Elsevier

Ad Hoc Networks

Volume 1, Issue 1, July 2003, Pages 107-123
Ad Hoc Networks

Effectiveness of RTS/CTS handshake in IEEE 802.11 based ad hoc networks

https://doi.org/10.1016/S1570-8705(03)00015-5Get rights and content

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. Two independent solutions are proposed in this paper. One is a simple enhancement to the IEEE 802.11 MAC protocol. The other is to utilize directional antennas. Simulation results verify that the proposed schemes indeed can help IEEE 802.11 resolve most interference caused by large interference range.

Introduction

In wireless networks, interference is location dependent. Thus, the hidden terminal problem may happen frequently [1]. 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 (under TWO-RAY GROUND pathloss model). 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 attack this problem, we investigate two techniques in this paper. The first technique is a simple MAC layer scheme with some minor modifications of IEEE 802.11 MAC DCF. Its major idea is to prevent the transmissions when the link quality is weak (e.g. transmitter–receiver distance is large) by selectively replying CTS packets. The major drawback of this MAC layer technique is the reduced effective transmission range. The second technique is to enhance the hardware, more precisely to use receiving beam forming (RBF) antennas. RBF antenna is a type of directional antennas, where the transmission is omnidirectional, but the reception is directional. It is capable to prevent interference by lock onto a specific direction for packet reception. In this paper, we prove that once the beam width of the RBF antenna is smaller than a certain value, it can totally bypass interference due to the large interference range. Both of the two techniques have their advantages and disadvantages, which will be discussed and investigated in this paper.

The rest of this paper is organized as following. In Section 2, we briefly review some related work in the literature. Section 3, 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 4, we identify the problems caused by large interference range. In Section 5, two independent solutions are proposed and discussed. Performance evaluations via simulation are given in Section 6 and we conclude the paper in Section 7.

Section snippets

Related work

Large interference range has been realized by more and more researchers in recent years [2], [3]. In [2], the influence of large interference range to the ad hoc network capacity is studied. In [3], large interference range is also recognized as one of the major factors which causing TCP unfairness/capture problem. However, so far from our knowledge, we have not seen any work trying to analyze and resolve this problem in detail. Thus, this paper presents a preliminary and original study on this

Effectiveness of RTS/CTS handshake

The RTS/CTS handshake of IEEE 802.11 MAC does not work as well as we expected in theory. It cannot prevent hidden terminal problems completely. In this section, we explain this through a simple theoretical analysis. For better understanding, we first define three radio ranges related to a wireless radio, 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

Problem caused by large interference range

In this section, we investigate how the large interference range affects the network performance. The effect of interference to the capacity of a single chain is discussed in [2], where NS2 simulator is used and the transmission range and interference range are set to 250 and 550 m respectively. The topology of a single chain is illustrated as in Fig. 4 and the distance between neighbor nodes is 200 m. Clearly, if not considering the large interference range, the capacity of this single chain

Proposed solutions

As shown in Section 3, the ineffectiveness of RTS/CTS handshake on resolving large interference range will cause significant data packet corruptions at the MAC layer and in turn wastes channel bandwidth and degrades the network performance. In this section, we propose two solutions to attack this problem. The first scheme is a simple MAC layer scheme based on the IEEE 802.11 MAC with some minor modifications. Another solution is to adopt the RBF antennas. RBF antennas are one kind of

Simulation platform and basic simulation scenario

All simulations in this paper are done using QualNet simulator [9], which is the successor of GloMoSim [11] simulation library. According to [12], QualNet incorporates a detailed model of the physical channel and of the IEEE 802.11 MAC layer. The TWO-RAY GROUND pathloss model and the RBF antenna are also implemented. Thus, it provides a good platform for our study of different radio ranges.

The simulation scenario configured in our experiments is consisting of 100 mobile nodes randomly deployed

Conclusion

This paper has three major contributions. First, we analyze the interference range for the open space environment in detail. The effectiveness of RTS/CTS handshake in terms of resolving such kind of interference is also explored. We believe that such a quantified analysis would be helpful to research in ad hoc networks, especially those works targeting the network capacity, scheduling and TCP fairness etc. in ad hoc networks. Second, frequent data packet corruptions due to large interference

Acknowledgements

This work is supported in part by ONR “MINUTEMAN” project under contract N00014-01-C-0016 and TRW under a Graduate Student Fellowship.

Kaixin Xu is a Ph.D. student of the computer science department at UCLA. He joined the Network Research Laboratory (NRL) of UCLA at 2000. His research focuses on the ad hoc wireless networking especially protocols at MAC, Network and Transport layers. His recently work includes enhancing TCP performance in multihop ad hoc networks, TCP performance in IEEE 802.11 MAC based ad hoc networks, as well as MAC protocols for utilizing directional antennas and mobility track. He is also working on

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Kaixin Xu is a Ph.D. student of the computer science department at UCLA. He joined the Network Research Laboratory (NRL) of UCLA at 2000. His research focuses on the ad hoc wireless networking especially protocols at MAC, Network and Transport layers. His recently work includes enhancing TCP performance in multihop ad hoc networks, TCP performance in IEEE 802.11 MAC based ad hoc networks, as well as MAC protocols for utilizing directional antennas and mobility track. He is also working on network protocols for building hierarchical ad hoc networks.

Mario Gerla was born in Milan, Italy. He received a graduate degree in engineering from the Politecnico di Milano, in 1966, and the MS and Ph.D. degrees in engineering from UCLA in 1970 and 1973, respectively. He joined the Faculty of the UCLA Computer Science Department in 1977. His research interests cover the performance evaluation, design and control of distributed computer communication systems; high speed computer networks; wireless LANs (Bluetooth); ad hoc wireless networks. He has been involved in the design, implementation and testing of wireless ad hoc network protocols (channel access, clustering, routing and transport) within the DARPA WAMIS, GloMo projects and most recently the ONR MINUTEMAN project. He has also carried out design and implementation of QoS routing, multicasting protocols and TCP transport for the Next Generation Internet. He is currently an associate editor for the IEEE Transactions on Networking.

Sang H. Bae is a Principal Engineer, Communications Networks, Boeing Phantom Works. Presently conducting simulation study of QoS-based composite routing for Joint Tactical Radio System (JTRS) system. Prior to Boeing, five years as researcher at Network Research Laboratory in UCLA responsible for wireless network systems development and designing, experimental studies of TCP and wireless MAC interaction and system engineering. Developed and implemented On Demand Multicast Routing Protocol. He received the Ph.D. form Computer Science, UCLA, and MS in Computer Science, California State University, and BS in Information and Computer Science, California State University.

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