CU-Boulder TLEN 5520 - Experimental Sensitivity Analysis of Wireless Protocols

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Experimental Sensitivity Analysis of Wireless Protocols in an Office Environment Gurpreet Sohal School of Computing, University of Leeds, Leeds LS2 9JT, UK [email protected] Larry Dowdy EECS Department, Vanderbilt University, Nashville, TN 37235, USA [email protected] Abstract This paper provides a methodical experimental evaluation of the 802.11a and 802.11b protocols, presenting an unbiased and independent performance review in a given office environment. Specifically, the bandwidths of the two protocols are compared as a function of distance between the wireless laptops and their access point (AP). In addition, the effects on bandwidth in the presence of interference from competing Bluetooth devices at various distances are also measured. Finally, the performance effects of using two different levels of security (i.e., 64 and 128 bit encryption schemes) are reported. 1. Introduction The emergence and growth of wireless local area networks (WLANs) has been dramatic over the past 20 years. Annual growth rates exceed 40% and forecasts indicate that this trend will continue and will likely accelerate over the next five years [1] [2]. See Figure 1. As the number of companies adopting wireless LANs increases, the benefits of such systems becomes easier to quantify. Compared with wired alternatives, the economic benefits of implementing a wireless network can result in an estimated savings in excess of $15,000 per year for each staff member [3]. Given the growth projections and the economic impact of wireless networks, it is important to analyze and optimize each aspect of wireless technologies. This paper presents an experimental sensitivity analysis of wireless protocols. Specifically, the 802.11a and 802.11b standards are compared in a given office environment. As a function of distance from the access points, issues of bandwidth, interference from competing Bluetooth devices, and encryption levels are addressed. The results indicate that 802.11a provides better bandwidth than 802.11b, but that its advantage decreases with distance. Also, 802.11b suffers from competing Bluetooth devices, but that this interference decreases with distance. The 802.11a standard does not suffer from such interference. Encryption levels do reduce bandwidth, but only in the 5%-10% range, depending on the size of the encryption keys. The results favor the 802.11a standard and indicate that it provides performance approaching that of wired 10Mbps Ethernet at close range. The outline of the papers is as follows. Section 2 provides an overview background of the key differences between wireless protocols. Section 3 describes the physical environment in which the experiments are conducted. The actual sensitivity analysis results are presented in Section 4, with a summary interpretation of the key findings given in Section 5. Conclusions and future work are provided in Section 6. 2. Background The most widely deployed WLAN protocol is the 802.11b IEEE standard. This protocol operates in the 2.4GHz frequency range with a maximum data link rate of 11 Mbps and exhibits a maximum throughput rate in the 4-5Mbps range. The 802.11a standard operates in the 5GHz frequency band with a maximum data link rate of 54Mbps and a throughput rate of approximately 26Mbps. Depending upon the modulation technique, higher frequency implies increased bandwidth, but at the cost of decreased range. Thus, 802.11a has higher bandwidth potential, but the performance of 802.11b does not degrade as rapidly with increased distance from the access point. The primary characteristics of these two protocols are summarized in Table 1. The more recent 802.11g protocol is an IEEE specification developed to be an extension of 802.11b, operating in the 2.4GHz frequency band, but employing improved modulation to increase its bandwidth capabilities. This standard offers a maximum data link Figure 1 - Worldwide Wireless LAN Chipset Units (in millions of units) Units in millions Year WCNC 2004 / IEEE Communications Society 1353 0-7803-8344-3/04/$20.00 © 2004 IEEErate of 54Mbps and a maximum throughput rate of 22Mbps. Thus, 802.11g seeks to capture many of the benefits of both 802.11a and of 802.11b. The propagation of radio signals by the 802.11a, 802.11b, and 802.11g protocols is influenced by many factors. As a result, performance figures stated by manufacturers can be misleading, depending on the specific office environment. Radio frequency barriers, such as wood, glass, and metal, all affect the ability to correctly demodulate a signal received at the destination AP. This consequently leads to an unpredictable quality of transmission for each device depending on its location within its relative environment within the office. Also, due to various inherent network overheads, the maximum raw data link rates and ranges specified by manufacturers are typically twice the “realistically” achievable data rates and ranges seen by end users. Thus, the actual effective throughput of a wireless enabled laptop can vary dramatically depending on the environment in which it is placed. A limited number of experimental studies have been reported in the literature that directly compares the various WLAN protocols [3][8]. The primary findings of these studies indicate the following. • The maximum effective range of both 802.11a and 802.11b is approximately 225 feet in a typical office environment. • At distances between 0 and 85.5 feet, 802.11a performs from 3 to 5 times better than 802.11b and outperforms 802.11b by at least a factor of 2 throughout the entire 225 feet range. • The 802.11b protocol is capable of maintaining a data link rate of 11Mbps for up to 107.5 feet. The 802.11b standard uses DSSS modulation, while 802.11a uses OFDM. The use of multiple sub-carriers provides increased bandwidth for OFDM devices. Simple physics indicates that range is proportional to wavelength, leading to the suggestion that the OFDM waveform at 2.4 GHz should result in a greater range than the 802.11a protocol, operating at 5 GHz. This is the motivation for the 802.11g protocol. The Atheros Corporation reports a performance comparison between the 802.11g and the 802.11a protocols [3]. This study also provides a general validation of the performance of the 802.11a protocol obtained by the Intersil Corporation [11]. These studies indicate


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