1. Remedy to the ‘Missing Ack Problem’
November 2003
Remedy to the ‘Missing Ack Problem’
Mathilde Benveniste, Avaya Labs – Research
Menzo Wentink, Globespan
Submission addresses D5.0 ballot comments: 441, 210, (209, 330, 331, 324, 325, 460, 466, 467, 526)
M. Benveniste (Avaya), M. Wentink (Globespan)

2. Problem of the ‘Missing’ Ack to a Downlink frame
November 2003
Problem of the ‘Missing’ Ack to a Downlink frame
If the AP does not receive the Ack to the last downlink frame sent to a station in power-save mode, the AP will retransmit the same frame
If the station received and acknowledged that frame, the station will be asleep during the retransmission and the AP will keep retransmitting till the Max Retry limit is reached
This is the ‘Missing Ack problem’, first encountered but not addressed in IEEE
It arises when the AP responds to a PS poll, and the Ack from the station is lost
Scheduled APSD is not vulnerable to this problem because the station keeps awake to send uplink frames after receiving downlink frames
But APSD with reverse polling suffers from this problem, as downlink frames are transmitted last
M. Benveniste (Avaya), M. Wentink (Globespan)

3. Remedy to the ‘Missing Ack Problem’
November 2003
Remedy to the ‘Missing Ack Problem’
The Ack is missing because either
the downlink frame was lost, or
the Ack was lost
Retransmission of the frame provides remedy for the lost frame
Since it is possible that the Ack was lost and the station has gone to sleep, exhaustive retransmission (to Max Retry Limit) is wasteful of bandwidth
M. Benveniste (Avaya), M. Wentink (Globespan)

4. November 2003
The QAP
The QAP that receives no acknowledgement to a frame sent to a power-saving station with the ‘last downlink transmission’ indication may retransmit that frame fewer times than the Max Retry Limit but at least once within the same service period, and may wait until the next service period to retransmit that frame.
M. Benveniste (Avaya), M. Wentink (Globespan)

5. Motion Adopt Normative Text changes in doc #03/796r1 November 2003
M. Benveniste (Avaya), M. Wentink (Globespan)