Currently, IEEE 802.11 Wireless LAN

Currently, IEEE 802.11 Wireless LAN (WLAN) Working Group (WG) is working on the Media Access Control (MAC) protocol enhancement, under the Task Group TGe [2], in order to support Quality-of-Service (QoS). One possible way to support QoS in 802.11 WLAN is to define/implement a contention-free MAC function on top of the Distributed Coordination Function (DCF), which is based on the Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) protocol. In fact, the current 802.11 MAC has such an entity residing on top of the DCF, named Point Coordination Function (PCF), which is based on a polling mechanism. However, the PCF is not capable of supporting QoS due to its limitations. The DCF and PCF share the media in the following way: there are network frames (or super-frames) that appear repeatedly over time, while a network frame is composed of a Contention Period (CP), when the DCF is used, and a Contention-Free Period (CFP), when the PCF is used. There have been at least two proposals to TGe, which are based on contention-free MAC protocols: one is AT&T MediaPlex proposal [2], which is based on the existing PCF, and the other is Sharewave Whitecap proposal [4], which is based on a dynamic Time-Division Multiple Access (D-TDMA).

One big problem with these contention-free MAC protocols is the operation in case of overlapping Basic Service Sets (BSSs). In this paper, “overlapping BSSs” are referred to as two or more BSSs running at the same channel, where the transmissions by some STAs belonging to one BSS affect some stations (STAs) in the other BSSs. In the situation of overlapping BSSs, the contention-free transmissions are not guaranteed due to the possible channel contentions from STAs in the co-located and/or adjacent BSSs. That means, consequently, it makes very difficult to support QoS in the situation of overlapping BSSs. However, this is an essential and very important problem, which should be solved in order to make supporting QoS within 802.11 WLAN more realistic.

In this paper, we first consider the features that should be included in 802.11e MAC enhancement, in order to support QoS in the BSS overlapping situation. Then, we propose a solution that can prevent the contention from STAs in overlapped BSSs in most cases. The solution is basically using the Request To Send / Clear To Send (RTS/CTS) mechanism during the CFP as part of the underlying contention-free MAC. Note that the RTS/CTS mechanism is defined as part of the DCF in the current standard. This solution is geared toward protecting a QoS-enabled BSS from STAs which belong to other overlapping BSSs, and do not understand the QoS-enabling functions.

In the following section, we list hierarchically a number of features, which should be included in the new MAC enhancement, in order to support QoS in situations of overlapping BSSs. While most of them are being considered within TGe already, this hierarchical view should help understanding the overlapping BSS problem. Section 3 presents the proposed use of the RTS/CTS mechanism during the CFP as part of the contention-free MAC. Then, we also consider when to use this mechanism as well as its limitations. The paper concludes with a brief summary in Section 4.