I am analyzing 802.11 block ACK operation. I am seeing the initial RTS/CTS exchange, followed by a block of QoS data packets, followed by a Block ACK. The capture appears to be missing the BlockAckReq following the block of data packets. Is there some commonly used way to trigger transmission of a block ACK that I am missing? I read something about an "implicit block ack request", but such beast appears to be related to A-MSDUs, which according to my packets are not allowed. Any thoughts? asked 22 Dec '11, 18:49  S_P |
One Answer:
I'm not an expert, but as far as I can tell, what you are seeing most likely is implicit Block Ack request. It is related to A-MPDU, not A-MSDU. If A-MPDU contains at least one MPDU with 'Ack Policy' field set to 'Block Ack' (zero), means it is a implicit Block Ack request. So the receiving station should respond with BlockAck frame. See 802.11n standard, 9.10.7.5 ('Generation and transmission of BlockAck by an HT STA') answered 16 Oct '12, 05:51  pcfist |
I'm not an expert for the new .11n features - but as far as I understand this topic the use of block ACKs is only requested once per session. Did you capture the whole traffic right from the association ?
By my reading of the 802.11 standard, section 9.10, Block ACK operation is initially established between devices using ADDBA Request and ADDBA Response messages, which I see in the capture. Reading 9.10 (e.g., as shown in Figure 9-21), it appears that during operation, the sender of the data block requests the Block ACK with a BlockAckReq message, which should then be followed by the Block ACK from the recipient of the block. It is this BlockAckReq message that I am failing to see in the capture.