Computing For Nation Development, February 25 26, 2010 Bharati Vidyapeeth s Institute of Computer Applications and Management, New Delhi Effectiveness of DSDV Protocol Under Hidden Node Environment Ruchi Garg 1, Himanshu Sharma 2 and Sumit Kumar 3 1,2 Maharishi Markandeshwar University Ambala, Haryana, India 3 Technology Education and Research Institute Kurukshetra, Haryana, India E-Mail: 1 garg.330@gmail.com, 2 himanshu.zte@gmail.com, 3 go2_sumit@yahoo.com ABSTRACT IEEE 802.11 is a standard for mobile ad hoc networks (MANET), implemented with various different protocols. Destination-Sequenced Distance-Vector Routing (DSDV) is one of the several protocols of IEEE 802.11 intended to provide various Quality of Service (QOS) parameters under acceptable range. To avoid the collision and interference the MAC protocol has only two solutions, one, to sense the physical carrier and second, to use the RTS/CTS handshake mechanism. But with the help of these methods DSDV is not free from the problem of hidden nodes like other several protocols. Under the hidden node environment, performance of DSDV depends upon various factors. The position of receiver and sender among the other nodes is very crucial and it affects the performance. Under the various situations the DSDV is simulated with the help of NS2 and the outcomes are discussed. 2. HIDDEN NODE OVERVIEW The nodes fall in the interference range of a receiver are called hidden nodes. From Fig. 1, there are five nodes; transmission range of n0 is shown by a bold solid circle, where just solid circles are representing the transmission range of n1 and n2, similarly dotted circles are representing the range of n4 and n3. The double dot dashed circle is representing the interference range of node n0. Here considerable part is the interference range, as interference range is more than the twice of the transmission range, so as the area depicted for the interference range is quite large in the Fig. 1. Now, n4 and n3 falls in the interference range of n0, so n4 and n3 are the hidden nodes for n0 when n0 is in the receiving mode. KEYWORDS DSDV; MANET; RTS/CTS; NS2; hidden nodes; MAC; protocol; IEEE 802.11; interference. 1. INTRODUCTION In ad hoc networks, dynamic topology and dynamic behavior of the nodes imposes an extensive contention for the shared wireless medium of 802.11. Simultaneous transmission of the packets in a network over the shared medium may cause a very poor performance. So if the virtual carrier sensing mechanism is not implemented properly the performance of any protocol may not be acceptable. If a protocol wants to transmit a packet from one node to another node successfully then there should be no interference of any other packet with this packet throughout the transmission. The interference caused by the other nodes can be avoided up to certain extent with the available methods of- RTS/CTS handshake and physical carrier sensing techniques. The power needed for interrupting a packet is much lower than the power needed by a packet for its successful delivery. The physical medium sensing prevents the interference up to a considerable level but it is not very effective all the time, as the carrier sensing is performed at the sender s end and has no idea about the exact situation at the receiver s end. Similarly RTS/CTS handshake is unable to prevent the delivery of packet from the interferences in all the situations. Furthermore the broadcast and multicast frames are not guided by any acknowledgement in RTS/CTS mechanism. Figure. 1. Hidden nodes (n3, n4) for the node n0. Interference range has a drastic effect on the capacity of the bandwidth. Consider the Fig. 1, when n0 wants to receive a packet from n1, at the same time n0, n2, n3 and n4 should not transmit any packet in the same medium. So, here, the capacity is reduced to 1/5 of the channel bandwidth. Now, again consider the Fig. 1, if n3 and n4 are outside the interference range of n0, the capacity is reduced to 1/3 of the channel bandwidth. In this paper, DSDV protocol is used for the analysis of performance of ad hoc network. DSDV has self- configuring and self- healing properties. It supports high mobility of nodes. For the simulation, testbed is using IEEE 802.11for physical layer, MAC layer and logical link layer to accomplish the wireless communication among the several nodes. In rest of the paper, related work, DSDV protocol, simulation environment and results are discussed and followed by concluding remarks.
