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Jurdak et al [1] give a survey on a set of MAC protocols which have been proposed to improve the performance of ad hoc wireless networks in different aspects.

An Interference Graph Based MAC Protocol for Ad Hoc Wireless Networks Weiming Lin, Chongqing Zhang, Minglu Li, Min-You Wu Department of Computer Science and Engineering Shanghai Jiao Tong University Shanghai, China {linweiming, zhangchongqing, li-ml, mwu}@sjtu.edu.cn

Abstract Due to using the simplified interference model, IEEE 802.11 MAC protocol introduces the hidden and exposed terminal problems which significantly decrease the performance of ad hoc wireless networks. In this paper, we propose a novel Interference Graph based MAC protocol (IG-MAC) to improve the throughput of ad hoc wireless networks. The key point is to model the interference information by means of Interference Graph and send busy tone with encoded communication information to prevent the potentially interfering nodes from initiating new transmissions. Through the simulation, our protocol can solve the above two problems caused by 802.11 and improve the network performance substantially. Keywords – ad hoc wireless network, MAC Protocol, 802.11, interference graph, busy tone

1. Introduction Medium Access Control (MAC) protocol plays a critical role in determining the throughput of the ad hoc networks. The primary design goal of MAC protocols is to coordinate the channel access among multiple nodes to achieve high channel utilization. To do this, an efficient MAC protocol needs to minimize or eliminate the incidence of collisions and maximize spatial reuse at the same time. Collisions can result from hidden terminals. A hidden terminal is the one that can neither sense the transmission of a transmitter nor correctly receive the reservation packet from its corresponding receiver. A hidden terminal may interfere with ongoing transmission by sending packet at the same time. Besides hidden terminal problem, to achieve high channel utilization, MAC also needs to maximize the spatial reuse level. Exposed terminal problem is an important factor that influences the spatial reuse. An exposed terminal is the one that could sense the

transmission of a transmitter but do not interfere with the reception at the receiver. However, it is not allowed to transmit simply because it senses a busy medium, which leads to bandwidth under-utilization. Jurdak et al [1] give a survey on a set of MAC protocols which have been proposed to improve the performance of ad hoc wireless networks in different aspects. Among these protocols, IEEE 802.11 DCF [2] is the most popular MAC protocol used in ad hoc wireless networks. In 802.11 DCF, two carrier sensing functions, physical carrier sensing (PCS) and virtual carrier sensing (VCS) functions, are used to determine the state of the medium. Though 802.11 DCF uses a four-way RTS/CTS/Data/Ack exchange to reduce collisions caused by hidden terminals in the network, but the performance evaluation shows that the throughput of 802.11 DCF may significantly degrade as the number of competing network nodes and number of hops increase [3][4]. Since, 802.11 DCF regards the transmission range and the interference range as the same while in fact interference range varies as the distance between the sender and the receiver changes. The four-way handshake with the VCS only partially solves the hidden terminal problem and does not solve the exposed terminal problem [5]. In this paper, by analyzing how the interference range changes with the distance between the sender and receiver, we show the RTS/CTS handshake mechanism adopted by IEEE 802.11 DCF protocol is not effective in achieve high network capacity. A novel Interference Graph based MAC Protocol (IG-MAC) is proposed. Compared with IEEE 802.11 DCF, the novel MAC protocol takes variable interference range into account to solve both of the hidden and exposed terminal problems. Simulation results show this MAC protocol can improve the performance substantially. The rest of the paper is organized as follows. In Section 2 we discuss related work. In section 3, we analyze the hidden and exposed terminal problems. In

Proceedings of The Sixth IEEE International Conference on Computer and Information Technology (CIT'06) 0-7695-2687-X/06 $20.00 © 2006

section 4, we present our Interference Graph model and describe the IG-MAC protocol in detail. Section 5 evaluates the performance of the new protocol and section 6 summarizes our conclusions.

node id into the busy tone to alleviate the deafness problem.

2. Related work

To explain the two problems more clearly, we first describe the transmission range, carrier sensing range and interference range. Considering both transmitter and receiver will send packets (transmitter sends RTS, DATA and receiver sends CTS, ACK), none of them can be free of the interference. We follow the definition of transmission and carrier sensing range from [6] and give our definition of the interference range: the Interference Range (Ri) is the range within which other transmitter interfere the ongoing communication. The interference range is not a static value, but a function of the distance between the transmitter and receiver. Assume the distance between transmitter and receiver is d, as [6] point out that if all the radio parameters are assumed to be the same, the interference range is 1.78 * d. So the interference area consists of two circles, one around the sender and the other around the receiver. These two circles’ radius is 1.78 times of the distance between transmitter and receiver. When the distance between transmitter and receiver is long, some potential interference sources may stand outside transmission range so that they cannot be notified by RTS/CTS and become a hidden terminal. On the other hand, when the distance between transmitter and receiver is small, some nodes that in transmitter or receiver’s transmission range actually will not interfere with the current communication because it is outside the interference range. This brings the exposed terminal problem and decreases the spatial reuse. For example, in both Figure 1 and Figure 2, node 1 is sending packet to node 2, and node 3 wants to send packet to node 4. We assume the four nodes in these two figures have the same transmission range and carrier sensing range.

