For an 802.11 station to send a Contention Free Burst – it needs to negotiate a Block ACK Session for whichever particular Traffic Identifier (tid) class of packets that intends to use the Block ACK mechanism.
The frame that are part of the Block ACK mechanism are
- ADDBA Request – ADD Block ACK Request – request for a Block ACK session for a particular Transmit Identifier (tid)
- ADDBA Response – ADD Block ACK Response – response from the responder (AP/STA) for an ADDBA request
- DELBA – Delete Block ACK Session – to remove an active Block ACK session
- Block ACK request – The frame sent by a transmitter station after a Contention Free Burst to obtain a Block ACK for the frames transmitted by it
- Block ACK – The frame that is sent by the receiver of the Contention Frame Burst on receipt of a Block ACK Request.
802.11 transmissions might contain fragments (prior to 802.11n). Each packet can be sent as multiple fragments (Maximum 16). Hence, the Block-ACK frame contains a bitmap of ((16 fragments) *(64 frames))/8 = 128 bytes, a bit for each potential fragment and 16 bits for each packet.
802.11n and 802.11ac standards do not support fragmentation and hence the block-ACK bitmap is only 64 bits – a bit for each frame. The smaller bitmap was termed as compressed bitmap.
The block-ACK ADDBA request/response and data transfer with Block-ACK request and Block-ACK and DELBA is pictorially depicted below
Fig Courtesy: 802.11 Standard
- The WLAN station sends an ADDBA request frame to the Access Point requesting a creation of a Block ACK session for a particular Traffic Identifier (TID)
- If the ADDBA request is accepted by the Access Point – the Access Point sends and ADDBA response with status code as success
- Once the block ack session is negotiated, the WLAN station can send a contention free Burst for the TID when it wins the medium (note: the TID for which Block ack session is negotiated is the TID which can take part in the Contention Free Burst)
- The station later sends a block ACK request frame requesting an ACK for the frames sent in the contention free burst
- The AP sends a Block ACK with a bitmap ACKing the different packets sent in the CFB
- A DELBA frame can be used to tear down an existing Block ACK session