Elsevier

Ad Hoc Networks

Volume 9, Issue 1, January 2011, Pages 16-27
Ad Hoc Networks

Optimal physical carrier sense in wireless networks

https://doi.org/10.1016/j.adhoc.2010.04.006Get rights and content

Abstract

We investigate the problem of maximizing Medium Access Control (MAC) throughput in Carrier Sense Multiple Access (CSMA) wireless networks. By explicitly incorporating the carrier sense threshold and the transmit power into our analysis, we derive an analytical relation between MAC throughput and system parameters. In homogeneous networks, we derive the optimal carrier sense range at a given node density as a function of the ratio between the transmit power and the carrier sense threshold. The obtained optimal carrier sense range is smaller than that for covering the entire interference range, which is in sharp contrast to what has been considered to be optimal in previous studies. Only when the node density goes to infinity, the optimal carrier sense range converges to that for exactly covering the interference range, thereby eliminating all the hidden nodes. For nonhomogeneous networks, any distributed algorithm for tuning the carrier sense threshold, in which each node tries to maximize its own throughput without coordination, may significantly degrade MAC throughput. In order to properly design a distributed algorithm, each node not only considers its own throughput, but also needs to take account of its adverse impact on others. Our analysis is verified by simulation studies under various network scenarios.

Introduction

Multi-hop wireless networks, e.g., wireless mesh networks, have emerged as a promising, cost-effective technology for next-generation wireless networking [1]. Their main advantage is the capability of building networks without a pre-installed infrastructure. Instead of coordinating the radio channel by a central entity, a distributed access mechanism is deployed at each node to arbitrate access to the channel. Caused by this convenience, most deployed multi-hop wireless networks are employing Carrier Sense Multiple Access (CSMA). Here, our main focus is on CSMA wireless networks.

A critical performance metric in CSMA wireless networks is network capacity, i.e., the average number of data bits that can be transported simultaneously in the network. This metric heavily depends on the level of spatial reuse characterized by carrier sense. For example, IEEE 802.11 Distributed Coordination Function (DCF) [2] has employed two types of carrier sense: mandatory physical carrier sense that monitors the signal strength of the channel, and optional virtual carrier sense that uses the Request-To-Send/Clear-To-Send (RTS/CTS) handshake to reserve the medium prior to transmission. In this paper, we mainly focus on physical carrier sense and will discuss the effect of RTS/CTS as an extension. For physical carrier sense, before each transmission, a sender listens to the channel and determines whether or not the channel is busy by comparing the received signal strength with the carrier sense threshold. If the signal strength is below the carrier sense threshold, the sender considers the channel to be idle and starts its transmission. Otherwise, the sender considers the channel to be busy and defers its transmission. Since the received signal strength is proportional to the transmit power of the corresponding sender, both the carrier sense threshold and the transmit power are major control knobs for physical carrier sense.

There have been a number of studies that focus on the impact of physical carrier sense on network capacity. (We will give a detailed summary of existing work in Section 2.) Most research efforts [3], [4], [5], however, have concentrated either on the relation between physical carrier sense and Shannon capacity, i.e., the achievable channel rate under the additive white Gaussian noise channel model (instead of Medium Access Control (MAC) throughput, i.e., the average rate of successful message delivery in the MAC layer) or on the derivation of a simple condition for eliminating all the hidden nodes (without considering sufficient details of how physical carrier sense operates) [6], [7]. What has not been fully investigated is the analytical relation between physical carrier sense and the MAC throughput. Hereafter, we interchangeably use network MAC throughput (aggregate MAC throughput over every node) and network capacity to denote MAC-level throughput under the saturation condition [8].

In this paper, we are interested in seeking solutions to the following questions: What is the analytical relation between network capacity and system parameters such as the carrier sense threshold and the transmit power? Is eliminating all the hidden nodes really optimal in terms of network capacity? If not, what is the optimal condition? As in the case of using Shannon capacity [3], [4], [5] to characterize network capacity, can we still quantify network capacity as a function of the ratio of the carrier sense threshold to the transmit power? Furthermore, is there any advantage of deploying nonhomogeneous networks (where the system parameters can be adjusted independently by each node) over homogeneous networks (where system parameters are set to the same values for every node)? We aim to answer the above questions in an analytical framework. Specifically, our contributions are as follows:

  • By explicitly incorporating the carrier sense threshold and the transmit power into our analysis, we establish an analytical relation between network capacity and the level of spatial reuse characterized by physical carrier sense. Although there have been considerable research efforts on modeling the performance of CSMA wireless networks [8], [9], [10], [11], none of them have explicitly incorporated the carrier sense threshold and the transmit power in their models.

