How Effective is the IEEE 802.11 RTS/CTS Handshake in Ad Hoc Networks? Kaixin Xu, Mario Gerla, Sang Bae University of California, Los Angeles Computer Science Department Los Angeles, CA 90095, USA Abstract - IEEE 802.11 MAC mainly relies on two techniques to combat interference: physical carrier sensing and RTS/CTS handshake (also known as “virtual carrier sensing”). Ideally, the RTS/CTS handshake can eliminate most interference. However, the effectiveness of RTS/CTS handshake is based on the assumption that hidden nodes are within transmission range of receivers. In this paper, we prove using analytic models that in ad hoc networks, such an assumption cannot hold due to the fact that power needed for interrupting a packet reception is much lower than that of delivering a packet successfully. Thus, the “virtual carrier sensing” implemented by RTS/CTS handshake cannot prevent all interference as we expect in theory. Physical carrier sensing can complement this in some degree. However, since interference happens at receivers, while physical carrier sensing is detecting transmitters (the same problem causing the hidden terminal situation), physical carrier sensing cannot help much, unless a very large carrier sensing range is adopted, which is limited by the antenna sensitivity. In this paper, we investigate how effective is the RTS/CTS handshake in terms of reducing interference. We show that in some situations, the interference range is much larger than transmission range, where RTS/CTS cannot function well. Then, a simple MAC layer scheme is proposed to solve this problem. Simulation results verify that our scheme can help IEEE 802.11 resolve most interference caused by large interference range. I. INTRODUCTION In wireless networks, interference is location based. Thus, the hidden terminal problem may happen frequently [10]. Resolving hidden terminal problem becomes one of the major design considerations of MAC protocols. IEEE 802.11 DCF is the most popular MAC protocol used in both wireless LANs and mobile ad hoc networks (MANETs). Its RTS/CTS handshake is mainly designed for such a purpose. However, it has an underlying assumption that all hidden nodes are within the transmission range of receivers (e.g. to receive the CTS packet successfully). From our study, we realize that such an assumption may not hold when the transmitter-receiver distance exceeds a certain value. Some nodes which are out of the transmission range of both the transmitter and the receiver may still interfere with the receiver. This situation happens rarely in a wireless LAN environment since there most nodes are in the transmission range of either transmitters or receivers. However, in an ad hoc network, it becomes a serious problem due to the large distribution of mobile nodes and the multihop operation. In this paper, we show that for the open space environment, the interference range of a receiver is 1.78 times the transmitter-receiver distance. This implies that RTS/CTS handshake cannot function well when the transmitter-receiver distance is larger than 0.56 (equal to 1/1.78) times the transmission range. We then further analyze the effectiveness of RTS/CTS handshake under such situations and its relationship with physical carrier sensing. Our study reveals that large interference range is a serious problem in ad hoc networks and may hurt the network capacity as well as the network performance significantly. This is confirmed via simulation experiments. To solve this problem, a simple MAC layer scheme is proposed to help IEEE 802.11 combat the large interference range. Simulation results show that our scheme is a great improvement over IEEE 802.11 MAC. The rest of this paper is organized as following. In section II, we compute interference range and analyze the effectiveness of RTS/CTS handshake using an analytical model. The relationship between interference range and physical carrier sensing range is also discussed. In section III, we identify the problems caused by large interference range. In Section IV, a simple MAC layer scheme based on IEEE 802.11 is proposed and evaluated. Related work is given in section V and we conclude the paper in section VI. II. EFFECTIVENESS OF RTS/CTS HANDSHAKE As we have pointed out the RTS/CTS handshake of IEEE 802.11 does not work well as we expected in theory. It cannot prevent hidden terminal problems completely. In this section, we explain this through a theoretical analysis. For better explanation, we first review the three radio ranges: namely transmission range (Rtx), carrier sensing range (Rcs) and interference range (Ri). Transmission Range (Rtx) represents the range within which a packet is successfully received if there is no interference from other radios. The transmission range is mainly determined by transmission power and radio propagation properties (ie, attenuation). Carrier Sensing Range (Rcs) is the range within which a transmitter triggers carrier sense detection. This is usually determined by the antenna sensitivity. In IEEE 802.11 MAC, a transmitter only starts a transmission when it senses the media free. * This work is supported in part by ONR “MINUTEMAN” project under contract N00014-01-C-0016 and TRW under a Graduate Student Fellowship. Interference Range (Ri) is the range within which stations in receive mode will be “interfered with” by an unrelated transmitter and thus suffer a loss. A. Large Interference Range and the Interference Area Nodes within the interference range of a receiver are usually called hidden nodes. When the receiver is receiving a packet, if a hidden node also tries to start a transmission concurrently, collisions will happen at the receiver. In this subsection, we investigate the interference range and its relationship to the transmission range. When a signal is propagated from a transmitter to a receiver, whether the signal is valid at the receiver largely depends on the receiving power at the receiver. Given transmission power, the
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