Effect of Node density and Mobility on On-demand routing protocols in MANET
Shailja R.Nayak1, Dr. Nisha Sarwade2
Electrical Engineering Department, V.J.T.I, Mumbai, INDIA1,2
Abstract— Mobile Ad-hoc Networks (MANET) are the assembly of mobile nodes that inter-communicate on shared wireless channels without any fixed infrastructure or any centralized control. It's a self configuring network where each node must function as a router. These mobile nodes move arbitrarily and form irregular topologies. Routing is consequently a prime challenge in ad-hoc networks and many routing protocols have been proposed to date where each one has its own advantages and pitfalls and thus used in different scenarios. Many experiments/simulations have been conducted comparing the performance of these routing protocols subjected to various conditions and constraints. The details of how density and mobility of nodes together, affects the performance of routing protocols needs consideration . This paper addresses this issue by simulating two routing protocols AODV and DSR using NS2.34 and comparing in terms of Packet delivery ratio (PDR). Simulation results illustrate that AODV performs well with less number of nodes and high mobility whereas DSR outperforms AODV for comparatively large number of nodes and moderatemobility.
keywords- MANET; PDR; AODV; DSR
I. Introduction
Ad-hoc means "formed for a particular purpose". Thus MANET's are the purpose specific networks which are configured on the fly when there exists limited or no communication infrastructure.They do not require a pre-existing architecture for communication purpose and do not rely on any type of wired infrastructure; thus in an ad hoc network all communication occurs through a wireless median. MANETs can be deployed to allow the communication devices to form a dynamic and temporary network among them. It is used in areas of Sensor networks for environmental monitoring, Rescue operations in remote areas, Remote construction sites, and Personal area Networking, Emergency operations, Military environments, Civilian environments [1]. Due to the dynamic nature of these networks and rapidly changing topologies routing is very crucial issue to deal with. An Ad-hoc routing protocol is a convention or standard that controls, how nodes come to agree which way to route packets between computing devices in a MANET [2]. The routing protocol must be able to keep up with the high degree of node mobility that regularly changes the network topology unpredictably. Hence, the protocols must be adaptive and be able to maintain routes despite the change in the topology of the network, caused by mobility of nodes.There are many routing protocols that are being used currently in MANET.They can be broadly classified as proactive, reactive and hybrid routing . In the proactive routing protocols, every node maintains one or more tables containing the routes to every other nodes in network. Some of the existing proactive routing algorithms are DSDV , Optimized link state routing (OLSR) . In reactive routing protocols routes are developed only when it is needed, which include the Dynamic Source Routing (DSR), Ad-hoc On Demand Distance Vector (AODV) .
Section II presents the related work & motivation required for this research. Section III provides an overview of the protocols evaluated in this paper. Section IV enumerates the different parameters used in the simulation. We describe the performance metrics used in our study and simulation based results in Section V. Conclusion and future work is presented in Section VI.
II. Related Work and Motivation
Many on-demand routing protocols have been proposed and are being used to facilitate effective and efficient routing in MANETs. However, these protocols have revealed a wide range of performance results under different network conditions and parameters. Therefore, it is quite difficult to determine which protocols may perform best under a number of different network scenarios, such as increasing pause time and node density nevertheless each protocol perform its best in some specific scenario. One of the several issues associated with routing protocols is the node mobility and node density. Due to these mobile nodes topology of the network changes unpredictably which in turn largely affects the routes between communicating and intermediate nodes and hence it will make a significant influence on the performance of the routing protocols. It causes frequent path breaks, packet collisions, transient loops etc. Node density is another subject associated with routing in MANETs. A routing protocol should be able to scale well (i.e. perform efficiently) in a network with large number of nodes. This requires minimization of control overhead and adaptation of the routing protocol to network size. Researchers have conducted many simulations comparing the performance of these routing protocols under various conditions and constraints [3]-[5]. Now, one question that arises is how node density and node mobility , together affects the performance of routing protocols being studied. The motivation of this research is consequent from this question. This paper has made an attempt to address this issue by studying the effect of mobility (using the pause time parameter) and node density(by varying number of nodes) through simulation of two on-demand routing protocols AODV [6] and DSR [7] using NS 2.34 simulator. The survey will definitely help designers choose the most adequate routing protocol for specific criteria while designing MANETs wherein mobility and scalability are the most important criteria under consideration.
III. Overview of Routing Protocols
A. Ad-Hoc on-Demand Distance Vector (AODV)
The Ad-hoc On-Demand Distance Vector (AODV) algorithm enables dynamic, self-starting, multihop routing between participating mobile nodes wishing to establish and maintain an ad-hoc network. AODV allows mobile nodes to obtain routes quickly for new destinations, and does not require nodes to maintain routes to destinations that are not in active communication [6]. When any source node wants to send a packet to a destination, it broadcasts a route request (RREQ) packet. Each node in turn forwards RREQ packet until the destination node itself is reached or the node which has a fresh route to destination is reached. A route reply (RREP) packet is then unicasted back to source node through established reverse route. Nodes monitor the link status of next hops in active routes. Whenever a link break in an active route is found, a route error (RERR) message is used to notify other nodes that the link is lost. The AODV routing protocol is a combination of DSDV and DSR algorithm. It uses the periodic broadcasting and sequence numbering procedure of DSDV and a route discovery procedure of DSR. However, there are two important differences between DSR and AODV. The most distinguishing feature is that in DSR the routing packet carries full routing information, whereas in AODV the packets carry the destination address only. This causes AODV to have potentially less routing overheads than DSR. The other difference is that the route reply packets in AODV carry the destination IP address and the sequence number whereas in DSR it contains the address of every node along the route. The advantage of AODV is that it is adaptable to high mobility networks. However due to route discovery latency AODV is not suitable for large size networks.
