), and l2J ðcs Á sild Þ Á ðcg Á gild Þ Á ðcw Á wild Þ (d) The pheromone amount at v 2 V, pv. ð2Þ(2) Ant decision table at node i, Ai : (adapted from [18]). An ant decision table is a data structure where J is the set of next hops at i to desti- that stores pheromone trail information for nation d. We then set cs = cg = cw = 1, and routing from node i to a destination d via k pos- arrive at sible next hop nodes J = {j1, j2, . . . , jk}. The link a b c ½sijd ðtފ ½gijd Š ½wijd Š ij in the ANSI network, between two nodes i aijd ðtÞ ¼ P a ð3Þ l2J ½sild ðtފ ½gild Šb ½wild Šc and j is assumed to be bidirectional. Routing tables are computed from ant decision tables. where a, b and c are chosen appropriately Each ant decision table entry Aijd for node i (see Section 4). The above formula was
  6. 6. ARTICLE IN PRESS6 S. Rajagopalan, C.-C. Shen / Journal of Systems Architecture xxx (2006) xxx–xxx adapted from Ant Colony Optimization ing the siJ 0 V 0 values on the trails iJ 0 that were techniques outlined in [18]. The intuition negatively reinforced, i.e., no ants that traveled behind this equation is that we want to V 0 ¼ fv01 ; v02 ; . . . ; v0m0 g were received) and recalcu- use the metrics of hop distance and path lated, the aijv values for all entries V in Sp0 are goodness and allow some flexibility as to recomputed and the new best next hops to destina- how much we rely on either metric by vary- tions V are computed again. This is followed by an ing a, b and c values. update of the routing table at node i. Negative rein- As soon as an ant p is received at a node i via forcement of routes also happens when a route isneighbor node j, i has the information about j’s con- explicitly invalidated by a route error message.gestion status from Sp . The pheromone sp depos- ijv (3) Routing table: The routing table at node i is aited by an ant p and the heuristic gijv to a table containing an entry for each destinationdestination v in the ant p traversing from node j d reachable from node i along with the best nextto node i via nodes v 2 V are given by the equations: hop, Jd , to d. The best next hop, Jd , to a des- i i 1 tination d is the next hop that contains the larg-sp ¼ ijv ðv; i; j 2 V Þ ð4Þ est aijd value in Ai . The value of Jd is thereby pj À pi i updated every time an ant visits a node i. Theand routing table also contains the distance of d 1 from i in hops, and this information is used togp ¼ 1 þ ijv ðv; i; j 2 V Þ ð5Þ set the number of hops for route discovery depthðvÞ when the routing table entry to d in i becomeswhere pi and pj are the pheromone amounts of ant p defunct.In the case of nodes which are part ofat nodes i and j, respectively, and V = {v1, v2, . . . , highly capable infrastructure, the routing is sto-vm} denotes the set of m nodes visited by p. The chastic, and the next hop is chosen directly fromvalue depth(v) is the depth of the node v in p’s nodes the ant decision table probabilistically. Specifi-visited stack.All s values in Ai are evaporated cally, a next hop j at node i for destination daccording to Eq. (7) each time another ant, p 0 , visits is chosen with a probability of aijd.node i. Let us say p 0 traverses the same trail ij attime (t + D) as traversed by p at time t. p 0 then pos-itively reinforces the trail ij "v 2 V in Sp . All other 3.2.2. Amplification of fluctuationstrails iJ 0 , (where J 0 is the set of all possible next hops The process of broadcasting ants during reactive/from i except j) in the ant decision table Ai are not proactive route discovery/recovery/maintenancepositively reinforced, and in the event no ant finds new routes to nodes and alters the informationtraverses through any of the other trails iJ 0 , the in the ant decision table accordingly. Because of thetrails iJ 0 eventually become invalidated (negatively nature of broadcast in the wireless medium, thereinforced) owing to pheromone evaporation. The routes found as a result of forward reactive antnew sijv at time (t + D) is calculated as follows: activity reflect the current status of the network 0 and accordingly amplify the current fluctuations insijv ðt þ DÞ ¼ evaporateðsijv ðtÞ; DÞ þ sp ijv ð6Þ the topology. Another mechanism amplifies the p0 fluctuations in the local area: when a node receiveswhere sijv is the pheromone deposited on the trailby p 0 over ij (see Eq. (4)). The function evapo- a unicast packet, it notes the neighbor node IDrate(sijv(t), D) returns the pheromone amount left and reinforces the path to the source of the packeton trail ij for destination v (after evaporation) due via the neighbor. In addition, when a data packetto the ants which traversed ij before p 0 . The phero- is sent along a next hop, the node reinforces the nextmone evaporation model used to calculate how hop as a valid next hop to the destination. Thismuch of the earlier pheromone trail, sijv(t) is left be- mechanism also amplifies local fluctuations of net-hind at (t + D) when p 0 traverses the trail ij is as work and topological characteristics and see to itfollows: that the nodes in the ANSI network use up-to-date network and topological information. sijv ðtÞevaporateðsijv ðtÞ; DÞ ¼ ð7Þ Some protocols, for e.g., [10], using SI mecha- 2D=c nisms for MANET argue for unicasting forwardwhere c is an arbitrary constant. After all the s val- reactive ants along one randomly chosen path toues in the ant decision table are evaporated (includ- the source and destination to amplify the fluctua-