3. RELATED WORK Number of protocols have been studied and designed to provide an effective protocol for wireless mobile ad hoc networks. DSDV is a pro-active distance vector routing protocol, assumes that all links are symmetric. In [6], authors have discussed DSDV and comparisons of it with other routing methods. It shows that DSDV is suitable for creating ad hoc networks with small number of nodes. In a network, nodes may fall in gray area [5], result of various overlapped interference zones. The nodes which act as links in between receiver and transmitter and fall in the gray area is known as unstable link, the link which gives the fluctuating packet loss rate. The interference not only creates unstable links but create several other problems and some time show the unpredictable behaviour also. In mobile ad hoc networks, the topology used is mesh topology. But the intermediate links in between the receivers and transmitters, within a network make a different topology each time. These topological changes create different patterns of overlapped interference zones and thus give a varying throughput. In contrast to the related work, this paper is focused primarily with the hidden nodes problem and what may be the possible outcomes of DSDV under this environment. The emphasis is on the different topological arrangements within the ad hoc networks and the behavior of hidden nodes when the nodes are mobile under certain circumstances. 4. IEEE 802.11 RTS/CTS IEEE 802.11 uses carrier sensing and virtual carrier sensing to avoid collisions. each other s transmission [4]. But it is not possible in DSDV under the standard IEEE 802.11. 5. DSDV PROTOCOL OVERVIEW Destination-Sequenced Distance-Vector Routing (DSDV) is a table-driven routing scheme for ad hoc mobile networks based on the Bellman-Ford algorithm. It was developed by C. Perkins and P. Bhagwat in 1994. Each entry in the routing table contains a sequence number. The number is generated by the destination, and the emitter needs to send out the next update with this number. Routing information is distributed between nodes by sending full dumps infrequently and smaller incremental updates more frequently. In DSDV the low packet delivery is due to the fact that, it uses stale routes in case of broken links. In DSDV the existence of stale route does not imply that there is no valid route to the destination. The packets can be forwarded through other neighbors who may have routes to the destination. When immediate links from the host say A to the destination say T breaks, the proposed protocol creates a temporary link through a neighbor which has a valid route to the desired destination. The temporary link is created by sending one-hop ROUTE- REQUEST and ROUTE-ACK messages. The host A upon finding the next hop broken link broadcasts a one-hop ROUTE- REQUEST packet to all its neighbors. In turn, the neighbors returns the ROUTE-ACK if it has a valid route to the destination and the host A is not the next hop on the route from the neighbor to the destination. Sourc e Node RT Destinatio n Node update RTS- CT DAT update CTS- AC Figure. 2. RTS/CTS Mechanism If a source node receives a CTS packet in response to its RTS packet, then the source transmits a data packet. If RTS/CTS packets are received by other nodes than the transmitter or receiver, these nodes defer these packets to avoid collisions and update their network allocation vector. Even with the availability of virtual sensing mechanism, the IEEE 802.11 is still suffering from the adverse effects of hidden nodes. The RTS/CTS can prevent a network from the hidden node problem if in a wireless LAN all nodes can sense Figure. 3. Creation of provisional route in Node A Each entry in the routing table has an additional entry for route update time. This update time is embedded in the ROUTE- ACK packet and is used in selecting a temporary route. In case of receiving multiple ROUTE-ACK with the same number of minimum hops, ad hoc host S chooses that route which has the latest update time. The Fig. 3 shows how host A creates a provisional route to the destination T, when the intermediate link from A to B breaks. Host A suspends sending packets
Effectiveness of DSDV Protocol Under Hidden Node Environment (Fig. 3(a)). After which it broadcasts ROUTE-REQUEST packets to its immediate one hop neighbors. The Ad Hoc hosts C, E, and G responds with ROUTE ACK packets along with hop count and the route update time to Ad Hoc host A (Fig. 3 (b)). Neighbor No. of Hops Via Node Update Time C 2 H 1668 E 2 F 1989 G 3 E 1067 I 3 A 800 Table 1: Route Update At Host A Table I shows the snapshot of the routing information received by Ad Hoc host A. From the table it can be seen that, Ad Hoc Host C and E have the same value for hop count metric, but the routing update time for E is greater than that of C, meaning the path through E is updated more recently. Therefore Host A resumes sending packets to the destination T (Fig. 3(c)). Later on whenever any Ad Hoc host moves in the range of the host A then the routing table of host A gets updated by the regular DSDV routing process. Then the updated route will be taken for forwarding the packets from the host A to the destination T. 6. TEST ENVIRONMENT In this paper, NS2 simulator is used to test the performance of DSDV protocol under various conditions. The version of NS2 used is 2.31. The common parameters are tabulated below in table II. For the test environment, certain assumptions have been made- All nodes have same transmission power. All nodes have a single wireless transceiver. Wireless antennas with all the nodes are omnidirectional. All nodes are working on one common channel. receiver (n0) is fixed in a grip of interferences due to some hidden nodes. transmitter n1 n0 d receiver Interference area not covered by RTS/CTS Figure.4. Arrangement of nodes In the arrangement of nodes of Fig. 4, when the distance d between receiver and transmitter exceeds more than 0.56 of R tx (where R tx is transmission