Fibonacci Backoff Algorithm for Mobile Ad Hoc Networks

Transcription

1 Fibonacci Backoff Algorithm for Mobile Ad Hoc Networks Saher S Manaseer Mohamed Ould-Khaoua Lewis M Mackenzie Department of Computing Science University of Glasgow, Glasgow G 8RZ, UK {saher, mohamed, lewis}@dcs.gla.ac.uk Abstract- The collision probability in the MAC layer could become very high when a poor backoff algorithm is used, especially in dense networks. The Binary Exponential Backoff () used in the IEEE 8. MAC protocol uses a uniform random distribution to choose the backoff value. This often leads to reducing the effect of window size increment. This paper proposes a new backoff algorithm called Fibonacci Increment Backoff () in which the differences between consecutive contention window sizes are reduced. Results from simulation experiments reveal that the proposed algorithm achieves higher throughput than the when used in a mobile ad hoc environment. Keywords IEEE 8., Ad hoc networks, Medium access control, Backoff algorithm, Throughput analysis. I. INTRODUCTION Since their emergence, wireless networks have become increasingly popular in the computing industry. This is particularly true within the past decade, which has seen wireless networks being widely adopted to enable mobility. Over the development process of computer networks, two main variations of mobile wireless networks have been introduced, infrastructure and ad hoc wireless networks []. Recently, a significant number of researchers have moved towards studying Mobile Ad Hoc Networks (MANETs). Interest in MANETs is due to many new characteristics provided only by this type of networks. Firstly, MANETs are easily deployed allowing a plug-and-communicate method of networking. Secondly, MANETs need no infrastructure [7], eliminating the need for an infrastructure reduces the cost of establishing the network. Moreover, such networks can be useful in disaster recovery where there is not enough time or resources to install and configure an infrastructure. Thirdly, MANETs also do not need central management. Hence, they are used in military operations where units are moving around the battlefield and a central unit cannot be used for synchronization [7]. Nodes forming an ad hoc network are required to have the ability to double up as a client, a server, and a router simultaneously [7]. Moreover, these nodes should also have the ability to connect to and automatically configure to start transmitting data over the network. As a result of having the characteristics mentioned so far,, protocols used for ad hoc networks generally function in a distributed manner []. The distributed Coordination Function (DCF) is used for synchronous, contention-based, distributed access to the channel []. MANETs use a shared medium to transfer data between its nodes. It is impractical to expect a MANET to be fully connected, where a node can directly communicate with every other node in the network. Typically, nodes are obliged to use a multihop path for transmission, and a packet may pass through multiple nodes before being delivered to its intended destination. The wireless medium used by MANETs has a number of problems. Bandwidth sharing, signal fading, noise, interference, etc. with such a public medium, a well-organized and effective Medium Access Control (MAC) is indispensable to organize sharing the scarce bandwidth resource [] [7]. Based on the features mentioned, the design of the medium access control (MAC) protocol is a significant factor affecting the performance of MANETs. Many researchers have proposed the mechanism of channel sensing, or packet sensing to avoid collision. The sensing mechanisms typically rely on the transmitter and receiver performing a handshake prior to the transmission of the data packet []. More specifically, The Medium Access Collision Avoidance (MACA) method proposed by Karn [] implements the handshake via a pair of Request-To-Send (RTS) and Clear-To-Send (CTS) messages. When a node has to send data to another, it first sends a short RTS to the destination. The receiver responds with a CTS packet []. On receipt of the CTS, the sender

2 sends its queued data packet(s). All other nodes overhearing the CTS message will defer from sending out any packet until the predicted transmission period indicated in the CTS packet, is passed. Any node that overhears the RTS signal but not CTS is allowed to send out packets in a certain time period as either the RTS/CTS handshake is not completed or it is out of range of the receiver. As a part of an efficient MAC protocol, a backoff algorithm is used to avoid collisions when many nodes try to access the medium []. Only one of the nodes is granted access to the channel, while other contending nodes are suspended into a backoff state for some period (BO) [8]. Many backoff algorithms have been developed in the literature [, 8]. One example is the Multiplicative Increase Linear Decrease (MILD) algorithm []. This algorithm improves the total throughput of the network, but the cost of this improvement is the need of a perfect knowledge regarding collisions happening over the network, which is high cost and hardto-acquire knowledge [7]. In a wired LAN the total number of nodes of the network is easily obtained. However, as nodes in MANETs are mobile, knowing the number of nodes may incur a high cost, since this knowledge needs to be updated due to the frequent change in the MANET topology. One approach to update and keep the knowledge consistent is by exchanging packets between neighboring nodes [].these packets form extra load, consume a part of the network resources, and cause a longer delay. Moreover, neighbor discovery requires more control processing, and even incurs more work to the backoff algorithm itself. Other backoff algorithms have tried to find a fixed optimum backoff value to use. Even though, the distributed functionality was incomplete []. In the IEEE 8. standard MAC protocol, the truncated Binary Exponential Backoff () is used [9]. In this algorithm, when a node over the network has a packet to send, it first senses the channel using a carrier sensing technique. If the channel is found to be idle and not being used by any other node, the node is granted access to start transmitting. Otherwise, the node waits for an inter-frame space and the backoff mechanism is invoked. A random backoff time is chosen in the range [, CW-]. A uniform random distribution is used here, where CW is the current contention window size. The following equation is used to calculate the backoff time (BO): BO = (Rand () MOD CW) * aslottime () The backoff procedure is preformed then, by putting the node on a waiting period of length BO. Using carrier sense mechanism, the activity of the medium is sensed at every time slot. If the medium is found to be idle then the backoff period is decremented by one time slot. Backoff time (BO) new = (BO) old - aslottime () If the medium is busy during backoff, then the backoff timer is suspended. That is, the backoff period is counted in term of idle time slots. Whenever the medium is found idle for longer than an inter-frame space, backoff is resumed. When the backoff period is finished with a BO value of zero, a transmission should take place. If the node succeeds to send a packet and receive an acknowledgment for that packet, then the CW for this node is reset to the minimum, which is equal to in the case of. The 'truncated' simply means that after a certain number of increases, the exponentiation stops; i.e. the retransmission timeout increases to a ceiling value, and thereafter does not increase any more. The ceiling is set at i = transmission failures, assuring that the maximum delay is slot times. Since these delays cause other nodes to collide as well, there is a possibility that on a busy network, many nodes may be caught in a single collision set. To overcome the possibility of starvation, the transmission process is aborted after failing attempts of transmission [] has a number of disadvantages []. One major disadvantage is the problem of fairness. tends to have a preference for most recent contention winner and new contending nodes over other nodes when allocating channel access. Determining backoff period is accomplished by choosing a random backoff value from a contention window (CW) which has smaller size for new contending nodes and contention winners. This behavior causes what is known as Channel capture effect in the network. Another problem of is stability. has been designed to be stable for large number of nodes [7]. However, a number of studies have shown that could suffer from instability [7]. In this paper, we present a new backoff algorithm, referred to as Fibonacci Increment Backoff () that can overcome the