  7. 7. ARTICLE IN PRESS S. Rajagopalan, C.-C. Shen / Journal of Systems Architecture xxx (2006) xxx–xxx 7tions in the network. Yet others, for e.g., [16], arguefor sending forward ants at regular intervals fromthe source while the source is sending packets tothe destination. We feel that the above methods willwork in low traffic scenarios and in wired networks,where there is little or no mobility, but not in highlymobile MANET with high loads. Besides, we feelthat the right model for amplification of fluctuationsin a MANET using SI mechanisms is the model weuse: that of broadcasting forward ants only when a babsolutely needed both at the source and intermedi-ate nodes (to perform local repair), and using these Fig. 1. Local reinforcement in ANSI. (a) Reinforcement by data packets. Node i, upon receiving a data packet from S via node j,mechanisms with a neighbor discovery mechanism, reinforces the path to node j via j and the source S via j.and applying the rules of SI on the data collected (b) Reinforcement in neighbor discovery mechanisms. Upon(viz., Eqs. (3)–(7)). By using the SI mechanisms receiving a HELLO beacon from j all nodes i reinforce trails via j.appropriately in ANSI, we are able to reduce thenumber of MAC layer resources used wastefully,as well as be responsive to a network with high traf- hello packets are used to perform local route man-fic rates, and provide better packet delivery rates agement by positively reinforcing previously knownand lower delay jitter characteristics. neighbors and new neighbors. The advantage of using this mechanism can be explained as follows:3.3. Protocol description If a direct route to a destination d is known at i via this process, then a previously known indirect A trail ij to destination d, sijd, is positively rein- route to d is less favored than the direct route byforced in ANSI when (a) a new route to a destina- the reinforcement mechanisms in ANSI. Note thattion d is found (via ant activity) at i via next hop HELLO messages are sent via all available interfaces(neighbor node) j, and (b) when i uses an already to facilitate neighbor discovery over all possibleknown nexthop node j again to route a packet to paths.d. A trail ij is negatively reinforced when (a) the trailij to destination d is subjected to evaporation (as per 3.3.2. Non-local route management and explicitEq. (7)), and (b) when next hop node j to d is no positive reinforcementlonger available (owing to MAC layer errors, route Reactive route discovery is performed by forwarderrors, or congestion at j). In the following sections, reactive ants, pf, and backward reactive ants, pb.we describe the various reinforcement mechanisms Reactive route discovery can be used both at theat work in ANSI. source of a data packet and at an intermediate node looking for an alternate route to the destination in3.3.1. Local route management—reinforcement by the event that previously known routes to the desti-data packets and the use of neighbor discovery nation have proved ineffective. A route request isHELLO messages sent by deploying a forward reactive ant pf and Local route management is made possible by the route reply is sent using a backward reactivereinforcement due to both movement of data pack- ant, pb. Even though multiple routes can be gath-ets and an explicit neighbor discovery mechanism. ered by a source sending forward reactive ants (byThese two concepts are illustrated in Fig. 1. allowing the destination to send backward reactive When a data packet arrives at a node i via a ants in response to all copies of the forward reactiveneighbor node p and is sent to the destination along ants received), we allow the destination to send anext hop j, both the trail to the previous hop, ip, and backward reactive ant only for the first forwardthe trail to the next hop, ij, are reinforced by the SI reactive ant received. This is because we found thatmechanisms at i. in a high traffic/mobility scenario in which a In addition, nodes in ANSI periodically broad- MANET node has many routes to the destination,cast a HELLO message. This message can contain packet delivery from source to destination cana variety of information about the node sending suffer invariably because using several routes willthe message, such as congestion status. In ANSI, spread the traffic over more nodes, and increase