B. Dynamic Source Routing (DSR)
Dynamic source routing protocol (DSR) [7] is an on-demand routing protocol that uses "source routing". It is composed of the two main mechanisms of "Route Discovery" and "Route Maintenance". In DSR, each node uses cache technology to maintain routes of all the nodes. When a source node wants to send a packet, it first checks its route cache, in case there exists a route to the destination the source node sends the packet along that route. Otherwise, the source node initiates a route discovery operation by broadcasting route request packets (RREQ). Upon receiving a route request packet, a node checks its route cache, if the node doesn’t have route for the requested destination, it appends its own address to the route record field of the route request packet and the request packet is forwarded to its neighbors. If the RREQ packet reaches the destination or an intermediate node that has routes to the destination, a route reply packet (RREP) is generated which comprises addresses of nodes that have been traversed by the route request packet. Otherwise, the route reply packet comprises the addresses of nodes the route request packet has traversed along with the route in the intermediate node’s route cache. Unlike AODV DSR does not require broadcasting of periodic packets of any kind at any layer within the network. For instance, DSR does not use any periodic routing table advertisement, link status sensing. This reduces the amount of overhead in transmitting broadcasts significantly when the network is stable. As nodes begin to move more or as topology pattern changes that are not affecting routes currently in use are ignored and do not trigger reaction from the protocol. An advantage of DSR is that nodes can store multiple routes in their route cache. Multiple routes are also advantageous for load balancing purposes. It is also very beneficial in network with low mobility. Since the routes stored in the route cache will be valid longer.
IV. Simulation and Performance Metrics
In this study, we have analyzed the performance of two on-demand routing protocols AODV and DSR, by varying pause time and number of nodes. The simulations have been performed using NS2.34.Mobility is an important issue affecting the performance and scalability of MANETs. Frequent link breakage due to the mobility of nodesbounds the scalability of mobile ad-hoc networks. To examine the consequence of node mobility and density, two different protocols are simulated using NS2.34 by varying pause time and number of nodes, with the following parameters (Table I).
To evaluate the efficiency of a routing protocol in MANET, we have evaluated the packet delivery ratio (PDR) metrics for both protocols. Packet delivery ratio is calculated by dividing the number of packets received by the destination through the number of packets originated by the application layer of the source. It specifies the Packet loss rate, which limits the maximum throughput of the network. The better the delivery ratio, the more complete and correct is the routing protocol.
TABLE I: SIMULATION PARAMETERS
PARAMETERS / VALUEProtocols / AODV, DSR
Simulation area / 700 x 700
Simulation time / 300 sec
No. of nodes / 30, 50 and 70 nodes
Mobility / 0-10 mps
Traffic type / CBR
Packet size / 512kb
Pause time / 0s, 30s, 60s, 90s, 120s, 150s, 200s, 300s
V. Simulation Results
We have examined the performance of routing protocols AODV & DSR by varying pause time from 0s to 300s for a MANET ranging from 30, 50 & 70 nodes. When the pause time is 0sec it denotes that nodes are continuously moving and hence the number of link changes are very high. It decreases with an increase in pause time and converges to 0 when the pause time reaches 300s which denotes a stable network, at this stage the routing table needs minimal updates. The results obtained from simulations are presented in fig. 1 to 3. The effect of variation in pause time and number of nodes on throughput of AODV & DSR is shown in fig. 1 & 2 respectively. Fig. 3 depicts the relative performance of both the protocols. As the network size grows, AODV’s PDR has declined because of increase in control overhead. However, there is an increase in the PDR when network size grows from 50 to 70 nodes. This is because of the increase in the number of neighboring nodes, any route reply will reach the source node faster, resulting in less control overhead. It is clear that AODV works well with high mobility networks and is quite unsuitable for low mobility networks where it's PDR is very low.
Figure 1. Effect of varying pause time and number of nodes on PDR of AODV for mobile nodes
Figure 2. Effectof varying pause time and number of nodes on PDR of DSR for mobile nodes
Fig. 2 reveals that DSR has higher PDR as compared to AODV for all network sizes and pause times. This is because of the use of route cache which thereby reduces the route discovery control overhead . AODV has more control overhead due to the requirement for issuing many route replies for a single route request in comparison with a single route reply required in the case of DSR. It is also clear from the results that DSR outperforms AODV for large number of nodes.
Relative performance of both the protocols is illustrated in fig. 3. On-demand protocols (DSR and AODV) drop a considerable number of packets during the route discovery phase, as route acquisition takes time proportional to the distance between the source and destination. AODV has a slightly lower packet delivery performance than DSR because of higher drop rates. AODV and DSR perform well at high mobility because both allow packets to stay in the send buffer for 30 seconds for route discovery and once the route is discovered, on that route data packets are sent to be delivered at the destination. Using AODV maximum PDR of 91.49% is obtained while with DSR maximum PDR of 99.8% is achieved . As mobility increases AODV performs better as it adopts hop-by-hop routing. DSR performs better at lower and moderate traffic load as it uses source routing.
Figure 3. Relative performance of AODV & DSR in terms of PDR
VI. Conclusion
In this paper, a simulation study was carried out to evaluate the performance of two most prominent on-demand routing protocols AODV & DSR, by creating a scenario of 30, 50 & 70 nodes and by varying the pause time. These protocols were compared based on packet delivery ratio (PDR) metrics. Results show that DSR achieves higher PDR than AODV with a variation in pause time and network size,but for higher mobility AODV performs better. The studies of the performance of AODV & DSR protocol hopefully can result in the development of a new optimal enhanced version of these protocols which can maximize the routing performance and overcome the limitation of the existing conventional protocols.
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