  • In the case of homogeneous networks, we show that network capacity depends on the ratio of the transmit power to the carrier sense threshold. By using the notion of the carrier sense range, we derive the optimal carrier sense range as an explicit function of system parameters such as node density, channel access probability, and duration of each channel state. We identify that the optimal carrier sense range is smaller than the value for exactly covering the entire interference range of the receiver, which implies that the hidden nodes will not be totally eliminated. This result is in sharp contrast to what has been considered to be optimal in previous studies. Only when the node density goes to infinity, the optimal carrier sense range converges to that for exactly covering the entire interference range, thereby eliminating all the hidden nodes. This analysis quantifies the intuitive tradeoff between the hidden node problem and the exposed node problem.

  • In the case of nonhomogeneous networks, the carrier sense threshold and the transmit power should be considered independently in order to determine network capacity. The problem of maximizing network capacity in a fully distributed manner is shown to be a non-cooperative game [12]. Any selfish distributed algorithm in which every node tunes its own parameters for maximizing its own throughput, without coordination with other nodes, will fail to maximize network capacity and may result in a poor system performance. Consequently, each node needs to consider not only its own throughput, but also needs to introduce a certain form of penalty as a price for the adverse impact on others.

The remainder of the paper is organized as follows: In Section 2, we give a detailed summary of related work and highlight the difference between prior work and ours. In Section 3, we introduce the propagation and interference models used in our analysis. Then, by focusing on physical carrier sense, we characterize the MAC throughput. In Section 4, we derive an analytical relation between network capacity and system parameters. Based on the relation, we find the optimal carrier sense range, which maximizes network capacity. Then, we discuss several related issues such as the effect of RTS/CTS on network capacity and multiple data rates. We present simulation results in Section 5, and conclude the paper in Section 6 with a list of research avenues for future work.

Section snippets

Related work

We categorize related work into the following three cases.

Network model

In this section, we introduce the network model used in our analysis.

Throughput analysis

In this section, we derive optimal operating condition of the carrier sense threshold and the transmit power for maximizing network capacity.

Simulation study

In this section, we carry out a simulation study to validate the derived relation between network capacity and system parameters. In particular, we corroborate the model presented in Section 3.2 as well as Proposition 1, Proposition 2.

First, we look into the relation between network capacity and system parameters such as the contention window size and the node density. Fig. 3 shows the numerical result on the normalized optimal carrier sense range, i.e., (X  d)/dI as a function of the node

Conclusion and future work

In this paper, we have investigated the issue of maximizing network capacity of CSMA wireless networks. In particular, we have explicitly incorporated the carrier sense threshold and the transmit power into the analysis. In a homogeneous network, we have found that the optimal carrier sense range is smaller than the value for exactly covering the entire interference range. In a nonhomogeneous network, the problem of maximizing network capacity in a fully distributed manner has been shown to be

Kyung-Joon Park received his B.S., M.S., and Ph.D. degrees all from the School of Electrical Engineering and Computer Science (EECS), Seoul National University (SNU), Seoul, Korea in 1998, 2000, and 2005, respectively. He is currently a research assistant professor in the School of EECS at SNU. He has been a postdoctoral research associate in the Department of Computer Science, University of Illinois at Urbana-Champaign (UIUC) from 2006 to 2010. He worked for Samsung Electronics, Suwon, Korea

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  • Kyung-Joon Park received his B.S., M.S., and Ph.D. degrees all from the School of Electrical Engineering and Computer Science (EECS), Seoul National University (SNU), Seoul, Korea in 1998, 2000, and 2005, respectively. He is currently a research assistant professor in the School of EECS at SNU. He has been a postdoctoral research associate in the Department of Computer Science, University of Illinois at Urbana-Champaign (UIUC) from 2006 to 2010. He worked for Samsung Electronics, Suwon, Korea as a senior engineer in 2005–2006, and was a visiting graduate student, supported by the Brain Korea 21 Program, in the Department of Electrical and Computer Engineering at UIUC in 2001–2002. He has current research interests in characterization and design of medical-grade protocols for wireless healthcare systems, analysis of malicious and selfish behavior for wireless network security, and design and analysis of self-adjusting protocols for wireless environments. He is the Gold Prize Winner of the 4th Inside Edge International Thesis Competition from Samsung Electro-Mechanics in 2008. He has received a Distinguished Paper Prize at the OPNET Conference in 2005. He is also a winner of the Human-Tech Thesis Prize from Samsung Electronics in 2003, 2004, and 2005.

