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A-MPDUs with U-APSD

A-MPDUs with U-APSD. Authors:. 2011-03-17. Abstract. This presentation proposes a mechanism by which BUs may be transmitted in A-MPDUs under U-APSD Note: this presentation does not cover BUs transmitted under other forms of block transfer, such as 802.11e Block Acknowledgement.

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A-MPDUs with U-APSD

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  1. A-MPDUs with U-APSD Authors: 2011-03-17 Mark RISON, CSR

  2. Abstract This presentation proposes a mechanism by which BUs may be transmitted in A-MPDUs under U-APSD Note: this presentation does not cover BUs transmitted under other forms of block transfer, such as 802.11e Block Acknowledgement Mark RISON, CSR

  3. U-APSD operation without A-MPDUs • Summary of non-11n behaviour: • AP sends MPDU to STA; if EOSP bit set then: • STA may in principle fall asleep after ACKing the MPDU • If the AP gets the ACK the SP is over • Nice and clean • AP may miss the ACK; in this case the AP retransmits the MPDU at least once but the STA will not ACK retransmissions • MPDU might time out, but that’s OK because STA has correctly received it • If the More Data bit is set then STA awakens (or doesn’t fall asleep) and sends another trigger Mark RISON, CSR

  4. Proposed analogous U-APSD operation with A-MPDUs • Proposal by analogy with non-A MPDU case: • AP sends A-MPDU to STA; all EOSP bits have the same value (see clause 9.12.1); if set and STA correctly receives at least one then: • STA may in principle fall asleep after BlockAcking the A-MPDU • Even if not all MPDUs in the A-MPDU were correctly received! • If the AP gets the BlockAck the SP is over • Even if not all MPDUs in the A-MPDU were acked! • The AP does not transmit an A-MPDU again (but if it does that’s OK) • AP may miss the BlockAck; in this case the AP transmits the A-MPDU again at least once but the STA will not BlockAck the transmission(s) • MPDUs might time out, which would be bad for those which the STA has not correctly received • If the More Data bit is set in the last MPDU it correctly receives then STA awakens (or doesn’t fall asleep) and sends another trigger • AP sets More Data bit to 0 or 1 in the last MPDU per the normal rules • AP sets More Data bit to 1 in MPDUs other than the last MPDU. This increases efficiency if the last MPDU transmitted is not correctly received -- otherwise, how would the STA know it needs to send another trigger to get the missing last MPDU? • If there is a hole in the sequence number space then STA awakens (or doesn’t fall asleep) and sends another trigger • This increases efficiency if an MPDU other than the last MPDU is not correctly received, and the last MPDU has the More Data bit set to 0 -- otherwise, how would the STA know it needs to send another trigger to get the missing non-final MPDU(s)? Mark RISON, CSR

  5. Thought experiments… • AP sends in an A-MPDU three MPDUs, all of which have the EOSP bit set, and all but the last of which have the More Data bit set • If the STA only correctly receives and BlockAcks the first two, it sees the EOSP bit but also the More Data bit in both, so sends another trigger • If the AP does not receive the BlockAck, then it will still be retransmitting the MPDUs and ignore the trigger • If the AP receives the BlockAck, then the SP is over and it does not transmit again; the trigger will cause it to (re)transmit the last MPDU • If the STA only correctly receives and BlockAcks the last, it sees the EOSP bit but also the hole in the sequence number space, so sends another trigger • If the AP does not receive the BlockAck, then it will still be retransmitting the MPDUs and ignore the trigger • If the AP receives the BlockAck, then the SP is over and it does not transmit again; the trigger will cause it to (re)transmit the first two MPDUs • If the STA correctly receives and BlockAcks all three, it sees the EOSP bit and the absence of the More Data bit in the last MPDU and the absence of a hole in the sequence number space, so falls asleep • If the AP does not receive the BlockAck, then it will just retransmit the MPDUs until timeout (unless the STA happens to send another trigger for some reason) Mark RISON, CSR

  6. Proposal • Add spec text to say that: • In A-MPDUs sent by the AP: • The More Data bit shall be set for all MPDUs other than the last MPDU • “last MPDU” means (to be decided): • the last MPDU logically, i.e. the one with the highest SN (mod 4096), so if the A-MPDU contains MPDUs with SN 3,2,1 in that order, the “last MPDU” is the one with SN 3, which is first in the A-MPDU • the last MPDU physically, i.e. the one which comes last in time, so if the A-MPDU contains MPDUs with SN 3,2,1 in that order, the “last MPDU” is the one with SN 1 • The More Data bit in the last MPDU shall be set if there is more data • No change from existing behaviour • The behaviour when the AP sends A-MPDUs under U-APSD is: • The STA should send another trigger if the More Data bit is set in the last MPDU it correctly receives, or if there’s a hole in the sequence number space • Informative advice • The AP shall consider the SP over and should not retransmit (until the next SP) if it receives a BlockAck for an A-MPDU containing MPDUs with EOSP set, even if that BlockAck does not ack all the MPDUs • Already mentioned as a NOTE (see clause 10.2.1.6.j) • The AP shall retransmit at least once if it does not receive a BlockAck for an A-MPDU containing MPDUs with EOSP set • Already implied (see clause 10.2.1.6.j) • The AP shall not include (until the next SP) any new MPDUs in retransmissions where EOSP had been signalled (it’s complicated enough as it is!) • New • To be confirmed: maybe it’s not that more complicated to allow new MPDUs to be added on (either the STA saw at least one of the existing ones, in which case it’s asleep/retriggering, or it didn’t, in which case it doesn’t know a new one has been added) Mark RISON, CSR

  7. References • Draft P802.11-REVmb/D7.03, March 2011 Mark RISON, CSR

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