  8. 8. ARTICLE IN PRESS8 S. Rajagopalan, C.-C. Shen / Journal of Systems Architecture xxx (2006) xxx–xxxthe contention in the network. In this case, it seemslike using one route deterministically, while keepingtabs on the congestion status of neighboring nodes(which is what ANSI does) is a better approach.3 Regardless, multiple routes are collected owingto the interaction of the ant information from thenodes in the network and HELLO beacons, andare used as and when older routes become defunct.Also, note that regardless of collecting informationabout multiple routes via other mechanisms, ANSIuses a deterministic choice of next hops when using Fig. 2. The propagation of the forward reactive ant (shown inpure MANET nodes (highly capable nodes collect solid arrows) and the return of the backward reactive ant (dashedmultiple routes and use stochastic routing, as we arrows). The rebroadcast from node 2, when received at node 1 iswill see later). This is because we found that stochas- killed immediately to prevent route loops. At each node X thetic approaches in MANET nodes using ANSI are forward reactive ant enters, it reinforces the path from X to all thenot suited to high data delivery in high traffic other nodes in the nodes-visited-stack. Thus, the forward reactive ant from S when received at node 4 reinforces the trail 4–2 toscenarios. both node 2 and node S. On the return path, all nodes in path of In Fig. 2, consider a node S which needs to route the backward reactive ant reinforce the trails to the paths to alldata packets to D, but does not have a route to D. the nodes in the path leading from the node upstream all the wayNode S buffers the data destined for D and broad- to the destination. Thus, when the backward reactive ant iscasts (over all interfaces) a forward reactive ant, received at S via path 1–3, . . . , D, S will reinforce trail S À 1 for destinations 1, 3, . . . , D.pSD (with a nodes visited stack Sf ), intended to dis- fcover the route to D. Because there is a good chancethat D has moved, the current implementation ofANSI sets the number of hops, /f, for the forward 2 and 4), but this is not done for pure MANETreactive ant (sent from S) to be a few hops larger networks.than the last known distance of S from D, which In the event that pDS is not received at S within a bcan be obtained from the routing table at S. If S timeout period, then the value of /f is increased by 2receives data intended to D after pSD has been f more hops and the search for the route resumesbroadcast, S buffers the data. When D receives again. The process of route discovery is continuedpSD , D copies the nodes visited stack, Sf , into a f again if a route is not found after the second backward reactive ant, pDS , and kills pSD . D b f ANSI retries twice for a route to destination.then sends pDS to S. pDS is not broadcast, it just b b To control the amount of MAC layer usage at abacktracks to the source S by using the nodes vis- node X, a scheduled HELLO packet is broadcast atited stack Sb in pDS . The ant, pDS , when visiting a b b X only if the last broadcast forward reactive antnode X along the path to S positively reinforces was sent before the last HELLO message.the route to all nodes v 2 Sb upstream from X toD, and adds an entry in AX to D via the next hop 3.3.3. Route errors, and negative reinforcementimmediately upstream (in the path from S to D). Route errors occur at a node X when X is unableAn intermediate node thereby knows what next to provide a route for the destination D owing tohop to use to route to D. In this way, backward non-availability of a routing table entry at X orreactive ants perform explicit positive reinforcement due to the non-availability of the next hop suggestedof routes to destination D. When S receives pDS b by the routing table entry at X. When a route errorfrom D, S sends the buffered packets intended for occurs at a node X in a network running ANSI, XD over the newly discovered route and flushes S’s first buffers the packet which X needs to forwardbuffer. Note that multiple paths may be readily col- and then sends a forward reactive ant to find thelected (for example, by sending another backward destination D. If X happens to be an intermediatereactive ant for the ant proceeding to D via nodes node, in addition to sending a forward reactive ant, X also sends a route error back to the source 3 Stochastic approaches to routing in pure MANET networks S of the packet. The packets buffered at X areis an effective approach when the mobility and traffic in the relayed across the network after a backward reac-network are low. tive ant from D reaches X.
