Performance Comparison of Ad Hoc Routing Protocols over IEEE 82.11 DCF and TDMA MAC Layer Protocols Govind. P. Gupta Computer Science Department R.K.G.I.T, Ghaziabad (India) er_gpgupta@yahoo.com A. K. Pandey MCA Department R.K.G.I.T, Ghaziabad (India) rmrcindia@gmail.com Abstract The developments of ubiquitous mobile computing devices have fueled the need for dynamic reconfigurable networks. Mobile ad-hoc network (MANET) routing protocols facilitate the creation of such networks, without centralized administration. Over the past few years, many researchers have conducted numerous simulations for comparing the performance of these protocols under varying conditions and constraints. Most of them have not considered MAC Protocols, which will impact the relative performance of routing protocols considered in different network scenarios. In this paper, we present a detailed simulation study of the performance of the, and routing protocols when run over IEEE 82.11 DCF and TDMA MAC protocols. The performance of these ad hoc routing protocols are compared in terms of their control packet overhead, and amount of data delivered using ns-2 simulation environment. Keywords: Ad hoc Routing, MAC protocols 1. Introduction A mobile ad hoc network (MANET) is a collection of mobile nodes connected by wireless links to form a temporary network without the use of any existing network infrastructure or centralized administration. Examples of ad hoc networks include: disaster situations such as earthquake and flooding, where the rescue teams need to coordinate themselves without the availability of fixed networks; soldiers in a battlefield exchanging tactical information; entrepreneurs in a meeting sharing business information [4]. In order to enable communication between any two nodes in such a MANET, a routing protocol is employed. The abstract task of the routing protocol is to discover the topology to ensure that each node is able to acquire a recent view of the network topology in order to construct routes. The comparison of ad-hoc routing protocols is not a new topic. Broch et al [1] have compared four ad-hoc routing protocols (, DSR,, TORA) through a detailed packet-level simulation using ns-2 with respect to metrics such as Packet Delivery Ratio, routing overhead and path optimality. Johanson et al [2] have compared the performance of DSR, and by varying traffic load and node mobility. These and other performance analysis papers, however, only offer comparative studies of protocol performance among different protocols. Royer et al [11] have compared the performance of WRP, FSR and by varying node mobility. In this paper, we compare the performance of, and the on IEEE 82.11 DCF and TDMA MAC Layers protocols. is the one of the most popular on-demand protocols to date, while is a representative table-driven protocol for ad hoc networks environment. The comparison is made in terms of data delivery ratio and control overhead, using the ns-2 simulation environment [7]. The Ad hoc Networks MAC protocols IEEE 82.11 DCF and TDMA are selected for simulation. It is likely that the performance of the protocols will be best when run over IEEE 82.11DCF, due to its channel acquisition characteristics. However, the question is whether protocols degrade proportionately to each other when run over the other MAC layer protocols. To determine whether the selection of MAC protocol is a factor when comparing routing protocols, this paper explores the behavior of, and routing protocols when run over varying MAC protocols. This paper is organized as follows. Section 2 reviews the key features of the three routing protocols under study. Section 3 presents overview of MAC protocols. Section 4 describes the common simulation environment and implementation parameters for the protocols. Section 5 presents the results of the simulation study. Section 6 present conclusions. 2. Ad Hoc Network Routing Protocols Studied In this section, we briefly describe the key features of the, and protocols studied in our simulations 2.1 Destination Sequenced Distance Vector Routing The Destination-Sequenced Distance-Vector Routing protocol () described by Perkins et al [12] is a tabledriven algorithm based on the classical Bellman-Ford routing mechanism. The improvements made to the Bellman-Ford algorithm include freedom from loops in routing tables. Every mobile node in the network maintains a routing table in which all of the possible destinations within the network and the number of hops to each destination are recorded. Each entry is marked with a sequence number assigned by the destination node. The sequence numbers enable the mobile nodes to distinguish stale routes from new ones, thereby avoiding the formation of routing loops.