    Jihyuk Choi received the B.S. and M.S. degrees from Seoul National University, Seoul, Korea in 1998 and 2000, respectively. He is a Ph.D. candidate at the Department of Electrical and Computer Engineering of the University of Illinois at Urbana-Champaign (UIUC). He was a research engineer at Electronics and Telecommunications Research Institute (ETRI), Korea from 2000 to 2003. He was also a senior researcher at LG Electronics Institute of Technology, Korea from 2003 to 2006. His research interests include wireless network protocols and network security.

    Jennifer C. Hou was born on September 26, 1964 in Taipei, Taiwan. She received her B.S.E. degree in Electrical Engineering from National Taiwan University, Taiwan, ROC in 1987, M.S.E degrees in Electrical Engineering and Computer Science (EECS) and in Industrial and Operations Engineering (I & OE) from the University of Michigan, Ann Arbor, MI in 1989 and in 1991, and Ph.D. degree in EECS also from the University of Michigan, Ann Arbor, MI in 1993. She was an assistant professor in Electrical and Computer Engineering at the University of Wisconsin, Madison, WI from 1993 to 1996, and an assistant/associate professor in Electrical Engineering at Ohio State University, Columbus, OH from 1996 to 2001. She joined the University of Illinois Computer Science faculty in 2001. She was a principal researcher in networked systems and served as the director of the Illinois Network Design and Experimentation (INDEX) research group. She has supervised several federally and industry funded projects in the areas of network modeling and simulation, network measurement and diagnostics, and both the theoretical and protocol design aspects of wireless sensor networks. She has published (with her former advisor, students, and colleagues) over 160 papers in archived journals, book chapters, and peer-reviewed conferences. Her work on topology control and performance limits in wireless networks has been widely cited. Dr. Hou has been involved in organizing several international conferences sponsored by professional organizations such as ACM Mobicom, IEEE INFOCOM, IEEE MASS, and IEEE RTAS, as well as editor in archival journals and magazines such as IEEE Trans. on Computers, IEEE Trans. on Wireless Communications, IEEE Trans. on Mobile Computing, IEEE Trans. on Parallel and Distributed Systems, IEEE Wireless Communication Magazine, Elsevier Computer Networks, and ACM Trans. on Sensor Networks. Dr. Hou was a recipient of an ACM Recognition of Service Award in 2004 and 2007, a Cisco University Research Award from Cisco, Inc., 2002, a Lumley Research Award from Ohio State University in 2001, a NSF CAREER award from the Network and Communications Research Infrastructure, National Science Foundation in 1996-2000 and a Women in Science Initiative Award from The University of Wisconsin-Madison in 1993–1995. She was elected as an IEEE Fellow and an ACM Distinguished Scientist in 2007. Dr. Hou passed away on December 2, 2007 in Houston, Texas at the age of 43.

    Yih-Chun Hu received his B.S. Magna Cum Laude in 1997 in Computer Science and Mathematics from the University of Washington. He received his Ph.D. in 2003 from the Computer Science Department at Carnegie Mellon University. He was a Postdoctoral Researcher at the University of California, Berkeley. He is currently an assistant professor in the Department of Electrical and Computer Engineering at the University of Illinois at Urbana-Champaign. His general research interests are in security and systems, with emphasis on the areas of secure systems and mobile communications. He has published papers in the areas of secure Internet routing, DDoS-resilient forwarding, secure routing in wireless ad hoc networks, security and anonymity in peer-to-peer networks, efficient cryptographic mechanisms for routing security, and the design and evaluation of multi-hop wireless network routing protocols, including Quality-of-Service mechanisms for ad hoc networks.

    Hyuk Lim received his B.S., M.S., and Ph.D. degrees all from the School of Electrical Engineering and Computer Science, Seoul National University, Seoul, Korea in 1996, 1998, and 2003, respectively. He is currently an assistant professor in the Department of Information and Communications, and the Department of Nanobio Materials and Electronics, Gwangju Institute of Science and Technology (GIST), Gwangju, Republic of Korea. He was a postdoctoral research associate in the Department of Computer Science, University of Illinois at Urbana-Champaign in 2003–2006. His research interests include analytical modeling and empirical evaluation of computer networking systems, network protocol design and performance analysis for wireless networks, measurement and diagnostics for wired/wireless networks, and location-aware applications in ubiquitous sensor networks.

    1

    K.-J. Park was with the Department of Computer Science, University of Illinois at Urbana-Champaign at the time of this work. He is now with Seoul National University, Seoul, Korea.

    2

    J. C. Hou was with the Department of Computer Science, University of Illinois at Urbana-Champaign at the time of this work. She is now deceased.

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