range of transmitter), the effectiveness of RTS/CTS falls sharply [1] in IEEE 802.11. A simulation has been carried out for the Fig. 4. The results of simulation are shown in Fig. 5 which proves that the deterioration is due to the distance and the hidden nodes is considerable. Antenna Type Transmission Range Topology IEEE standard Interference Range Test Area Traffic Type Packet Size Propagation Model Routing Protocol Table 2: Test Parameters Omni Directional 250 m Mesh IEEE 802.11 550 m 800 x 800 m CBR (UDP) 512 bytes Two Ray Ground DSDV 7. RESULTS AND ANALYSIS A number of tests have been conducted with the help of NS2 to understand the behaviour of DV under hidden node environment and to understand the possible reasons. In the first test case, as shown in Fig. 4, sender (n1) is mobile while Figure.5.Throughput vs. distance between n0 and n1 Another experiment is performed to understand further, the degradation of the performance in DSDV due to hidden nodes. Now two pairs of a transmitter and a receiver exists as shown in Fig. 6. In the Fig. 6, node n0 is a sender for n1 and n3 is a sender for n2. By this arrangement, n3 is a hidden node for n1; similarly n0 is a hidden node for n2. During the simulation, distance d between the two pairs are going to vary. A very strange behaviour is observed as shown in the Fig. 7. It shows the throughput at n1 due to n0 with the varying distance d. As per the expectations, as the distance d will increase, the reception
of packets at node n1 due to node n0 should not fall or it should increase even. But the graph in Fig. 7 shows that after a particular distance the curve falls and then after a certain gap it again achieves a remarkable height. n0 Figure. 6. Two pairs of transmitters and receivers This is contributed by the larger interference range and ineffectiveness of RTS/CTS for resolving the hidden nodes problems. When node n3 is out of the transmission range of the node n1, it cannot successfully receive the CTS packet of node n1. However, since it is still in the interference range of the node n1, transmission from the node n3 will interrupt any packet reception at the node n1. Only when node n2 and node n3 are all out of interference range of node n1, the two connections are fully separated from each other. Since the pairs are moving away from each other they will come out of reach of each other by any means so the number of packets received at node n1 achieves a remarkable strength. Figure. 7.Throughput vs. distance between n1 and n2 n1 n2 8. CONCLUSION In the IEEE 802.11 standard, the RTS/CTS mechanism is only optional. Thus, in practice, wireless networks are often exposed to the hidden node problem. As wireless networks proliferate, it is therefore vital to analyze and accurately quantify the impact d n3 of hidden nodes on network performance. Therefore number of tests have been simulated and studied for the protocol DSDV. A common problem observed is a performance degradation and unexpected behavior due to topological changes in network nodes. Similarly performance of DSDV also depends upon the carrier sensing techniques. RTS/CTS is one of the techniques to solve the problem of interference but the results are satisfactory within certain limitations only. In totality paper highlighted the inconsistent behavior of wireless network under hidden node environment with the help of DSDV protocol. Also it is vital to understand the behavior of current networks to help the design of future networks. 9. FUTURE SCOPE Wireless networks are still based on IEEE 802.11, which was not designed for wireless networks originally [4]. To improve the working of IEEE 802.11 for wireless networks, number of protocols are suggested, and studied so far. In this paper a protocol, DSDV is studied and discussed in a purview of a major problem faced by any wireless network i.e. hidden nodes. The discussion throws a light on this problem and shows the degradation of DSDV under various scenarios. Since the discussion is able to define a problem out of various problems with the possible reasons, so the discussion may further lead to the possible solutions of the discussed problem. 10. REFERENCES [1]. Kaixin Xu Gerla, M. Sang Bae, How effective is the IEEE 802.11 RTS/CTS handshake in ad hoc networks, In Global Telecommunications Conference, 2002. GLOBECOM '02. IEEE, pp. 72-76 vol.1, 17-21 Nov.2002. [2]. Pirzada, A.A. Wishart, R. Portmann, M., Congestion Aware Routing in Hybrid Wireless Mesh Networks, In 15th IEEE International Conference on Networks, 2007. ICON 2007., pp. 513-518, 19-21 Nov. 2007. [3]. http://www.isi.edu/nsnam/ns/tutorial/index.html. NS-2Simulator. [4]. Shugong Xu and Tarek Saadawi, Does the IEEE 802.11 MAC Protocol Work Well in Multihop Wireless Ad Hoc Networks?, IEEE Communications Magazine, pp. 130-137, June 2001. [5]. Fei Ye, Zhi Zhou and Zhisheng Niu, Exploit the capacity of unstable links in AODV-based ad hoc networks,in Asia-Pacific Coference on Communications, 2006 APCC -06, pp. 1-5, August 2006. [6]. Perkins, Charles E. and Bhagwat Pravin. Highly Dynamic Destination-Sequenced Distance-Vector Routing (DSDV) for Mobile Computers, in ACM SIGCOMM Computer Communication Review, Volume24, Issue 4, pp. 234 244, October 1994. Continued on Page No. 136
Effectiveness of DSDV Protocol Under Hidden Node Environment Continued from Page No. 132 [7]. Abdul hadi A. Rahman and Zuriati A. Zukarnain, Performance Comparison of AODV, DSDV and I- DSDV Routing Protocols in mobile Ad Hoc Networks, European Journal of Scientific Research, Vol.31 No.4 (2009), pp.566-576. [8]. C. E. Perkins & E. M. Royer, Ad-hoc On-Demand Distance Vector Routing, In 2 nd IEEE Workshop on Mobile Computing Systems and Applications, pp.90-100, Feburary 1999. [9]. Ur Rahman Khan, K. Reddy, A.V. Zaman, R.U. Reddy, K.A. Harsha, T.S., An Efficient DSDV Routing Protocol for Wireless Mobile Ad Hoc Networks and its Performance Comparison, in Computer Modeling and Simulation, 2008. EMS '08, 8-10 Sept. 2008, pp. 506-511.