We will now take a look at the frame formats of the different frames that are transmitted in a Block ACK session negotiation and termination.  The Three frames that are used in the Block Ack session negotiation setup and termination are

• ADDBA Request frame
• ADDBA Response frame
• DELBA frame

All three frames are action frames

ADDBA request frame format

The Frame is shown pictorially below

Fig Courtesy: 802.11 Standard

• The Category field is set to 3 – indicating Block Ack
• Block Ack Action field is set to 0 – representing ADDBA Request
• Dialog token is set to a non-zero value that is sent back in the ADDBA response
• Block ACK Parameter set – contains information
  • Whether A-MSDU is supported in the aggregation (this field was a reserved field pre-802.11n)
    • 1 – A-MSDU permitted
    • 0 – A-MSDU not permitted
  • The block ACK Policy – the block ACK policy is set to
    • 1 – immediate Block Ack
    • 0 – Delayed Block Ack
  • The TID for which the Block Ack session is requested
  • A Buffer size field that indicates the number of buffers that should be allocated for this TID.
    • If A-MSDU field is 0 – each buffer can contain the number of octets equivalent to the maximum size of the MSDU supported
    • If A-MSDU field is 1 – each buffer can contain the number of octets equivalent to the maximum size of the A-MSDU supported

The Block ACK Parameter set is shown below

Fig Courtesy: 802.11 Standard

• Block Ack Timeout Value – this indicates the maximum time after which the Block Ack session will be terminated if no frame exchanges are seen for that duration
  • The Block Ack timeout field is provided below (2 bytes long)

Fig Courtesy: 802.11 Standard

• Block Ack Starting Sequence control – this field indicates the starting sequence number of the first frame to be sent after the Block Ack session is negotiated. The fragment number is always set to 0.

Fig Courtesy: 802.11 Standard

Block ACK Frame Formats – ADDBA Response