Characterizing the IEEE 802.11 Traffic: The Wireless Side

Many studies on measurement and characterization of wireless LANs have been performed recently. Most of these measurements have been conducted from the wired portion of the network based on wired monitoring or SNMP statistics. In this paper we argue that traffic measurements from a wireless vantage point in the network are more appropriate than wired measurements or SNMP statistics, to expose the wireless medium characteristics and their impact on the traffic patterns. While it is easier to make consistent measurements in the wired part of a network, such measurements can not observe the significant vagaries present in the wireless medium itself. As a consequence constructing an accurate measurement system from a wireless vantage point is important but usually quite difficult due to the noisy wireless channel. In our work we have explored the various issues in implementing such a system to monitor traffic in an IEEE 802.11 based wireless network. We show the effectiveness of the wireless monitoring by quantitatively comparing it with SNMP and measurements at wired vantage points. We also show the analysis of a typical computer science department network traffic using the wireless monitoring technique. Our analysis reveals rich information about the PHY/MAC layers of the IEEE 802.11 protocol such as the typical traffic mix of different frame types, their temporal characteristics, correlation with the user activities and the error characteristics of the wireless medium. Moreover, we identify anomalies in the operation of the IEEE 802.11 MAC protocol. Our results show excessive retransmissions of some management frame types reducing the useful throughput of the wireless network. We also find that some features of the protocol, which were designed to reduce the retransmission errors, are not used. In addition, most of the clients fail to adapt the data rate according to the signal condition between them and the access point, which further reduce the useful throughput.