102	5. Evaluation of IEEE 802.11e with the IEEE 802.11a Physical Layer

5.3Radio Resource Capture

This section is based on Mangold et al. (2002c). Stations that are hidden to each other operate simultaneously, without mutually synchronized transmission, contention and CCA procedures. The contention-based channel access works only efficiently if all stations are in detection range of each other. There, if the channel gets idle at a specific point in time, the CCA processes of all stations indicate the channel idle. Then, stations will start their backoff procedures synchronously, and the backoff entity that first counts down its backoff counter determines the station which transmits next. The other backoff entities will defer from channel access. If this synchronized contention for channel access is not maintained owing to hidden stations, some stations may capture the radio channel for long time durations. This leads to problems in QoS support, as discussed in the following, where two scenarios are discussed. Note that the resource capture phenomenon exists with or without the 802.11e MAC enhancement.

5.3.1Radio Resource Capture by Hidden Stations

The resource capture by hidden stations was identified in Benveniste (2001; 2002), and there referred to as neighborhood capture. A station that is located in the detection range of other stations that are hidden to each other can only initiate a transmission if all of the hidden stations are idle.

BSS 2 may experience heavily loaded channel

BSS 3

	BSS 2
BSS 1	BSS 1 and BSS 3 perform
	carrier sensing independently

Figure 5.29: Scenario where two (Q)BSSs that are not in receive range of each other (BSS 1 and BSS 2) perform their CCA independently. Stations of (Q)BSS 2 may not detect an idle channel for undesirable long periods.