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Mobile Ad Hoc Networks
Posted Date: 08 May 2008 Resource Type: Articles/Knowledge Sharing Category: Computer & Technology
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Posted By: durga Member Level: Silver
Rating:  Points: 3
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Modeling, Simulation and Performance Evaluation of
Mobile Ad Hoc Networks
Abstract
A mobile ad hoc network (MANET) is a collection of wireless mobile nodes dynamically forming a temporary network without the use of any existing network infrastructure or centralized administration. It is a multi-hop wireless network, which consists of number of mobile nodes. Due to a dynamic nature of ad hoc networks, traditional fixed network routing protocols are not viable and therefore several proposals for routing protocols are proposed through various RFCs. Ad hoc mobile networks have found many applications in various fields like military, emergency, conferencing and sensor networks.
In this paper, we have tried to simulate three different ad hoc routing protocols for ad hoc mobile networks, compared their performance based on a number of factors such as average routing overhead, packet delivery ratio and end-to-end delay using ns-2 simulator. The simulations are later on extended to different military scenarios for analyzing routing protocol performances.
1. INTRODUCTION
Wireless networks can generally be classified as wireless fixed networks and wireless ad-hoc networks. MANETs are based on the idea of establishing a network without taking any support from a centralized structure. By nature these types of networks are suitable for situations where either no fixed infrastructure exists or deploying network is not possible. Communication in mobile ad-hoc networks is normally achieved through other mobile devices in the network. The multi-hop support in ad-hoc networks makes it possible to communicate between nodes outside direct radio range of one another. It's also the main difference between mobile ad-hoc networks and wireless LANs. Ad hoc mobile networks have found many applications in various fields like military, emergency, conferencing and sensor networks. Each of these application areas has their specific requirements for routing protocols.
The initial portion of this paper gives a brief introduction to Mobile Adhoc Networks, routing protocols and modeling concepts in MANETs and then it discusses the simulation results. Apart from this, an endeavor has been made to simulate Mobile Ad Hoc Network for performance evaluation in real military scenario by taking various examples from military environment where MANET can effectively be used.
2. MOBILE AD HOC NETWORK
Mobile adhoc networks are emerging as the next generation of networks and defined as a collection of mobile nodes forming a temporary ( spontaneous ) network without the aid of any centralized administration or standard support services.
Mobile ad hoc network is usually thought of as a network with nodes that are relatively mobile compared to a wired network. Hence the topology of the network is much more dynamic and the changes are often unpredictable. This fact creates many challenging research issues in routing protocols which have to work under various constraints like bandwidth, battery power and latency etc. The routing protocols used in ordinary wired networks are not well suited for this kind of dynamic environment.
3. ROUTING IN MANET
Due to the dynamic nature of ad hoc networks, traditional fixed network routing protocols are not viable and therefore several proposals for routing protocols are proposed through various RFCs. Some of the popular MANET routing protocols are Destination-Sequenced Distance-Vector (DSDV), Dynamic Source Routing (DSR) and Ad Hoc On-Demand Distance Vector Routing (AODV). This section gives a brief introduction to these protocols.
3.1 Destination-Sequenced Distance-Vector
The Destination Sequenced Distance Vector Protocol (DSDV) is a proactive, distance vector protocol, which uses the Bellmann-Ford algorithm [1,2]. Compared to RIP one more attribute is added to the routing table i.e. sequence number as new attribute that guarantees loop-freedom. It makes it possible for the mobile nodes to distinguish stale routes from new ones and that is how it prevents loops. Every mobile station maintains a routing table that lists all available destinations, the number of hops to reach the destination and the sequence number assigned by the destination node. The stations periodically transmit their routing tables to their immediate neighbors. A station also transmits its routing table if a significant change has occurred in its table from the last update sent. So, the update is both time-driven and event-driven.