  9. 9. ARTICLE IN PRESS S. Rajagopalan, C.-C. Shen / Journal of Systems Architecture xxx (2006) xxx–xxx 9 In addition, when a route error is received at anintermediate node between X and S, the nodeexplicitly invalidates the routing table entries to D.The packets received at X before the route error isreceived at S are X’s responsibility (to forward),but the packets generated after the time when theroute error is received at S from X are S’s responsi-bility—S generates a forward reactive ant to find theroute to D.3.3.4. Proactive routing within highly capable sectionsof the network Fig. 3. In a hybrid network (nodes 1–9 are part of a highly As mentioned in Section 3.1, nodes belonging to capable network in this case, and are connected by the grid shown), nodes belonging to more capable infrastructure are ablenon-mobile, highly capable infrastructure, such as to perform stochastic routing. In this figure, two possible pathscellular networks engage in proactive routing as well that 1 can take to route to D are one via nodes 1 ! 4 ! 7 !as reactive routing because these nodes are not con- 8 ! 9 (P1) and another via nodes 1 ! 2 ! 5 ! 6 ! 9 (P2).cerned about topological fluctuations. These nodesalso maintain a list of mobile nodes which are acces-sible from each other, thus assisting the reactive the changes in the network. Hence, a choice for a,routing process within the mobile nodes as and b, and c should be made carefully to allow forwhen possible. Nodes in non-mobile, highly capable responsiveness of the system. Using insights frominfrastructure send proactive ants periodically to all our preliminary results, we arrived at a value ofthe other highly capable nodes they are connected a = b = c = 2, and these are the values we use into. Proactive ants are not returned like forward our implementation.reactive ants, and they reinforce the route to theproactive ant sender along the path the proactive 4. Simulation resultsant takes. Proactive ants, apart from carrying anodes-visited-stack for gathering information about ANSI was simulated in QualNet (Version 3.7),the nodes that were visited, are fixed in hop length and the performance of ANSI was compared withand also carry a data structure for indicating the a popular routing protocol, AODV [1], for the samemobile nodes which are accessible from the proac- network and load characteristics. We chose to com-tive ant sender. These nodes engage in proactive pare ANSI with AODV because AODV has beenroute collecting activity using all their interfaces, shown to perform well in a vast majority of adand so are able to combine routes found via differ- hoc network scenarios. In addition, AODV alsoent interfaces effectively during the routing process. works on hybrid ad hoc networks. Our work here Because nodes belonging to a highly capable net- is an extension of our earlier work [19] which onlywork need not be concerned about the issues due to tested ANSI under UDP loads over a puremobility, these nodes are able to effectively utilize MANET. As we mentioned earlier, ANSI functionsthe benefits due to stochastic routing (see Fig. 3). as a purely reactive protocol in a pure MANETAs mentioned before, ant-based routing naturally environment.lends itself to stochastic routing because multipleroutes are found and maintained. 4.1. Simulation and network model3.3.5. Driving the routing process via more desirable 4.1.1. ANSI parametersnodes The current implementation of ANSI used By choosing higher values for a, b, and c, the a = b = c = 2. In both AODV and ANSI, the reac-process of next hop selection in ANSI favors the tive route recovery is retried twice, and for ANSI,next hops with higher values for s, g, and w, respec- the last try uses /f = 15. For the first two trials intively. However, by choosing values which are too ANSI, /f is determined according to the informa-high, the route selection is too skewed towards the tion available about the unknown destination: ifbest next hops and it becomes very difficult for the the destination had a valid entry in the routing tableSI mechanisms at the nodes to respond quickly to earlier, then /f is set to one more than the earlier
  10. 10. ARTICLE IN PRESS10 S. Rajagopalan, C.-C. Shen / Journal of Systems Architecture xxx (2006) xxx–xxx a bFig. 4. Hybrid network topologies used for Experiments 1–3: (a) The hybrid network topology for Experiments 1 and 2. (b) The hybridnetwork topology for Experiment 3.number of hops to the destination. Otherwise, (thus, one stream will be an ‘‘internal’’ stream)./f = 5. The evaporation constant, c, used in Eq. There are thus, altogether, 16 traffic streams in this(7) is 15 s. In hybrid networks, the nodes which experiment. Each of these streams send 512-byteare part of the high-speed Ethernet (see Section packets at a uniform rate of 1–20 packets/s.4.1.2) used a proactive route update interval of 10 s. In Experiment 3, we studied the performance of ANSI and AODV in a larger hybrid network