Performance Comparison of AODV and AOMDV Routing Protocols in Mobile Ad Hoc Networks

Transcription

1 International Research Journal of Applied and Basic Sciences 2013 Available online at ISSN X / Vol, 4 (11): Science Explorer Publications Performance Comparison of AODV and AOMDV Routing Protocols in Mobile Ad Hoc Networks Mina Vajed Khiavi *1, Shahram Jamali 2 1. Department of Computer Engineering, Science and Research Branch, Islamic Azad University, Ardabil, Iran 2. Computer Engineering Department, University of Mohaghegh Ardabili, Ardabil, Iran Corresponding Author m_vajed@yahoo.com ABSTRACT: One of the main challenges of Mobile Ad Hoc Networks is the design of robust routing algorithms that adapt to the frequent and randomly changing network topology. In this paper we compare AODV and AOMDV routing protocols for MANETs. The AODV is a unipath routing protocol and AOMDV is a multipath version of AODV. We analyses these routing protocols by extensive simulations in ns-2 simulator and show that how number of nodes, pause time and traffic rate affect their performance. Performance of AODV and AOMDV is evaluated based on Packet Delivery Ratio, Network Life Time, System Life Time and End-to-End Delay. Keywords: AODV, AOMDV, MANET, Routing. INTRODUCTION A mobile ad hoc network (MANET) is a collection of wireless mobile nodes that dynamically establishes the network in the absence of fixed infrastructure. One of the distinctive features of MANET is each node must be able to act as a router to find out the optimal path to forward a packet. As nodes may be mobile, entering and leaving the network, the topology of the network will change continuously. MANETs provide an emerging technology for civilian and military applications. One of the important research areas in MANETs is establishing and maintaining the ad hoc network through the use of routing protocols (Kiess W and Mauve M, 2007; Ramanathan R and Redi J, 2002). Ad hoc routing protocols can be divided into two categories: proactive routing protocols and reactive routing protocols. Proactive (table-driven) routing protocol is an approach where each router can build its own routing table based on the information that each router or node can learn by exchanging information among the network s routers. This is achieved by exchanging update messages between routers on a regular basis to keep the routing table at each router up-to-date. Then, each router consults its own routing table to route a packet from its source to its destination. When a source node or an intermediate node consults the routing table, the path information, which is up-to-date, is immediately available and can be used by the node. This is because each router or node in the network periodically updates routes to all reachable nodes via broadcasting messages that the node received from the other nodes in the network (Boukerche A et al, 2011). In a Proactive routing protocol, although getting the path information is fast, the maintenance of the up-todate network information requires high overhead traffic and needs some significant amount of bandwidth. In addition, the process of maintaining the routes to the reachable nodes is continuous even if there is no data traffic flowing on these routes. Reactive (on-demand) routing protocol is an approach where the routing process needs to discover a route whenever a packet arrives from a source and needs to be delivered to a destination. Here, each node has no pre-built routing table or global information to be consulted. Due to the node s mobility in a wireless network, maintaining the existing route is an important process. In a Reactive routing protocol, the route discovery process happens more often, but this process requires low control overhead traffic compared to the Proactive routing protocol. Therefore, the Reactive is considered to be more scalable than the Proactive routing protocol. In addition, using a Reactive, the node has to wait for the discovery process each time the node attempts to send a message; this increases the overall delay (Abolhasan M et al, 2004; Eiman A and Biswanath M, 2012). There are many routing protocols available. This paper considers AODV and AOMDV for performance comparisons due number of nodes, pause time and traffic rate. These protocols are analyzed based on the