3.2 Dynamic Source Routing
The key distinguishing feature of DSR is the use of source routing [3,4,5]. That is, the sender knows the complete hop-by-hop route to the destination. These routes are stored in a route cache. The data packets carry the source route in the packet header. When a node in the ad hoc network attempts to send a data packet to a destination for which it does not already know the route, it uses a route discovery process to dynamically determine such a route. Route discovery works by flooding the network with route request (RREQ) packets. Each node receiving an RREQ rebroadcasts it, unless it is the destination or it has a route to the destination in its route cache. Such a node replies to the RREQ with a route reply (RREP) packet that is routed back to the original source. RREQ and RREP packets are also source routed. The RREP routes itself back to the source by traversing this path backward. If any link on a source route is broken, the source node is notified using a route error (RERR) packet.
3.3 Ad Hoc On-Demand Distance Vector Routing
The Ad-hoc On demand Distance Vector routing protocol (AODV) [6,7] joins mechanisms of DSR and DSDV. The periodic beacons, hop-by-hop routing and sequence numbers (guarantee of loop-freedom) of DSDV and the pure on-demand mechanism of Route Discovery and Route Maintenance from DSR are combined. However, AODV adopts a very different mechanism to maintain routing information. Without source routing, AODV relies on routing table entries to propagate an RREP back to the source and, subsequently, to route data packets to the destination. AODV uses sequence numbers maintained at each destination to determine freshness of routing information and to prevent routing loops. All routing packets carry these sequence numbers.
In contrast to DSR, RERR packets in AODV are intended to inform all sources using a link when a failure occurs. Route error propagation in AODV can be visualized conceptually as a tree whose root is the node at the point of failure and all sources using the failed link as the leaves.
4. MODELING OF MANET
The network simulator NS supports various models for simulating mobile adhoc networks. This section describes the radio propagation and random waypoint mobility model used in the simulation.
4.1 Radio Propagation Model
The “free space” model assumes exactly one path between the transmitter and the receiver. The path must be clear from obstacles and is given as:
(1)
Where:
Pr is the received signal power (in W), Pt is the transmitted signal power, Gr and Gt are the gains of the receiving and the transmitting antennas respectively, ? is the wave length, L is the system loss, and d is the distance between the transmitter and the receiver.
According to [12], a single direct path between the communicating partners exists seldom at larger distances. The “two-ray ground” model considers both the clear path and the ground reflected path and given as: -
(2)
In addition to the parameters of the free space model, the equation contains hr and ht, which are the heights of receiving and the transmitting antennas respectively. Similar to the free space model, the model neglects obstacles of the propagation environment. Figure 1. Below shows the two-ray ground reflection model.
Figure 1:Two ray ground reflection
Where,
Tx - Transmitter
Rx - Receiver
However, the two-ray ground model is too optimistic for the short transmitter-receiver separation distances. Hence, in most applications, the two models are combined. The free space model is used at small distances, while the two-ray ground model is used at larger distances.
The distance at which both models give identical results
(3)
Where is the crossover distance and for the distance less than this, free space model defined by equation (1) should be used.
For the simulation model, the transmitter antenna height and receiver antenna height have been taken as 1.5 meter respectively from the ground level assuming that a person carrying his mobile device on his lap or listening to his ear and considering average Indian human height.
The wavelength,
? = C / fc
Here,
C = 3x m/s
fc = 2.4 GHz ( it is the frequency of wireless network as defined by 801.11b standard )
Hence,
? = 3x / 2.4 x
= 0.125 meter
We know,
= 226.188 meter
Hence, two-ray ground reflection model, as radio propagation model has been used in the simulation because transmission range between two nodes is 250 meter, which is more than 226 meter as calculated by above formula.
4.2 Random Waypoint Mobility Model
Random waypoint mobility is a model that includes pause times between changes in destination and speed [12]. Once this time expires, it chooses a random destination in the simulation area as well as a speed by which it travels. Mobile nodes in this model tend to choose a destination near the center of the simulation area, or a destination that requires travel through the center of the simulation area. Random Waypoint scenarios are characterized by a pause time value that affects how often nodes move during the scenario, which in turn affects the amount of topology change.