con-4.1.2. Network and application parameters sisting of 360 pure MANET nodes spread over 9 We performed five experiments, in which we mobile regions uniformly located in a 5000 m ·studied the performance of ANSI and AODV with 5000 m terrain, each of size 1000 m · 1000 m andincreasing traffic and increasing number of nodes serviced by one highly capable, immobile nodeof both UDP and TCP flows in both hybrid and (nodes 361–369) located at the center of each mobilepure MANETs. In all these experiments, the source region. The highly capable nodes are all connectedand destination are chosen randomly and are pair- via a 100 Mbps Ethernet link. The topology of ourwise-distinct for each trial. experiment is shown in Fig. 4(b). Each highly capa- In Experiments 1 and 2, we studied the perfor- ble node has both Ethernet interfaces and an 802.11mance of ANSI vs. AODV in a hybrid network, interface. The size of the data packets sent wasfor both UDP (Experiment 1) and TCP (Experiment 512 bytes. Six UDP streams are randomly gener-2) flows. In these experiments, the non-mobile nodes ated, with the following profile of the source–desti-are connected to each other over a 100 Mbps Ether- nation pairs: (a) regions 1–4, (b) regions 1–7, (c)net link. Fig. 4(a) shows the simulation topology. regions 8–5, (d) regions 8–2, (e) regions 3–6, andThe size of the entire terrain is 2000 m · 2000 m. (f) regions 3–9. The data sources generated packetsInside this terrain, there are four MANET at the uniform rate of 2 to 20 packets/s in steps of‘‘regions’’, each of which contain 20 MANET nodes 2 packets/s.inside a terrain of size 500 m · 500 m, and ‘‘ser- In Experiment 4, we studied the effect of increas-viced’’ by one highly capable, immobile node (nodes ing TCP traffic in a pure MANET network. In this81–84) located in the center of the mobile region. experiment, 50 nodes were placed uniformly in aThis highly capable node, located in the center of network of size 1100 m · 1100 m. This maintains aeach of the regions, manages both an Ethernet inter- node density4 of 8.15 mÀ2, which, according toface and an 802.11 interface, and is connected to the [20], is sparse for a network with mobile nodes.others by another highly capable node, node 85, The experiment simulates 25 streams of TCP trafficwhich has 4 Ethernet interfaces. Note that MANET sending 64-byte packets at a uniform packet ratenodes within a region are not able to communicate varying from 1 to 20 packets/s.with MANET nodes of other regions directly (theclosest they can get is around 353 m, which is beyondthe transmission range of the MANET nodes). Four streams are chosen for each region, with 4 Node density is defined as the number of nodes in an areaone stream headed towards each of the regions covered by the transmission range of a node.
  11. 11. ARTICLE IN PRESS S. Rajagopalan, C.-C. Shen / Journal of Systems Architecture xxx (2006) xxx–xxx 11 In Experiment 5, we studied the performance of aged over the number of source–destinationANSI and AODV under UDP loads in a pure pairs. For TCP flows, the above described quan-MANET environment with an increasing number tity is the measured packet delivery ratio. Theof nodes. The number of nodes was varied from actual packet delivery for TCP flows is calculated50 to 250 and exactly half the number of nodes were using the expected number of packets that shoulddata sources. The terrain size was such that the node be sent at the application layer at the datadensity was constant at 8.15 mÀ2 (for example, for sources. UDP does not perform congestion-con-50 nodes, the terrain size was 1100 m · 1100 m). trol so the expected and measured number ofThe data sources generated one 64-byte packet a packets sent at the application layer of the datasecond to be sent to the data sink. source are the same. In all the experiments, the MANET nodes were 2. (End-to-end metric 2) End-to-end delay: measureduniformly distributed initially in the terrain and as the average delay in sending packets fromthe mobile nodes moved as per the Random Way- source to destination and averaged over the num-point Model with a minimum speed of 0.001 m/s, ber of source–destination pairs.maximum speed of 20 m/s, with a pause time of 3. (End-to-end metric 3) Delay jitter: measured as10 s. In hybrid networks (Experiments 1–3), the the average variance of the interarrival times atmobile nodes were restricted to move only within the destinations and averaged over the numbertheir region (bounded by a 500 m · 500 m terrain of source–destination pairs.for Experiments 1 and 2 and a 1000 m · 1000 m ter- 4. (End-to-end metric 4) Number of packets sent byrain for Experiment 3). The MANET nodes in the Super application sender: measured as the totalexperiments used one 802.11 interface with omnidi- number of packets which are actually sent byrectional antennas and a transmission range of Super Application senders. For Super Applica-250 m at the physical layer and 802.11DCF at the tion