After a Block ACK Session is negotiated between an Access Point and a WLAN station connected to the Access Point, the AP and the WLAN station can partake in a Contention Free Burst within the scope of the block ACK session.
After the Frame burst is complete, the WLAN station would send a Block ACK request to the Access Point requesting the Access point to ACK the frames that the WLAN station just sent. The Access Point would respond with a Block ACK frame with a bitmap that would contain the number of packets properly received by the AP.
If the Access Point were to send Data packets to the WLAN station, then the AP would send a Block ACK Request to the WLAN station and the Block ACK frame will be sent by the WLAN station to the Access Point.
Both the Block ACK Request and the Block ACK frame are discussed in the current and the next article
Block Ack Request Frame
The Block Ack Request frame is used for the following purposes
- To obtain the ack for the previous frames sent in the TXOP (via aggregation/or frame burst in SIFS duration)
- To clear the receive re-order buffer at the receiver
Fig Courtesy: 802.11 Standard
- Duration field carries the time required to transmit one Block Ack + one SIFS duration
- The Receiver address contains the address of the recipient station
- The Transmitter address contains the address of the station sending the block Ack Request frame
- The BAR control field is shown below
Fig Courtesy: 802.11 Standard
- The Bar Ack Policy field is reserved in the case of legacy block ACK agreement (non-aggregation case). In the case of a HT immediate Block ack and Delayed block ACK, the BAR ack policy field indicates the following as described below in the standard
Fig Courtesy: 802.11 Standard
- The Multi TID field indicates if the aggregation and block ack arrangement works for multiple TIDs. The Multi-TID field works in conjunction with the compressed bitmap fieldThe Compressed Bitmap field indicates whether a compressed block ACK will be sent or a non-compressed block ACK will be sent. The description of the encoding of the compressed bitmap field and the Multi-TID field is shown below
Fig Courtesy: 802.11 Standard
- The TID_INFO and the BAR information field work in conjunction to provide information on the number of TIDs in the request and starting sequence control and per TID info in the case of Multi-TID aggregation.
- When Multi-TID aggregation is not supported – The TID_INFO field contains the TID for which the block ACK request is sent out. The BAR information field contains the Starting sequence number of frame from which the block Ack is desired.
- In the case of a Multi-TID block ack – the TID_INFO field indicates the number of TID+1 that are present in the block ACK request. As an example – if the TID_INFO is 2 – then 3 TIDs are present. The BAR Information field indicates the Per-TID info and the Block Ack Starting Sequence control.
Fig Courtesy: 802.11 Standard
- The Per TID info field contains the TID for which Block ACK is requested. Each TID request will have a separate Per-TID information field. The Per-TID information field is shown below from the standard