Routing table updates are periodically transmitted throughout the network in order to maintain table consistency. To help alleviate the potentially large amount of network traffic that such updates can generate, route updates can employ two possible types of packets. The first is known as a full dump. This type of packet carries all available routing information and can require multiple network protocol data units (NPDUs). During periods of occasional movement, these packets are transmitted infrequently. Smaller incremental packets are used to relay only that information which has changed since the last full dump. Each of these broadcasts should fit into a standardsize NPDU, thereby decreasing the amount of traffic generated. The mobile nodes maintain an additional table where they store the data sent in the incremental routing information packets. New route broadcasts contain the address of the destination, the number of hops to reach the destination, the sequence number of the information received regarding the destination, as well as a new sequence number unique to the broadcast. The route labeled with the most recent sequence number is always used. In the event that two updates have the same sequence number, the route with the smaller metric is used in order to optimize (shorten) the path. Mobiles also keep track of the settling time of routes, or the weighted average time that routes to a destination will fluctuate before the route with the best metric is received. By delaying the broadcast of a routing update by the length of the settling time, mobiles can reduce network traffic and optimize routes by eliminating those broadcasts that would occur if a better route was discovered in the very near future. 2.2 Ad-hoc On-demand Distance Vector Routing [6] is an improvement of to on-demand scheme. It minimize the broadcast packet by creating route only when need. In, every node in network should maintain route information table and participate in routing table exchange. When source node wants to send data to the destination node, it first initiates route discovery process. In this process, source node broadcasts Route Request (RREQ) packet to its neighbors. Neighbor nodes, which receive RREQ forward the packet to its neighbor nodes. Neighbor nodes, which receive RREQ forward the packet to its neighbors, and so on. This process continues until RREQ reach to the destination or the node that know the path to destination. When the intermediate nodes receive RREQ, they record in their tables the address of neighbors, thereby establishing a reverse path. When the node which knows the path to destination or destination node itself receive RREQ, it send back Route Reply (RREP) packet to source node. This RREP packet is transmitted by using reverse path in formation in route table of each intermediate node. When the source node receives RREP packet, it can know the path to destination node and it store the discovered path information in its route table. That is the end of route discovery process. And then perform route maintenance process. In route maintenance process, each node periodically transmits a Hello message to detect link breakage. 2.3 Optimized Link State Routing () [8] is an optimization of pure link state algorithm in ad hoc network. It is designed to reduce duplicate retransmission in the same region. The routes are always immediately available when needed due to its proactive nature. Hop by hop routing is used in forwarding packets. Nodes exchange topology information with other nodes periodically. The use of MPRs (Multipoint Relay) selectors in is the distinctive feature over other classical link state protocols where every node retransmits each message. In, only nodes selected as MPRs forward control traffic that causes reducing the size of control message and minimizing the overhead from flooding control traffic. MPRs advertise link state information for their MPR selectors periodically in their control messages. MPRs are also used to form a route from a given node to any destination in route calculation.each node periodically broadcasts Hello message for the link sensing, neighbor s detection and MPR selection process. Each node can get topology up to 2 hops from Hello messages. The information about the symmetric one hop and two hops neighbors is used to calculate the MPR set. Each node selects set of neighbor nodes as MPRs from among 1-hop neighbors with symmetric link that covers all the two hop neighbors and records in MPR selector table. MPR is recalculated when a change in one-hop or two-hops neighborhood topology is detected. Every node periodically broadcasts list of its MPR selectors instead of the whole list of neighbors. Upon receipt of MPR information, each node recalculates and updates routes to each known destination. In order to exchange the topological information, the Topology Control (TC) message is broadcasted throughout the network. Only MPRs need to forward TC messages. Each node maintains the routing table in which routes for all available destination nodes are kept because of the proactive nature. 3. MAC Protocols The main job of the MAC protocol is to regulate the usage of the medium, and this is done through a channel access mechanism. A channel access mechanism is a way to divide the main resource between nodes, the radio channel, by regulating the use of it. It tells each node when it can transmit and when it is expected to receive data. The channel access mechanism is the core of the MAC protocol. In this section, we describe IEEE 82.11 DCF and TDMA, which are the two main classes of channel access mechanisms for ad hoc network. 3.1 IEEE 82.11 DCF: The medium access control in the IEEE 82.11 DCF standard [1] is based on carrier sensing (CSMA) and collision detection (through acknowledgements). A node wanting to transmit a packet must first test the radio channel to check if it is free for a specified time called the Distributed Inter Frame Space (DIFS). If so, a DATA packet is transmitted, and the receiver waits a Short Inter Frame