using TCP, this number depends on howMAC layer. The link bandwidth for the mobile long the TCP connection lasts.nodes using 802.11 was 2 Mbps. In addition to using 5. (End-to-end metric 5) Variation of the congestion802.11, the non-mobile nodes also used Ethernet window of a sender: measured as the TCP conges-with a capacity of 100 Mbps. The simulations used tion window (snd_cwnd) at one sender for onea two-ray pathloss model and no propagation fad- flow for one trial as it varies with simulation model was assumed. The application used was 6. (Network-wide metric 1) Total number of routeCBR, and sources and destinations were pairwise errors initiated: is the total number of routedistinct and chosen randomly. Both TCP and errors generated in the network.UDP-based CBR flows were studied. Super applica- 7. (Network-wide metric 2) Total number of 802.11tion was used for generating a reliable CBR traffic DCF MAC layer unicasts sent: is the total num-stream using TCP (regular CBR application uses ber of all (successful) 802.11DCF unicast trans-UDP). missions sent in the network. For AODV, this All experiments were run for a simulated time of measures the total number of data packets,5 min and all sources started sending packets at RREP and RERR sent out at the 802.11DCFexactly 40 s into the simulation and ended data gen- interface. For ANSI, this measures the totaleration at exactly 260 s. TCP-LITE, used for the number of data packets, backward reactive ants,TCP flows in our experiments, is a variant of and RERRs sent at the 802.11 interface.TCP-RENO, and used an MSS of 512 bytes, maxi- 8. (Network-wide metric 3) Total number of 802.11mum send/receive buffer of 16384 bytes each, and DCF MAC layer broadcasts sent: is the totaldelayed ACKs. number of all 802.11DCF broadcasts sent by all We studied the following end-to-end and net- nodes in the network. For AODV, this measureswork-wide characteristics: the total number of RREQ and Hello packets sent at the 802.11DCF interfaces, and for ANSI,1. (End-to-end metric 1) Packet delivery fraction: this measures the total number of forward reac- measured at the application layer as the ratio of tive ants, proactive ants and the Hello packets the total number of packets which were received sent at the 802.11 interfaces. (at the application layer) at the data sinks to the total number of packets that were sent from the We do not report end-to-end delay and delay jit- data sources (at the application layer), and aver- ter for TCP flows as these metrics are typically not
  12. 12. ARTICLE IN PRESS12 S. Rajagopalan, C.-C. Shen / Journal of Systems Architecture xxx (2006) xxx–xxxreported for TCP flows, because of the fact that AODV send a comparable number of MAC uni-TCP has to deal with out-of-order deliveries casts. ANSI sends fewer MAC broadcasts whenand the large delays (as compared to UDP flows) the packet rate is low to moderate, but as the packetowing to reliability and congestion-control mecha- rate increases, ANSI sends more MAC broadcasts.nisms. The reason why ANSI performs better than We analyzed the results from the above experi- AODV—delivering more packets with better met-ments and show them using graphs with 95% confi- rics such as delay, jitter and number of routedence intervals of the measured values. errors—is because ANSI manages the local network information better than AODV does, and performs4.2. Simulation results congestion-aware routing. This is why ANSI shows lower route errors as compared to AODV (see4.2.1. Experiment 1: Hybrid network—effect of Fig. 5(d)). Owing to the above reasons, routes breakincreasing the UDP packet rate less often and result in fewer route request opera- Fig. 5 shows the results for the performance of tions in ANSI as compared to AODV. When routesANSI vs. AODV over a hybrid network using do break in ANSI, they are managed by the proto-UDP flows. We see that ANSI consistently outper- col mechanisms locally rather than a network-wideforms AODV in terms of packet delivery, delay, jit- flooding. This in turn results in lower congestionter and number of RERR initiated. ANSI and at the nodes. This is why, even though ANSI shows Packet delivery at the CBR layer (%) 1 0.3 AODV 0.99 0.25 End-to end delay (s) ANSI 0.2 0.98 0.15 0.97 0.1 AODV 0.96 0.05 ANSI 0.95 0 0 5 10 15 20 0 5 10 15 20 a Packet rate (pkts/s) b Packet rate (pkts/s) 600 Total number of RERR initiated AODV AODV 500 ANSI ANSI 0.1 Delay jitter (s) 400 300 0.05 200 100 0 0 0 5 10 15 20 0 5 10 15 20 c Packet rate (pkts/s) d Packet rate (pkts/s) 4 4 x 10 x 10 15 5 802.11DCF,Broadcasts sent 802.11DCF,Unicasts sent 4 10 3 2 5 AODV 1 AODV ANSI ANSI 0 0 0 5 10 15 20 0 5 10 15 20 e Packet rate (pkts/s) f Packet rate (pkts/s)Fig. 5. Experiment 1: Performance studies of ANSI vs. AODV in a hybrid network with UDP flows: (a) Packet delivery ratio, (b) end-to-end delay, (c) delay jitter, (d) number of RERR initiated, (e) 802.11DCF, Unicasts sent and (f) 802.11DCF, Broadcasts sent.