2 important metrics such as Packet Delivery Ratio, Network Life Time, System Life Time and End-to-End Delay. The rest of the paper is organized as follows. Section 2 describes two routing protocols AODV and AOMDV of MANETs. Section 3 describes performance metrics. The simulations and results of simulations present in section 4. Finally section 5 concludes the paper. Background On demand routing protocols work on the principle of creating routes when required between a source and destination node pair in a network topology. Our discussion is limited to two on-demand ad-hoc routing protocols, AODV and AOMDV, as follows. Ad hoc On-demand Distance Vector Routing (AODV) AODV (Perkins CE, 1997) combines the use of destination sequence numbers as in DSDV (Perkins CE and Bhagwat P, 1994) with the on-demand route discovery technique in DSR to formulate a loop-free, on-demand, single path, distance vector protocol. In contrast to DSR, AODV uses hop-by-hop routing instead of source routing. Below we review some of the key features of AODV to provide sufficient background for AOMDV described in the next section. Route Discovery When a traffic source needs a route to a destination, it initiates a route discovery process. Route discovery typically involves a network-wide flood of a route request (RREQ) for the destination and waiting for a route reply (RREP). Duplicate copies of a RREQ at every intermediate node are discarded. Source attaches a strictly increasing broadcast id with each RREQ it generates. Source id along with the broadcast id of the RREQ is used to detect duplicates. An intermediate node receiving a non-duplicate RREQ first sets up a reverse path to the source using the previous hop of the RREQ as the next hop on the reverse path. If a valid route is available, then the intermediate node generates a RREP, else the RREQ is rebroadcast. When the destination receives a nonduplicate RREQ for itself, it generates a RREP. The RREP is routed back to the source via the reverse path. As the RREP proceeds towards the source, a forward path to the destination is established. Note that the route discovery procedure just described requires that a bidirectional path exists between the source and the destination. Latest AODV specification describes a technique to find at least one such bidirectional path in the presence of unidirectional links. In the rest of our discussions, we will assume that there are no unidirectional links and that bidirectional paths exist between every pair of nodes. Also, several optimizations have been proposed in the literature to contain the scope of the flood and to reduce the redundancy of the broadcasts during the flood. However, these are somewhat orthogonal to our interest, and we will limit our discussion to pure flooding in this paper. Sequence Numbers and Loop Freedom Sequence numbers in AODV play a key role in ensuring loop freedom. Every node maintains a monotonically increasing sequence number for itself. It also maintains the highest known sequence numbers for each destination in the routing table (called destination sequence numbers ). Destination sequence numbers are tagged on all routing messages, thus providing a mechanism to determine the relative freshness of two pieces of routing information generated by two different nodes for the same destination. The AODV protocol maintains an invariant that destination sequence numbers monotonically increase along a valid route, thus preventing routing loops. This is explained below further as it will play a crucial role in the understanding of the multipath protocol. In AODV, a node can receive a routing update via a RREQ or RREP packet either forming or updating a reverse or forward path. The update rule in Figure 1 is invoked whenever such a routing packet is received. It is easy to see why loops cannot be formed if this rule is followed. Consider the tuple where represents the sequence number at node for the destination. Similarly, represents the hopcount to the destination from node. For any two successive nodes and on a valid path to the destination, being the downstream node, the route update rule in figure 1 enforces that: ( ) Where the comparison is in lexicographic sense. Thus, the tuples ( ) along any valid route are in a lexicographic total order, which in turn implies loop freedom. 3278