Space (SIFS) before acknowledging the reception of the data by sending an ACK packet. Since the SIFS interval is set shorter than the DIFS interval, the receiver takes precedence over any other node attempting to send a packet. If the sender does not receive the acknowledgement, it assumes that the data was lost due to a collision at the receiver and enters a binary exponential backoff procedure. At each retransmission attempt, the length of the Collision Window (CW) is doubled. Since contending nodes randomly select a time from their CW, the probability of a subsequent collision is reduced by half. To bound access latency somewhat, the CW is not doubled once a certain maximum (CWmax) has been reached. Node movement is modeled by the Random Waypoint mobility model [7]. In this mobility model when the node arrives at its randomly chosen destination, it rests for some pause time. It then chooses a new destination and begins moving once again. The pause times are varied between and 2 seconds. The node transmission range is 25 m. Different network scenario for different pause times are generated. Each simulation is run for 2 seconds and models a network of 25 nodes. The propagation model is the Two way ground model. The bandwidth is 2 Mb/s, the data packet size is 512 bytes and packets are sent at a rate of four per second by each source. We have evaluated, and over IEEE 82.11 DCF and TDMA. The MAC parameters impact differently on routing protocol performance. 5. Results and Performance Comparison 3.2 TDMA Figure 1: IEEE 82.11 DCF access control. Unlike contention based MAC protocol (82.11, for example), a TDMA MAC protocol is a schedule-based protocol that allocates different time slots for nodes to send and receive packets. In TDMA systems the channel is divided into slots, which are grouped into frames. The fivephase reservation protocol (FPRP)[9] is a single-channel time division multiple access (TDMA) based broadcast scheduling protocol. Nodes use a contention mechanism in order to acquire time slots. The protocol is fully distributed, that is, multiple reservations can be simultaneously made throughout the network. No ordering among nodes is followed; nodes need not wait for making time slot reservations. The slot reservations are made using a fivephase reservation process. FPRP also ensure that no collisions occur due to the hidden terminal problem. Time is divided into frames. There are two types of frames: reservation frame (RF) and information frame (IF). Each RF is followed by a sequence of IFs. Each RF has N reservation slots (RS), and each IF has N information slots (IS). In order to reserve an IS, a node needs to contend during the corresponding RS. Based on these contentions, a TDMA schedule is generated in the RF and used and used in the subsequent IFs until the next RF. 4. Simulation Environments For our simulations, we have used the ns-2 Network Simulator. The ns-2 particularly popular in the ad hoc networking community, and protocols used in ad hoc networks have been supported. A mobile ad hoc network is generated as follows: there are 25 nodes in the network and they are confined to rectangular area 1 m by 5 m. In this section we have determined whether the selection of MAC protocols affects the relative performance of the protocols. The MAC protocols IEEE 82.11 DCF and TDMA are selected for simulation. For performance analysis, two results are examined: the packet delivery ratio and the control packet overhead. These metrics are the most widely used for representing performance. When we design a routing protocol, higher data delivery and lower control overhead are always desired, but they can seldom come together. a) IEEE 82.11 DCF: In this section we have focused on the performance evaluation of the routing protocols over the IEEE 82.11 DCF MAC protocol. Figure 2 shows the results of data packets delivered to destination with varying pause time. Packet Delivery Ratio(%) 1.2 1.8.6.4.2 Packet Delivery Ratio over 82.11 MAC Fig 2: Packet Delivery Ratio over IEEE 82.11 DCF MAC It is clear from the experimental result that has the highest packet delivery ratio, when run over IEEE 82.11 DCF MAC layer. performs best for the higher mobility scenarios and outperforms the other protocols. This is because the collision avoidance mechanism incorporated into IEEE 82.11 for the transmission of RTS packets aids in the reduction of the number of collisions. Consequently, more data packets
reach their destinations. performs better than and fails to outperform. In networks with more or less static connectivity (i.e., little mobility), and had the best in data delivery results. Another important performance metric control packet overhead is evaluated. Figure 3 shows the results of control packet overhead with varying pause time. Control Packet Overhead 14 12 1 8 6 4 2 Control Packet Overhead over 82.11 MAC Fig. 3: Control Packet Overhead over IEEE 82.11 DCF The amount of control overhead generated by is directly related to the number of routes it is maintaining. does not maintain as many routes. Most control overhead in is related to route discovery, which is initiated when a path break occurs. In networks with low mobility, path breaks occur less frequently, making perform well. It is clear from the above experimental result that performs better when run over IEEE 82.11 DCF. has approximately constant control packet overhead, regardless of movement rate. This constant behavior arises because each destination broadcasts a periodic update with a new sequence number every 15 seconds. performs better than other routing protocols with less control overhead. performs better than and fails to outperform. As must maintain an up-to-date routing table at all times, a decrease in network performance is expected, as more network overhead is needed. In networks with more or less static connectivity (i.e., little mobility), performs better with less control packet overhead. The control overhead is kept at a minimum, so both bandwidth and energy consumption by control overhead is greatly reduced. These points make more suited to resource and bandwidth critical situation. b) TDMA In this section we focus on the performance evaluation of the routing protocols over the TDMA MAC protocol. Figure 4 shows the results of data packets delivered to destination with varying pause time. Packet Delivery Ratio (%) 1.2 1.8.6.4.2 Packet Delivery Ratio over TDMA MAC Fig. 4: Packet Delivery Ratio over TDMA MAC As in case of IEEE 82.11 DCF, the over TDMA MAC protocol does not generate messages for knowing the neighbors. This is because TDMA MAC uses intrinsic neighbors discovering mechanism. We have considered the variation of the pause time, which is shown in figure 5.3. It is very interesting to note that if we vary the pause time, it creates the difference in the performance. As the mobility increases, we can see the decrease in the packet delivery ratio of the routing protocol. outperforms all the other protocols and faces heavy loss in the delivery of packets. outperforms and is better than and protocols. Another important performance metric control packet overhead is evaluated. Figure 5 shows the results of control packet overhead with varying pause time. Control Packet Overhead 8 6 4 2 Control Packet Overhead over TDMA MAC Fig. 5: Control Packet Overhead over TDMA MAC In the Figure 5, it can be observed that performs better when run over TDMA MAC. This is because of TDMA MAC protocol support less message overhead to maintain the routes. The amount of control overhead generated by is directly related to the number of routes it is maintaining. does not maintain as many routes. performs better than other routing protocols with less control overhead. has the worst overhead of generating the control packets.
