The access point (AP) should be able to support a large number of associated stations (STAs) (e.g. over 6000) which may operate on a very strict energy budget. Such devices could be battery powered sensors which transmit and receive data very rarely and stay in the low power operation mode for relatively long periods of time. The basic operation mode is the distributed coordination function (DCF) mode. In order to support a large number of STAs operating in random access mode the access point may utilize specific techniques to restrict contention to the channel to avoid of collisions of simultaneous transmissions. One such technique to reduce contention and collisions of transmission is a method of grouping STAs to multiple groups and assigning certain parameters for each group to indicate the specific group that can access content for the channel. Such grouping information and parameters for the operation can be delivered to the STAs in an association phase or in broadcast information such as beacons.
The STAs may operate in low power mode for prolonged periods of time and therefore the grouping related parameters may be not be valid anymore when a STA wakes up and resumes the channel access operation. Broadcast messages such as beacons are transmitted only occasionally and if a STA stays in low power mode for extended periods of time it may lose synchronization (due to the internal clock drift in the AP and in the STA) and is not able to estimate when the next beacon would be transmitted. This may cause the STA to stay awake for relatively long period trying to receive a beacon transmission.
In 802.11 protocol the AP buffers data frames if the STA is in the low power state. The AP informs the STAs about the buffered frames by indicating buffered data in the traffic indication map (TIM) which is transmitted in the beacon message. Once the STA awakes it can retrieve the buffered frames by transmitting some buffered uplink (UL) data which implicitly notifies the AP that the STA is awake, or the STA can transmit a power save poll (PS-poll) to indicate to the AP that it is awake and ready to receive data. Alternatively any frame that is classified to be a so called ‘trigger frame’ indicates that STA is awake.
To improve data exchange between AP and STAs, speed frame exchange can be initiated between AP and STA (or between STA and another STA). AP may have data in the DL buffer but is not able to transmit all the data in the same session since it may have some pending management frames in the buffer (such as a beacon transmission) or other control/data frames it may need to deliver.