3 If ( ) (( ) ( )) End If Figure1. AODV route update rule. Use of Soft State and Route Maintenance AODV uses a timer-based technique to remove stale routes promptly. Each routing entry is associated with a soft state timer called route expiration timeout. This timer is refreshed whenever a route is used. Periodically, newly expired routes are invalidated. Route maintenance is done using route error (RERR) packets. When a link failure is detected (by a link layer feedback, for example), routes to destinations that become unreachable are invalidated. A RERR is broadcast that includes the list of unreachable destinations and their sequence numbers. The RERR propagation mechanism ensures that all sources using the failed link receive the RERR. RERR is also generated when a node is unable to forward a data packet for lack of a route. A node upon receiving a RERR from a downstream neighbor for some destination invalidates the corresponding route and updates the sequence number from the RERR. RERR is rebroadcasts if at least one destination becomes unreachable. Ad hoc On-demand Multipath Distance Vector Routing (AOMDV) The key concept in AOMDV ( Marina M. K anddas. S. R, 2001) is computing multiple loop-free paths per route discovery. With multiple redundant paths available, the protocol switches routes to a different path when an earlier path fails. Thus a new route discovery is avoided. Route discovery is initiated only when all paths to a specific destination fail. For efficiency, only link disjoint paths are computed so that the paths fail independently of each other. Note that link disjoint paths are sufficient for our purpose, as we use multipath routing for reducing routing overheads rather than for load balancing. For the latter, node disjoint paths are more useful, as switching to an alternate route is guaranteed to avoid any congested node. Link disjoint paths, on the other hand, may have common nodes. Since node disjointness is stricter than link disjointness, we use link disjointness in the hope to find more alternate routes in the network. AOMDV Route Discovery Several changes are necessary in the basic AODV route discovery mechanism to enable computation of multiple link disjoint routes between source destination pairs. Note that any intermediate node I on the route between a source S and a destination D can also form such multiple routes to D, thus making available a large number of routes between S and D. Recall that in the route discovery procedure a reverse path is set up backwards to the source via the same path the route request (RREQ) has traversed. If duplicates of the RREQ coming via different paths are ignored as before, only one reverse path can be formed. To form multiple routes, all duplicates of the RREQ arriving at a node are examined (but not propagated), as each duplicate defines an alternate route. See Figure 2(a). However, each of these alternate routes may not be disjoint. For example, in Figure 2(b) three copies of RREQ reach destination D, two of which are not via disjoint paths. How do we differentiate between duplicate RREQs that come via disjoint routes and that do not? Reverse routes should be formed only using the former type. Note that the copies of a RREQ reaching D via node disjoint paths must take different first hops from S. Were their trajectories to meet again at a node (e.g., node A in Figure 2(c)), the copy arriving later in that node will not be propagated further. Thus, all trajectories of a RREQ between any pair of nodes with unique first hops are guaranteed to be disjoint. To determine this, however, the first hop information needs to be included in the RREQ packet as an additional field. Each node remembers the first hop of each RREQ (in a first hop list) it has 3279

4 seen with the same source id and broadcast id. A reverse path is always formed when the first hop is unique. However, as in regular AODV, only the first copy of the RREQ is forwarded. Thus there is no additional routing overhead. All these reverse paths can be used to propagate multiple RREPs towards the source so that multiple forward paths can be formed. Note that all such paths are node disjoint. In the hope of getting link disjoint paths (which would be more numerous than node disjoint paths) the destination node adopts a looser reply policy. It replies up to k copies of RREQ arriving via unique neighbors, disregarding the first hops of these RREQs. Unique neighbors guarantee link disjointness in the first hop of the RREP. Beyond the first hop, the RREP follows the reverse route that have been set up already which are node disjoint (and hence link disjoint). Each RREP arriving at an intermediate node takes a different reverse route when multiple routes are already available. Note that because of the looser reply policy it is possible for the trajectories of RREPs to cross at an intermediate node. See Figure 3. Figure 2. Several network configurations explaining various protocol features. Figure 3. The second copy of RREQ via B is suppressed at intermediate node I. The parameter k is used to prevent a RREP explosion. Also, our earlier observation indicated that additional routes beyond a few provide only marginal benefit, if any. We have used k=3 in our experiments. Sequence Numbers and Loop Freedom Revisited If only the destination replies to a RREQ as in the preceding treatment, loops are not possible. This is because RREQs cannot loop as only the first arriving copy is propagated further. This prevents any loop in the reverse or forward paths either. But are we still free from loops when intermediate nodes choose to reply to a RREP? Recall that AODV uses a sequence number-based invariant to guarantee loop freedom. A similar invariant 3280

