5.3 Radio Resource Capture

103

However, this is usually not the case, especially at times of high traffic load. See Figure 5.29 for an illustration of this problem. Three BSSs are shown, where each BSS can only detect transmissions of its respective neighbor BSSs. BSS 1 and BSS 3 are hidden to each other. Hence, BSS 1 and BSS 3 can independently operate at the same time and are uncoordinated without synchronized channel access. Once a station of BSS 1 started a frame exchange, stations of BSS 2 will defer from channel access. However, this implies that a station of BSS 3, which does not detect the ongoing frame exchange in BSS 1, starts its frame exchanges independently during the ongoing frame exchange. This can continue with any following frame exchange as long as BSS 1 and BSS 3 have data to deliver. As a result, the stations of BSS 2 may not be able to transmit for longer time durations, and the channel is captured by stations in the neighborhood of BSS 2. This leads to increased MSDU Delivery delays and reduced throughput in BSS 2.

5.3.2Radio Resource Capture through Channels that Partially Overlap in the Spectrum

Similarly, the same capture effect can occur also when frequency channels overlap in the frequency domain. See Figure 5.30 for an illustration of this problem. Now all stations are in close vicinity to each other such that any transmission is detected by any station, across all BSSs. However, it is here assumed that every BSS operates at a different frequency channel, as indicated in Figure 5.30. This implies that the three BSSs can operate without mutually interfering each other, as long as the used frequency channels are orthogonal, i.e. as long as frequency channels do not overlap in the frequency domain. When frequency channels overlap, the same resource capture as described in the last section may occur. In the example of Figure 5.30, stations of BSS 1 operating at channel 2 and stations of BSS 3 operating at channel 4 can initiate frame exchanges without mutual interference. The two BSSs can operate independently. The BSS 2 operates on channel 3, which overlaps with the two other channels. Therefore, the CCA processes of all stations in BSS 2 may not find an idle channel for longer time durations, for the same reason as explained in the last section. Note that overlapping frequency channels are not standardized for the 5 GHz band.

5.3.3Solution

To reduce the unwanted effects of the resource capture, Benveniste (2001) proposes an efficient and simple solution based on synchronous time division, here referred to as slotting. In the following, a way to introduce such a slotting in 802.11(e) is presented.

104	5. Evaluation of IEEE 802.11e with the IEEE 802.11a Physical Layer
	BSS 2 may
	experience heavily
	loaded channel	BSS 1
	BSS 2	BSS 3

20 MHz per channel, 0 overlap assumed

OFDM power density spectrum

out of channel emissions

			BSS 1 and BSS 3
			perform carrier
			sensing independently
	2	4	6
1	3		5

frequency

Figure 5.30: Resource capture with frequency channels that overlap in the frequency domain. As before, stations of (Q)BSS 2 may not detect an idle channel for undesirable long periods.

5.3.3.1Mutual Synchronization across QBSSs and Slotting

Two modifications of the 802.11e protocols are required to enable coexisting QBSSs or coexisting IBSSs to synchronize the access. Figure 5.31 illustrates the two modifications. One is that beacons must be transmitted in contention by all stations of a QBSS, as is the case in an IBSS. Stations that are associated to an HC also must transmit the beacon. It is not necessary for such stations to deliver all information an HC usually transmits with its beacon. For the purpose of synchronization of neighbored BSSs, only the TSF information is required.

The second modification to the standard is that all stations, regardless of whether they are an AP or a station of a QBSS or IBSS, must update their timers according to the rules of the IBSS. Exactly this will guarantee the synchronization of neighbored BSSs. By applying these two modifications in 802.11, overlapping QBSSs are mutually synchronized without loosing any functionality of the protocol. Synchronous time division, i.e., slotting, is possible by defining a slot dwell time. With a slot dwell time, frame exchanges of a BSS are allowed to start only if they can be finished before the end of the respective slot, i.e., without exceeding the slot dwell time. After expiry of a slot dwell time, stations of the BSSs contend for the next frame exchange in parallel. Because the slot dwell times are synchronized, all stations of all BSSs content at the same time, which provides a fair ac-