3 limitation of the existing MAC solutions in. In the algorithm the difference between consecutive contention window sizes are reduced according to a Fibonacci sequence. Results from simulation experiments reveal that the proposed algorithm achieves higher throughput than the when used in a mobile ad hoc environment. The rest of the paper is organized as follows. Section presents the new algorithm. Section presents the simulation model. Section then analyses the performance results. Finally, Section concludes this study. II. THE PROPOSED ALGORITHM Most backoff algorithms suffer from the following shortcoming due to their inherent operations. Increasing the contention window in case of failure to transmit tends to rapidly increase large contention windows to even larger sizes. Reaching such large window sizes dangerously decrease the possibility of gaining access to the channel. Moreover, a large window size tends to contribute to increasing channel idle times, leading to a major waste the shared limited communication channel. Motivated by this above observation, we propose a new backoff algorithm to improve the performance of the backoff algorithm. One of the most famous series in math is the Fibonacci series defined by the following rule []: F (n) = F (n -) + F (n - ). F () =, F () =, n >=. The Fibonacci series has a number of useful characteristics. One specific property is a special value, closely related to the Fibonacci series, is called the golden section. This value is obtained by taking the ratio of successive terms in the Fibonacci series. Figure illustrates this property. After a certain number of terms, the ratio tends to converge into a limit of. []. In our proposed algorithm, we used the F (n) as the new contention window size, leading to a smaller increment on large window sizes as shown in figure. Figure Ratio of successive Fibonacci terms. Figure Pseudo code of Algorithm

4 III. SIMULATION The proposed algorithm has been evaluated using the Network Simulator NS.9 []. The original standard MAC protocol has been modified to implement our proposed backoff algorithm. Modifications have mainly targeted the mac8_.h and mac8_.cc files. Several topologies and mobility scenarios have been created to test the algorithm as intensively as possible. Firstly, we have varied the total number of nodes in the network. Changing the number of nodes is our method of predicting the performance of our algorithm for all sizes of networks. Simulations have been carried out for networks having total number of node varying between and mobile nodes. Secondly, in order to address the main drawback of MANETs, we have used different values for mobility speed. Testing for speed values, ranging from ms - to ms - has given us useful information concerning the efficiency of our algorithm for static and highly mobile MANETs as well. Other simulation parameters are the area of m m, simulation time is 9 seconds, nodes transmission range is m and the traffic generated is CBR traffic. Table summarizes simulation parameters. Table Simulation Parameters Parameter Nodes Speeds (ms - ) Area Simulation time Transmission range Value,,,,,, m X m 9 s m IV.RESULTS As shown in the following figures, the new has improved the total throughput of the network simulated in our work. When the number of nodes is increased, the contention is higher to gain access to the channel. Because of the reduced amount of increment on the window size, a larger size of data was successfully received by nodes over the network. The same enhancement is noticed even while increasing mobility speed. One of the major obstacles in the way of developing a MAC protocol for MANETs is mobility. Having a long backoff value allows the node to move outside the transmission range before being allowed to retry accessing the channel. With, the ceiling of backoff periods is controlled to prevent extremely long backoff periods. One more factor participating in increasing throughput is reducing idle times. With smooth increments of contention window size, idle time is reduced. Total received data for speed m /s 7 Data receievd MB 8 Number of nodes Figure Received Data at speed m/s

5 Total data received at m/s Number of nodes Figure Received Data at speed m/s Total Data Received for Nodes.. Data MB... speed m/s Figure Received Data at nodes Total Data Received for Nodes 8 7 Data (MB) 8 Speed m /s Figure Received Data at nodes V. CONCLUSION The Binary Exponential Backoff () is used by the IEEE 8. Medium Access Control (MAC) protocol. uses uniform random distribution to choose the backoff value. In this paper, we have proposed a new backoff algorithm to reduce the increment factor for large contention window sizes. Results from simulations have demonstrated that the proposed algorithm increased the total throughput of mobile ad hoc networks especially when the system size is large. The total throughput has been increased for highly mobile networks.

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