In this context, a slot is a channel time period earmarked for the polled frame exchange sequence with a particular station. But unless all uplink and downlink frames are sent at the same PHY rate, taking the exact same amount of channel time, then the slots will have variable duration. That contradicts maintaining the fixed timing relationship needed for effective power-save synchronization.
Some vendors prefer to operate all the stations at a fixed rate (6 Mbits/s) to avoid this problem. But, if variable rates are used, then either the schedule must be changed and communicated reliably to each associated stationnot a good idea because of extra overhead and reliability issuesor the AP must transmit something in the unused time of each slot so that non-polled stations won't fill in the dead air with Wi-Fi packets, thus further perturbing the attempt to make a synchronous schedule.
Lastly, suppose that an AP is supporting a mix of handsets using different codec intervals, a likely scenario. In this situation, it may be impossible to construct a polling schedule that doesn't include periodic and frequent timing conflicts between different CBR clients, leading to a right-shift of the schedule.
Other effects that will perturb the ideal HCCA schedule are the occasional need to send more than one downlink VoIP frame to a station. This happens when packets arrive in bunches at the AP because of Internet or routing queuing behavior. When this happens, the AP must transmit multiple downlink frames instead of the usual single frame. This will right-shift the schedule unless every conceptual slot has sufficient extra time budget. The same right-shift happens if there's an uplink retransmission. It's doubtful that the AP designer will reserve extra slot time for every station in every slot. That wastes channel time. Instead, the likely decision is to allow the schedule to shift to the right at the expense of extending the power-on time for each affected downlink station.
The HCCA approach can be characterized as an N-body synchronization scheme, whereby the AP sets up a polling schedule for N stations that attempt to stay synchronized with the AP in spite of schedule perturbations. It's fair to characterize this as an N-body problem because timing anomalies with any station on the polling for either uplink or downlink traffic effect the timing of the other N-1 participants. The timing interdependence of polling must be compared to the timing independence power-save methods applied to EDCA, which is the other QoS extension to Wi-Fi defined within WMM.
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