Figure 2: Random waypoint model
5. NETWORK SIMULATOR
The entire simulations were carried out using ns 2.28 network simulator which is a discrete event driven simulator developed at UC Berkeley [8] as a part of the VINT project. The goal of ns2 is to support research and education in networking. It is suitable for designing new protocols, comparing different protocols and traffic evaluations. NS2 is developed as a collaborative environment. It is distributed as open source software. A large number of institutes and researchers use, maintain and develop ns2. NS2 Versions are available for FreeBSD, Linux, Solaris, Windows and Mac OS X.
5.1 Structure of ns2
NS2 is built using object oriented language C++ and OTcl (object oriented variant of Tool Command Language). NS2 interprets the simulation scripts written in OTcl. The user writes his simulation as an OTcl script. Some parts of ns2 are written in C++ for efficiency reasons. The data path (written in C++) is separated from the control path (written in OTcl). Data path object are compiled and then made available to the OTcl interpreter through an OTcl linkage. Results obtained by ns 2 (trace files) have to be processed further by other tools like Network Animator (NAM), perl, awk script etc.
6. SIMULATION METHODOLOGY
NS-2 simulator [8] has been used in this simulation since it supports simulating an ad-hoc mobile environment. Table 1 below lists the various parameters that were used in the simulation.
6.1 Traffic Generation models
Traffic-scenario generator script ‘cbrgen.tcl’ is used to create CBR traffic connections between wireless mobile nodes. During the course of the simulation 20 connections were setup between the nodes in the network with the traffic rate of 4 packets per seconds where each packet size was 512 bytes.
6.2 Mobility Generation models
The movement scenario files used for each simulation are characterized by a pause time. The simulation carried out with movement patterns generated for different pause times: 0, 20, 40 and 100 seconds. A pause time of 0 seconds corresponds to continuous motion, and a pause time of 100 corresponds to almost no motion. A set of five movement scenario files were used for each value of pause time to improve the accuracy of the results and to smooth out spikes due to extremely favorable or unfavorable movement scenarios. The ‘setdest’ program of NS-2 simulators used which generates node-movement files using the ‘random waypoint algorithm’.
Table 1
Simulation Parameters
Ser No Parameters Value
1 Nodes 50
2 Area 500*500m2
3 Simulation Time 100 sec
4 Max Speed 20 m/s
5 Pause Time (sec) 0,20,40,100
6 Traffic Source CBR
7 Packet Size 512 Bytes
8 Packet Send Rate 4 Packets/s
9 Max Connections 20
7. PERFORMANCE EVALUATION METRICS
The metrics have been chosen in order to evaluate the routing protocols for Quality-of-Service in terms of short packet delays, low percentage of packet loss and low routing load. The main attention was given to evaluate the routing layer performances. The following three metrics capture the most basic overall performance of Routing protocols studied in this paper: -
7.1 Average End-to-End delay of data packets (E2E Delay)
The end-to-end delay is defined as time between the point in time the source want to send a packet and the moment the packet reaches it destination. It includes all possible delays caused by buffering during route discovery latency, queuing at the interface queue, retransmission delays at the MAC, and propagation and transfer times
end to end delay = T(destination receives packet) - T(source wants to sent packet)
7.2 Normalized Routing Load (NRL)
Routing overhead is the number of routing packets transmitted per data packet delivered at the destination. Each hop-wise transmission of a routing packet is counted as one transmission.
Normalized Routing load =
? Number of Routing Packets sent
? Number of Data Packets Received
7.3 Packet Delivery Fraction (PDF)
The packet delivery ratio in this simulation is defined as the ratio between the number of packets sent by constant bit rate sources and the number of packets received by the CBR sink at destination.
Packet Delivery Fraction (PDF) =
? CBR packets received by CBR sinks
? CBR packets sent by CBR sources
8. PERFORMANCE COMPARISON
8.1 Results of Simulation to analyse the effect of mobility
To analyse the effect of mobility, pause time was varied from 0 seconds (high mobility) to 100 seconds (no mobility).