  13. 13. ARTICLE IN PRESS S. Rajagopalan, C.-C. Shen / Journal of Systems Architecture xxx (2006) xxx–xxx 13larger MAC broadcasts in higher packet rates, it As packet rate increases, for both AODV andstill shows delays and jitter lower than that for ANSI, the mean time before links break owing toAODV. The higher number of broadcasts when node mobility is still the same, but because the ofthe packet rate increases is because of the conges- the use of highly capable nodes (which are withintion-aware properties of ANSI, which allow it to 2 hops away for any MANET node), the percentagedrop badly congested routes and look for new ones. of packets delivered with lower variation in end-to-This, while delivering packets more quickly and end delay increases (in comparison to the number ofsmoothly, obviously makes ANSI incur more route packets delivered at higher variations in end-to-enddiscovery overheads, which is what we see in terms delay) at both AODV and ANSI, thus bringing theof larger MAC broadcast overheads. The new, con- overall variation down. This is why we see agestion-free (or low congestion) routes are then used decrease in delay jitter as packet rate increases forto deliver more packets in ANSI. Note that AODV, both ANSI and AODV.does not show an appreciable increase in the num-ber of MAC broadcasts as the packet rate increases 4.2.2. Experiment 2: Hybrid network—effect ofbecause it does not perform congestion-aware rout- increasing the TCP packet rateing, but owing to this, the performance of AODV Fig. 6 shows the results for Experiment 2. Fordegrades. The fact that the number of ANSI’s TCP flows, we see that ANSI’s measured and actualMAC unicasts are comparable to that of AODV packet delivery ratio is higher than the same metrics(in the context of better performance metrics), along for AODV. We also see that for AODV, the mea-with its fewer route errors is a clear indication of the sured packet delivery ratio improves as the packetfact that ANSI is engaged in providing/finding bet- rate increases, but the actual packet delivery ratioter routes as compared to AODV. decreases. ANSI’s actual packet delivery is nearly The reason why delay jitter decreases with 5–10% more than AODV’s actual packet deliveryincreasing packet rate in both ANSI and AODV ratio. ANSI also sends more packets during the sim-(see Fig. 5(b) and (c)) is as follows: Delay jitter is ulation as compared to AODV—we see that thea measure of the variation of interarrival times at number of packets which ANSI sends is very closethe destination. Thus, if end-to-end delay measured to the number expected to be sent.5 In terms ofat the destination varies very little, then delay jitter the effect of the routing protocol on TCP, the con-is bound to be low. ANSI, being congestion-aware, gestion window for the output queue at node 53chooses congestion-free routes and delivers packets (for packet rate 1 packets/s, sent from node 53 toat the destination with little variation in end-to- node 48) shows steady growth, while AODV’s con-end delay. AODV, because it is not congestion- gestion window (for the same stream, output queueaware, delivers packets along congested routes, at node 53) shows substantially slower growth.which results in higher end-to-end delays because ANSI, as before, shows a lot fewer route errorsa node running AODV does not react to congestion (see Fig. 6(d)). ANSI shows more MAC unicastuntil a congested node along the path is no longer traffic as compared to AODV. Though ANSI showsable to receive or transmit packets. Thus, for a sin- lower MAC broadcast traffic when the packet rate isgle stream of UDP traffic from one source to desti- low, it shows more MAC broadcast overheads whennation in AODV, the destination first experiences the packet rate increases.low variation in end-to-end delay, but thereafter, The reason why ANSI performs better (with 5–10%the path becomes more congested and the variation higher actual packet delivery ratio) than AODV underin end-to-end delay progressively increases until the TCP loads is because of the congestion-aware routingpath breaks. AODV then engages in route discovery in ANSI. Owing to this property, ANSI is able to sup-and finds a congestion-free path, and once again the ply congestion-free routes which allow for the smoothmeasurement of end-to-end delay at the destination passage of ACKs back to the data source, allowingshows low variation until the new path becomes TCP operations to perform smoothly.congested again. Statistically, the value of delay jit- We would like to draw attention to the graphster depends on the percentage of the packets that showing the measured packet delivery ratio inare delivered at the destination with low variation Fig. 6(a). These results for measured packet deliveryin end-to-end delay, and so if a higher percentageof packets are delivered with a higher variation, 5 This amount is 16 · 220 · x = 3520x packets total (x is thethe jitter is bound to be larger. packet rate), and indicated by the straight line graph in Fig. 6(b).
