Block ACK Request
Understanding Block ACK Request
The BA session maintains a sliding window of sequence numbers. The receiver tracks which MPDUs within the window have been received. The BAR's SSN advances this window: any MPDUs with sequence numbers before the SSN are considered complete (acknowledged or abandoned). This prevents the window from stalling when frames are lost.
In 802.11n+ with A-MPDU, each aggregated burst implicitly triggers a BA response. The BAR is needed mainly for error recovery when a BA is lost or when the transmitter needs to advance the window without sending new data.
SIFS: 16 μs
BA response: 32 bytes at 6 Mbps = 43 μs
Total exchange: ~91 μs
Implicit BA with A-MPDU avoids this overhead
Block ACK Frame Exchange
| Step | Frame | Direction | Duration |
|---|---|---|---|
| 1 | A-MPDU (data burst) | TX → RX | Variable |
| 2 | SIFS | — | 16 μs |
| 3 | Block ACK (bitmap) | RX → TX | ~43 μs |
| Alt | BAR (if BA lost) | TX → RX | ~32 μs |
Frequently Asked Questions
When is BAR sent?
When implicit BA was lost, to advance BA window, or to keep the session alive. Rare in 802.11n+ implicit BA mode.
Frame format?
24 bytes: Frame Control, Duration, addresses, BAR Control (TID), Starting Sequence Number. SSN advances the BA window.
Airtime impact?
~91 μs per BAR exchange. Excessive BARs indicate packet loss or BA instability. Good troubleshooting metric.