Performance Analysis and Evaluation of IEEE 802.11E MAC in WLANs with Hidden Stations and Multimedia Applications

Introduction

The wireless local area network (WLAN) is a flexible data communication system that can either replace or extend a wired LAN to provide location-independent network access between computation and communication devices using waves rather than a cable infrastructure (IEEE Computer Society, 1999; Gast, 2002). WLAN is becoming more widely recognized as a general-purpose connectivity alternative for a broad range of business organizations owing to its simplicity, flexibility, and accessibility independent of location, as well as its ability for wireless stations to roam throughout the business organizations. Many practical WLANs have employed the Institute of Electrical and Electronics Engineers (IEEE) 802.11 standards ratified in 1997 that can operate at data rates up to 2 Mbps in the 2.4-GHz industrial, scientific and medical (ISM) band. But the most general business requirements cannot be well supported by the slow data rate of the legacy IEEE 802.11 standard. Recognizing the critical need to support higher data-transmission rates, the IEEE ratified both 802.11a and 802.11b standards with the rates up to 54 and 11 Mbps in the 5 and 2.4-GHz ISM band, respectively (IEEE Computer Society, 1999; Gast, 2002). Moreover, both standards identify the specification of the Medium Access Control (MAC) protocol, which is responsible for efficient control access to the wireless transmission medium.

The IEEE 802.11 MAC protocol offers two different methods to support share access to wireless channels: a Distributed Coordination Function (DCF) and an optional Point Coordination Function (PCF) (Gast, 2002). The former is the dominant MAC mechanism implemented in the IEEE 802.11-compliant products and is based on the Carrier Sense Multiple Accesses (CSMA) protocol making certain that the stations sense the medium prior to data-transmission. Moreover, the DCF applies a collision avoidance (CA) mechanism, which can reduce the collision probability using an additional random binary exponential time called back-off time. The main objective of CSMA/CA is to avoid stations transmitting at the same time, which can lead to collisions and corresponding retransmissions (IEEE Computer Society, 1999; Choi et al., 2003; Gast, 2002; Lindgren et al., 2001). However, hidden stations in WLANs often cause serious problems and performance degradation. A pair of stations are said to be hidden from each other if the transmission from one station cannot be heard by the other. As a result, two or more hidden stations may transmit data at the same time, causing a collision and MAC failure. Thus, additional collision avoidance protocol becomes necessary to combat the hidden station problem. Apart from the common CSMA/CA techniques, the DCF further reduces the possibility of collisions using the popular collision avoidance scheme that consists of channel reservation frames (i.e., Request-To-Send and Clear-To-Send). Different from DCF, the optional coordination function PCF is a centralized scheme designed for infrastructure networks that have a point coordinator operating at the access point (AP) to poll and select the next wireless station for data-transmission (Gast, 2002).