(a)
(b)
b)
(c)
Figure 3: Simulation results to analyse the effect of mobility
8.2 Results of Simulation to analyse the effect of Traffic Load
To study the effect of traffic load on the network, numbers of connections were varied as 10,20,25 and 30 connections. The network was simulated for high mobility scenario keeping the pause time 0 seconds.
(a)
(b)
(c)
Figure 4: Simulation results to analyse the effect of Traffic Load
9 ANALYSIS OF SIMULATION RESULTS
The simulation results bring out some important characteristic differences among these routing protocols. The presence of high mobility implies frequent link failures and each routing protocol reacts differently during link failures. The different basic working mechanism of these protocols leads to the differences in the performance.
DSDV fails to converge at lower pause times hence performance of the protocol decreases as mobility increases. At higher rates of mobility (lower pause times), DSDV performs poorly, dropping to a 70% packet delivery ratio. Nearly all of the dropped packets are lost because a stale routing table entry directed them to be forwarded over a broken link. As DSDV maintains only one route per destination hence consequently, each packet that the MAC layer is unable to deliver is dropped since there are no alternate routes. For DSR and AODV, packet delivery ratio is independent of offered traffic load, with both protocols delivering between 95% and 100% of the packets in all cases. The reason for having better packet delivery fraction of DSR and AODV is that both allow packets to stay in the send buffer for 30 seconds for route discovery and once the route is discovered, data packets are sent on that route to be delivered at the destination.
We see that DSDV has least end-to-end delay as compared to DSR, AODV. This is because DSR and AODV keep the packets in send buffer for longer duration for route discovery for the packets to be delivered at the destination hence the delay. But if we see DSR and AODV deliver more packets at the destination as compared to DSDV because these two protocols try to provide some sort of guarantee for the packets to be delivered at the destination by compromising at the delay. Where as DSDV, try to drop the packets, if it is not possible to be delivered hence the lesser delay and lesser packet delivery ratio.
Since DSDV uses the table-driven approach of maintaining routing information, it is not as adaptive to the route changes that occur during high mobility. DSDV sends periodic routing updates at every 15 seconds in the network. This periodic broadcast could be ‘full dump’ or just an update of the routing information. These periodic broadcasts increase routing load in the network. Hence for DSDV we will have more routing overhead irrespective of mobility and traffic load and this increases more if we simulate a network for longer duration as DSDV sends periodic updates at regular intervals. Secondly as the number of nodes increases in the network, we will have more nodes to broadcast the periodic routing information, which will increase the overheads. In contrast, the lazy approach used by the on-demand protocols, AODV and DSR to build the routing information as and when they are created make them more adaptive and result in better performance (high packet delivery fraction and lower average end-to-end packet delays). The reasons for AODV to have more routing overheads as compared to DSR is frequent ‘REQUEST’ packets initiated by AODV where as DSR limits the scope of broadcasting ‘REQUEST’ packets by caching of routes hence less routing load.
In summary, it can be said that for robust scenario where mobility is high, nodes are dense, area is large, the amount of traffic is more and network is for longer period, AODV performs better among all studied routing protocols. For the normal situations where a network is of general nature with moderate traffic and moderate mobility DSR would be the right choice as it delivers more packets at the destination with lowest routing overheads. For low mobility and less number of nodes, DSDV can also be used as it gives less end to end delay but the problem with DSDV is that, it delivers less number of data packets as compared to others two protocols. Hence it is advisable that if it is possible to avoid DSDV, it should be avoided for the mobile ad hoc networks.
10. MANET IN DEFENCE SE|CENERIO
Ad hoc mobile networks have found many applications in various civil sector like emergency, conferencing and sensor networks etc. MANET can also be effectively used in various military operations like Patrolling, Ambush and Raid etc. The meaning of these terms is explained in the following section.
10.1 Patrolling
Patrolling is an operation in which army unit operates as an independent entity. The main aim of the patrolling is to acquire timely, accurate and detailed information. In Counter Insurgency operations, patrol is significant in order to dominate area and combat with insurgents.
10.2 Ambush
Ambush is also another area where MANET can be very effectively used. In ambush, troops move and deploy themselves to destroy enemy or terrorists.