6. Conclusions This paper presents the performance comparison of, and routing protocol over two kinds of MAC protocols for ad hoc networks: IEEE 82.11 DCF and TDMA. We simulated each protocol in ad hoc networks of 25 mobile nodes moving about and communicating with each other and presented the results for six different pause times:,4,8,12,16,2 seconds. When performance is measured over IEEE 82.11 DCF, the routing protocol outperforms all other protocols with respect to packet delivery ratio. perform better than other routing protocols with less control packet overhead. performs better than with respect to packet delivery ratio. When performance is measured over TDMA MAC, the routing protocol suffers with more control overhead packets when compare to. The over TDMA MAC protocol does not generate messages for knowing the neighbors. This is because TDMA MAC uses intrinsic neighbors discovering mechanism. It is very interesting to note that with the variation of pause time there is difference in the performance. As the mobility increases, we see the decrease in the throughput of the routing protocol. The amount of control traffic generated with IEEE 82.11 DCF MAC protocol is considerably greater than TDMA MAC protocols. performs better when run over TDMA MAC. This is because TDMA MAC protocol supports less message overhead in order to maintain routes. Table-driven and on-demand protocols react differently depending upon the MAC protocol used. The results of simulation study show that the choice of MAC protocol is a key component of the performance of a routing protocol, and this aspect must be taken into consideration when doing comparative studies of the performances of routing protocols. [6] C. Perkins, E. Royer, S. Das. Ad Hoc On-Demand Distance Vector () Routing, http://www.ietf.org/internet-drafts/draft-ietf-manetaodv-5.txt, March 2, IETF Internet Draft. [7] CMU Monarch Project, "The CMU Monarch Project s Wireless and Mobility Extensions to ns", August 1999, http://www.monarch.cs.cmu.edu [8] E.M. Royer & C.K. Toh, A Review of Current Routing Protocols for Ad-Hoc Mobile Wireless Networks, IEEE Personal Communications Magazine, April 1999, pp. 46-55. [9] C.Zhu, M.S. Corson, A Five-Phase Reservation Protocol (FPRP) for Mobile Ad Hoc Networks, Wireless Networks, Vol.7, Issue.4, August 21. [1] IEEE Standards Department. Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications, IEEE Standard 82.11.1997, 1994. [11] Elizabeth M. Royer, Sung-Ju Lee, and Charles E. Perkins, The Effects of MAC Protocols on Ad hoc Network Communication, Proceedings of the IEEE Wireless Communications and Networking Conference, 2. [12] C. E. Perkins and P. Bhagwat, Highly Dynamic Destination-Sequenced Distance-Vector Routing () for Mobile Computers, Comp. Commun.Rev, Oct. 1994, pp. 234 44. 7. References [1] J. Broch, D. Maltz, D. Johnson, Y. Hu, and J. Jetcheva, "A Performance Comparison of Multi-Hop Wireless Ad Hoc Network Routing Protocols, Proceedings of the Fourth Annual ACM/IEEE International Conference on Mobile Computing and Networking (MobiCom'98), pages 85-97, October 1998 [2] P. Johansson, T. Larsson, N. Hedman, B. Mielczare, M. Degermark, Scenario-based performance analysis of routing protocols for mobile ad-hoc networks, In Proc. of the ACM/IEEE MobiCom, August 1999. [3] S. Das, C. Perkins and E. Royer, "Performance Comparison of Two On-demand Routing Protocols for Ad Hoc Networks" [4] S. Corson and J. Macker, " Mobile Ad hoc Networking (MANET): Routing Protocol Performance Issues and Evaluation Considerations", RFC 251 MANET. January 1999 [5] The network simulator - ns-2. http://www.isi.edu/nsnam/ns/