5 is maintained in the multipath technique is as well. However, its design is trickier and needs a somewhat elaborate description. Unlike the single path case, different routes for the same destination will now have different hop counts. Nodes must be consistent regarding which of these multiple routes it advertises to others. (An advertisement occurs when an intermediate node replies to a RREQ, or propagates a RREQ to its neighbors, for example). If two nodes on a route advertise routes such that the advertisement from the upstream node has a smaller hopcount, it presents a sure recipe for loops. But how do we prevent such situations given that each node maintains more than one routes in general? Note that a route can be formed through an intermediate node only when the latter advertises it. This is regardless of how many routes this node actually maintains. We establish a strict route advertisement policy to prevent loops. It is controlled by a field, advertised hopcount in the routing table entry, which is initialized each time the sequence number of this route entry is updated. The basic structure of a routing table entry in the AOMDV in comparison with AODV is shown in Figure 4. There are two main differences: (i) the hopcount is replaced by advertised hopcount in the AOMDV and (ii) the nexthop is replaced by the route list. The route list is simply the list of nexthops and hopcounts corresponding to different paths to the destination. The advertised hopcount represents the maximum of the hopcounts of each of those multiple paths so long as a strict route update rule is followed. This update rule is presented in Figure 5. This rule is invoked whenever a node receives a RREQ or RREP packet from a neighbor. As in AODV, routes corresponding to only the highest known sequence number for the destination are maintained. However, AOMDV allows for multiple routes for the same destination sequence number. Multiple routes can form via any neighbor upon receiving a RREQ or RREP from that neighbor. Lines (9)-(10) ensure loop freedom. A proof is provided in the appendix. Destination Sequence number Hopcount Expiration_timeout Nexthop Destination Sequence number Advertised_hopcount Expiration_timeout Route_list (a)aodv (b) AOMDV Figure 4. Structure of routing table entries for AODV and AOMDV (Marina M. K and Das. S. R, 2001). If ( ) If ( ) Else End if Else If ( ) End if Figure 5. AOMDV route update rule (Marina M. K and Das. S. R, 2001). 3281

6 The node updates its advertised hopcount for a destination whenever it propagates a RREQ from or when it generates/forwards a RREP for. Specifically, it is updated as follows: A key observation here is that similar to AODV the following condition holds well for two successive nodes and on any valid route to destination ( ) ) Where the comparison is in the lexicographic sense. We complete our description of AOMDV with mentions of a few other details. As shown in Fig. 4, each routing table entry has one common expiration timeout regardless of the number of paths to the destination. If none of the paths are used until the timeout expires, then all of the paths are invalidated and the advertised hopcount is reinitialized. We also investigated with having individual timeouts per path similar to the expiration mechanism in AODV. But simulation experiments did not show any additional benefit with this added complexity. Route maintenance in AOMDV is similar to AODV except that a destination is declared unreachable only when all routes to it break. Also, it is necessary to keep track of the maximum sequence number heard from RERRs through multiple downstream neighbors so that when all paths break, the destination sequence number is updated to this maximum value. Performance Metrics For MANET simulation, there are many performance metrics which are used to analysis the various proposals (Maqbool et al, 2011). In this paper we have used four performance metrics that evaluate routing protocols in all important aspects. Packet Delivery Ratio Packet delivery ratio is the ratio of number of packets received at the destination nodes to the number of packets sent from the source nodes. The performance is better when packet delivery ratio is high (Gupta et al, 2010). Network Life Time Network life time is the time when a node finished its own battery for the first time. The performance is better when network life time is high (Jamali and Jahanbaksh, 2011; Jahanbakhsh et al, 2011). System Life Time System life time is the time when 20% of nodes finish their own battery. The performance is better when system life time is high (Jamali and Jahanbaksh, 2011; Jahanbakhsh et al, 2011). End-to-End Delay End-to-end delay is the average time delay for data packets from the source node to the destination node. To find out the end-to-end delay the difference of packet sent and received time was stored and then dividing the total time difference over the total number of packet received gave the average end-to-end delay for the received packets. The performance is better when packet end-to-end delay is low (Gupta et al, 2010). Simulation Results The simulations were performed using Network Simulator 2 (Ns-2) ( particularly popular in the ad hoc networking community. The mobility model used is Random Way point Model ( The traffic sources are CBR (continuous bit rate), number of data connections is 10, data packet size is 512 byte and data sending rate is 4 packet/second. The source-destination pairs are spread randomly over the network in a rectangular filed of 1000m x 1000m. During the simulation, each node starts its journey from a random spot to a random chosen destination. Once the destination is reached, the node takes a rest period of time in second and another random destination is chosen after that pause time. This process repeats throughout the simulation, causing continuous changes in the topology of the underlying network. The simulation time is 500 seconds and maximum speed of nodes is 20 m/s. The primary energy of all nodes is 40 J. The interface queue is 50- packet drop-tail priority queue. The following three simulations were conducted to evaluate the performance of the AODV and AOMDV protocol under varying conditions: Simulation 1: Varying number of nodes. Simulation 2: Varying transmission rate. 3282