10.3 Raid
Raid is another classic example of mobile ad hoc network. Here troops work in groups and they need to communicate with each other. They operate in enemy territory hence high power radio will not be advisable to use due to various reasons.
11. PERFORMANCE EVALUATION OF MILITARY SCENARIO
After getting the results for various scenarios, mobility, traffic rates, packets size and simulation time we have extended the simulations for all the three military scenarios discussed above.
From the simulations it was observed that AODV and DSR perform better in most of the situations. However none of these routing protocols could deliver 100% packets except in few odd cases. Hence, it is recommended that for such scenarios, DSR or AODV protocols can be used as per the situations till new protocol or modifications to existing routing protocols are made. An effort should be made to modify or write new routing protocol, which can work in all kinds of scenarios
12. CONCLUSION
As it can be seen, there are large number of different kinds of routing protocols. The use of a particular routing protocol in Mobile Ad hoc Network depends upon factors like size of the network, load, mobility requirements etc. This paper compared the performance of DSDV, AODV and DSR routing protocols for mobile ad hoc networks using ns-2 simulator. The simulation results shows that both AODV and DSR generally perform better under high mobility situations than DSDV.
Based on the simulation results different military network scenarios were evaluated for the performance and QoS. The analysis of these results suggests that there is a requirement of better routing protocol.
REFERENCES
[1] Charles E. Perkins and Pravin Bhagwat, “Highly dynamic Destination-Sequenced Distance-Vector routing (DSDV) for mobile computers”. In Proceedings of the SIGCOMM ’94 Conference on Communications Architectures, Protocols and Applications, pages 234–244, August 1994.
[2] Elizabeth Royer and C-K Toh, “A Review of Current Routing Protocols for Ad-Hoc Mobile Wireless Networks”, IEEE Personal Communications Magazine, April 1999, pp. 46-55.
[3] S. Desilva and S. Das. Experimental evaluation of a wireless ad hoc network. In Proceedings of the 9th International Conference. On Computer Communications and Networks, 2000.
[4] D. Johnson, D. Maltz, Y. C.Hu, and J. Jetcheva. The dynamic source routing protocol for mobile ad hoc networks (DSR). IETF Internet-Draft, draft-ietf-manet-dsr-07.txt, Feb. 2002.
[5] D. B.Johnson and D. A.Maltz. Dynamic source routing in ad hoc wireless networks. In T. Imielinski and H. Korth, editors, Mobile Computing, volume 353. Kluwer Academic Publishers, 1996. , chapter 5, pages 153–181
[6] C. E. Perkins, E. M. Royer, and S. R. Das. Ad hoc on demand distance vector (AODV) routing. IETF Internet-Draft, draft-ietf-manet-aodv-10.txt, works in progress, Jan. 2002.
[7] Charles Perkins. Ad Hoc On Demand Distance Vector (AODV) routing. Internet-Draft, draft-ietf-manet-aodv-00.txt, November 1997. Work in progress.
[8] The network simulator - ns-2. http://www.isi.edu/nsnam/ns/2002.
[9] M. Scott Corson and Joe Macker. Mobile Ad hoc Networking (MANET): Routing Protocol Performance Issues and Evaluation Considerations. RFC 2501, January 1999.
[10] K. Fall, K. Varadhan, and the VINT project. (2003, December) The ns manual Page 164-168. [Online]. Available:http://www.isi.edu/nsnam/ns/ns-documentation.html
[11] Freely adapted from a portion of Jean-Paul M. G. Linmartz's
Wireless Communication, The Interactive Multimedia CD-ROM,
Baltzer Science Publishers, P.O.Box 37208, 1030 AE Amsterdam,
ISSN 1383 4231, Vol. 1 (1996), No.1
[12] T. Camp, J. Boleng, and V. Davies, “A survey of mobility models for ad hoc network research,” Wireless Communications & Mobile Computing (WCMC): Special issue on Mobile Ad Hoc Networking: Research, Trends and Applications, vol. 2, no. 5, pp. 483–502, 2002.
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