  14. 14. ARTICLE IN PRESS14 S. Rajagopalan, C.-C. Shen / Journal of Systems Architecture xxx (2006) xxx–xxx 4 Packet delivery at the app. layer (%) x 10 1 Packets sent by the app. layer 6 AODV(m) 0.95 AODV(a) ANSI(m) 4 0.9 ANSI(a) AODV 2 ANSI 0.85 Ideal 0.8 0 0 5 10 15 20 0 5 10 15 20 a Packet rate (pkts/s) b Packet rate (pkts/s) 250 Total number of RERR initiated ANSI Congestion window (bytes) 15000 AODV 200 10000 150 100 5000 AODV 50 ANSI 0 0 100 200 300 0 0 5 10 15 20 c Simulation time (s) d Packet rate (pkts/s) 5 4 x 10 x 10 3 4 802.11DCF,Broadcasts sent 802.11DCF,Unicasts sent 2.5 3 2 1.5 2 1 AODV 1 AODV 0.5 ANSI ANSI 0 0 0 5 10 15 20 0 5 10 15 20 e Packet rate (pkts/s) f Packet rate (pkts/s)Fig. 6. Experiment 2: Performance studies of ANSI vs. AODV in a hybrid network with TCP flows: (a) measured (m) and actual(a) packet delivery ratio, (b) packets sent by the Super Application Layer, (c) congestion window for TCP output at node 53 for 1 pkt/s,(d) number of RERR initiated, (e) 802.11DCF, Unicasts sent and (f) 802.11DCF, Broadcasts sent.ratio are counter-intuitive. While the measured that we had fixed the TCP send buffer to bepacket delivery ratio of AODV increases with 16,384 bytes, and the congestion window cannotpacket rate, we note that the percentage of packets grow beyond this size. In this figure, we see howsent to the data sink increasingly decreases. Thus, Super application works TCP when sending CBRthe actual packet delivery ratio, measured as the traffic. Note that this is traffic inside a mobile regionpercentage of packets that are received to the per- (both node 53 and node 48 are inside the samecentage of packets that are expected to be sent (in mobile region as per Fig. 4(a)). TCP, when workingthis case x · 16 · 220 = 3520x, where x is the packet on top of ANSI, is able to increase the congestionrate), actually decreases. So, the traditional packet window as per congestion avoidance algorithms,delivery ratio metrics, defined as the ratio of the but in AODV, congestion avoidance is quicklynumber of application layer packets delivered to thwarted by congestion occurring along the paththe number of application packets sent, is actually from node 53 to node 48, which is why the TCP stacka misleading metric to measure when studying at node 53 shows fast recovery behavior [21] for theMANET performance under TCP loads. TCP output queue. This is the case owing to losing a The behavior of ANSI and AODV under TCP lot of ACKs in AODV. Indeed, we see that the con-loads can be summarized clearly by Fig. 6(c). Note gestion window in AODV does not grow/change
  15. 15. ARTICLE IN PRESS S. Rajagopalan, C.-C. Shen / Journal of Systems Architecture xxx (2006) xxx–xxx 15after a certain point into the simulation (around More packets are sent by ANSI at the Super200 s) for AODV. Whereas, for ANSI, we see a application layer as a result of larger congestion‘‘healthy’’ growth of the congestion window, con- windows, and subsequently, more MAC unicaststrolled by the congestion-avoiding sender (linear are sent for these packets, which is why we see thegrowth of congestion window) rather than being number of MAC unicasts for ANSI is more. Morecontrolled by congestion elsewhere in the network. MAC broadcasts are sent in ANSI as a response This behavior for TCP running over ANSI to finding newer routes which are less congested.results from ANSI’s congestion awareness, which As before, AODV does not respond to congestion,constantly maintains routes with low congestion and so it shows only a small increase in the numberand chooses them in favor of the ones with higher of MAC broadcasts as the packet rate increases.congestion. This permits TCP running over ANSIto receive ACKs more frequently and regularly than 4.3. Experiment 3: Large hybrid network—effectin the AODV case, where losing ACKs causes fast of increasing UDP packet raterecovery behavior. AODV, not being congestionaware, chooses congested routes frequently because Fig. 7 shows the results for the performance ofit has no way of knowing which routes are con- ANSI and AODV in a larger hybrid network. Asgested and which ones are not, making the passage we can see, the results are similar to the results ofof ACKs more difficult. Experiment 1, shown in Fig. 5. We also see ANSI’s Packet delivery at the CBR layer (%) 1 AODV AODV 0.4 0.95 ANSI End-to-end delay (s) ANSI 0.9 0.3 0.85 0.2 0.8 0.1 0.75 0.7 0 5 10 15 20 0 5 10 15 20 a Packet rate (pkts/s) b Packet rate (pkts/s) 0.4 Total number of RERR initiated AODV 1500 AODV 0.3 ANSI ANSI Delay jitter (s) 1000 0.2 500 0.1 0 0 0 5 10 15 20 5 10 15 20 c Packet rate (pkts/s) d Packet rate (pkts/s) 4 5 x 10 x 10 10 802.11DCF,Broadcasts sent 3 802.11DCF,Unicasts sent 8 2.5 6 2 1.5 4 1 2 AODV AODV ANSI 0.5 ANSI 0 0 0 5 10 15 20 0 5 10 15 20 e Packet rate (pkts/s) f Packet rate (pkts/s)Fig. 7. Experiment 3: Performance studies of ANSI vs. AODV in a (larger) hybrid network with UDP flows: (a) packet delivery ratio,(b) end-to-end delay, (c) delay jitter, (d) number of RERR initiated, (e) 802.11DCF, Unicasts sent and (f) 802.11DCF, Broadcasts sent.