Xu et al [6] indicates that the interference range can be modeled as a function of the distances between the source and destination nodes, and the physical layer techniques. The interference range may result in more variable number of hidden terminals. They also investigate how effective is the RTS/CTS handshake in terms of reducing interference. He et al [7] investigates the performance of IEEE 802.11 DCF MAC protocol in multi-hop wireless networks. Different from some other analytical models, it takes the impact of the variable interference range into account. Cesana et al [8] proposes a novel MAC protocol which insert information about receiving power and interference levels into CTS packets. By computing an estimation of the interference increasing due to an eventual transmission, it increases the spatial reuse. But due to the interference range can exceed the transmission range, not all the node can overhear the CTS package, thus this protocol cannot solve the hidden terminal problem. Ye et al [9] indicates that the area that should be reserved by the VCS depends on the distance between the transmitter and receiver. And they propose a distance-aware virtual carrier sensing protocol by incorporating distance information in the decision making process for the channel reservation which consequently increases the spatial reuse. But it cannot solve the hidden terminal problem as well. Some existing proposals have utilized “busy tones” to solve hidden and exposed terminal problems. In protocol Busy Tone Multiple Access (BTMA) [10], a base station broadcasts a busy tone signal to keep the hidden terminals from accessing the channel when it senses a transmission. Dual Busy Tone Multiple Access (DBTMA) [11] extends the work of BTMA using distributed approach to send busy tone. In DBTMA, RTS packets are used to initiate channel request. Two out-of-band busy tones, the transmit busy tone (BTt) and the receive busy tone (BTr), are used to protect the RTS packets and the data packets respectively. It can solve hidden terminal problem, but cannot solve the exposed terminal problem. Compare to these two protocols that do not encode any information in the busy tone, Choudhury et al [12] proposes ToneDMAC which encodes the transmitter’s

3. Hidden and exposed terminal problems

Figure 1. Illustration of the hidden terminal problem

Proceedings of The Sixth IEEE International Conference on Computer and Information Technology (CIT'06) 0-7695-2687-X/06 $20.00 © 2006

4. Proposed protocol In this section, we propose the novel Interference Graph based MAC protocol (IG-MAC). Firstly, we define the Interference Graph and describe how to generate it. Secondly, we explain how to notify the potentially interfering nodes using busy tone with encoded communication information. Lastly, we describe the operation procedure of the IG-MAC. Figure 2. Illustration of the exposed terminal problem

4.1. Interference Graph Figure 1 illustrates the hidden terminal problem. In Figure 1, the distance between node 1 and 2 is the transmission range (Rt), thus the interference range of node 1 and 2 is about 1.78 * Rt. We assume node 3 is outside the carrier sensing range of node 1 and 2 but inside the interference range of node 2. When node 1 wants to send packet to 2, 802.11 uses RTS/CTS to block the two transmission range that around node 1 and 2. In other words, every node that in these two transmission range cannot start communication with other nodes during the node 1 and 2’s transmission. Since node 3 is outside the node 1’s transmission range and carrier sensing range, so it can neither receive the RTS or CTS packet nor sense the busy media. Under the 802.11, node 3 considers that it can send packet to node 4. While actually node 3 is in the interference range of node 2, sending packet from 3 to 4 will interfere with the communication between node 1 and 2. Exposed terminal problem is explained in Figure 2. In Figure 2, the distance between node 1 and 2 is very short, so the node 1 and 2’s interference range locates inside each other’s transmission range. As node 3 is outside the two nodes’ interference range, the transmission between node 3 and node 4 will not interfere with node 1 and 2’s communication. But under 802.11, the CTS packet that sent by node 2 will prevent node 3 and 4’s communication and reduce the spatial reuse. As illustrated before, 802.11 uses a simplified interference model that does not consider the variable interference range as a function of the distance between the transmitter and receiver but regard it is the same as the transmission range. This simplified model causes the above two problems, which increases the MAC layer interference and reduces spatial reuse. In 802.11, if a potential transmitter node senses a current communication between a sender and a receiver, it knows neither the distance between the sender and the receiver nor the distances between itself and these two nodes. This problem causes the trouble that the potential transmitter node cannot judge whether it will interfere the ongoing transmission or not.

Before we give the definition of the Interference Graph, we first define the Interference Set which is the basic element of Interference Graph. Interference Set: For node m, if node k and node j can communicate (within each other’s communication range) and their communication can be interfered by node m’s transmitting (in the other words, node m is within node k or node j’s interference range), then we call this node pair (node k and j) a node m’s Interference Link. Node m’s Interference Set is a set which include all of the node m’s Interference Link. We define Nm, Nn as node m and n respectively, IL (Nk, Nj) as a node m’s Interference Link that composed by node k and j, Sm as node m’s Interference Set. We use the following equation to describe the Interference Set: (1) S m = S (IL (N k , N j )) Interference Graph: The Interference Graph is defined as a set of all nodes in the network and their Interference Set. We can also consider the Interference Graph as a set of tuple (Nm, Sm). We use the following equation to describe the Inference Graph: (2) G = S (N m , S m ) We define Rt as the node transmission range, Rc as the node carrier sensing range and D (a, b) as the distance between node a and b. As illustrated in section 3, the interference range is 1.78 times of the distance between sender and receiver. Then we can generate the Interference Graph using following scheme: 1. For each node m in the network 2. For each node k that D (m, k)

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