7 Simulation 3: Varying pause time. Simulation 1 Figure 6 shows the four performance metrics as a function of node numbers. In this simulation number of nodes considered 20, 50, 80, 100 and 150, pause time considered 80 seconds and transmission rate considered 10 packets/seconds. Simulation 2 Figure 7 shows the four performance metrics as a function of node numbers. In this simulation transmission considered 5, 10 and 15 packets/seconds, number of nodes considered 70, pause time considered 80 seconds. Figure 6. Performance with varying number of nodes. Simulation 3 Figure8 shows the four performance metrics as a function of node numbers. In this simulation pause time considered 0, 50, 100, 300 and 500, number of nodes considered 70 and transmission rate considered 10 packets/seconds. 3283

8 Figure 7. Performance with varying transmission rate Figure 8. Performance with varying pause time 3284

9 We can conclude from simulations outputs as folow. Increasing number of node cause packet delivery ratio increase but network life time and system life time decreased. Increasing transmission rate cause performance of network decrease. In some cases AOMDV has better performance and in some cases AODDV has better performance. CONCLUSION This paper evaluated the performances of AODV and AOMDV using ns-2 simulator. Comparison was based on of packet delivery ratio, network life time and system life time and end-to-end delay. Simulation results are shown by figures. In first simulation number of nodes is varying but pause time and transmission rate are fixed. In second simulation transmission rate is varying but number of nodes and pause time are fixed. In third simulation pause time is varying but number of nodes and transmission rate are fixed. REFERENCES Abolhasan M, Wysocki T, Dutkiewicz E A review of routing protocols for mobile ad hoc networks. Ad Hoc Networks 2: Boukerche A, Turgut B, Aydin N, Ahmad MZ, B_l_ni L, Turgut D Routing protocols in ad hoc networks: A survey. Computer Networks: Eiman A, Biswanath M A survey on routing algorithms for wireless Ad-Hoc and mesh networks. Computer Networks 56: Gupta AK, Sadawarti H, Verma AK Performance analysis of AODV, DSR & TORA Routing Protocols. IACSIT: Jahanbakhsh S, Jamali Sh, Zeinali E NISR: A Nature Inspired Scalable Routing Protocol for Mobile Ad Hoc Networks. IJCSET: Jamali Sh, Jahanbakhsh S BA-TORA: a Multipath Routing Protocol for MANETs by Inspiration from Bee and Ant Colonies. Electrical Review: Kiess W, Mauve M A survey on real-world implementations of mobile ad-hoc networks. Ad Hoc Networks: Marina MK, Das SR On-Demand multipath distance vector routing in ad hoc networks. Proceedings of the 9th IEEE International Conference on Network Protocols (ICNP). Perkins CE, Bhagwat P Highly dynamic Destination-Sequenced Distance-Vector routing (DSDV) for mobile computers. Paper presented at the Proceedings of the SIGCOMM 94 Conference on Communications Architectures, Protocols and Applications. Perkins CE Ad hoc On demand Distance Vector (AODV) routing. IETF Internet draft, Ramanathan R, Redi J A Brief Overview of Ad Hoc Networks: Challenges and Directions. IEEE Communications Magazine: