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A-MPDU Delimiter Changes

A-MPDU Delimiter Changes. Authors:. Date: 2010-09-12. A-MPDU Delimiter Changes. Increase Length field to accommodate longer MPDU Support ACK response to QoS Data frame. Increase Length field. The Length field in current A-MPDU delimiter is 12 bits Supports a 4KB MPDU

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A-MPDU Delimiter Changes

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  1. A-MPDU Delimiter Changes Authors: Date: 2010-09-12 Robert Stacey, Intel

  2. A-MPDU Delimiter Changes • Increase Length field to accommodate longer MPDU • Support ACK response to QoS Data frame Robert Stacey, Intel

  3. Increase Length field • The Length field in current A-MPDU delimiter is 12 bits • Supports a 4KB MPDU • At VHT data rates, we need to support larger MPDUs to maintain same MAC efficiency • Up to 12KB MPDU is needed to maintain efficiency with 64 MSDU BA window limit Robert Stacey, Intel

  4. MAC Efficiency Calculations (1) TXOP Limit = 3ms (w/ truncation), AIFS=34us, Avg CW = 67.5us, MSDU 1500B, BA buffer limit = 32 Marvell

  5. MAC Efficiency Calculations (2) TXOP Limit = 3ms (w/ truncation), AIFS=34us, Avg CW = 67.5us, MSDU 1500B, BA buffer limit = 32 Marvell

  6. A-MPDU Delimiter format • Length extension occupies 2 reserved bits and contains high order length bits • Note – all fields are LSB first so LSB of length is in B4, LSB of length extension is in B2 • EOF (already in spec framework) occupies a currently reserved bit Robert Stacey, Intel

  7. Support ACK response to QoS Data • VHT frame format always contains A-MPDU and has no Aggregate bit • 11n has a protocol rule: • If Aggregate = 0 and QoS Control Ack Policy = ‘normal ack’ • Respond with ACK • If Aggregate = 1 and QoS Control Ack Policy = ‘normal ack’ • Respond with BA • We need a signaling mechanism in 11ac that supports the same protocol rule • Maintain a common protocol engine between 11n and 11ac • Support a slightly more efficient protocol at low MCS • Support QoS data transfer without BA agreement Robert Stacey, Intel

  8. MAC efficiency with ACK vs BA 440us 44us 68us • 29.25 Mbps (1x1, 80 MHz, BPSKr1/2) • Includes contention overhead and 10% PER • Using ACK instead of BA provides a small efficiency increase • Between 1% and 3% • Slightly more if we consider TCP traffic with 40B ack packets 1500B Data ACK MAC efficiency = 61.0% BA MAC efficiency = 58.7% 2.1ms 5 x 1500B Data (A-MSDU) ACK MAC efficiency = 81.1% BA MAC efficiency = 80.2% Robert Stacey, Intel

  9. Data transfer before BA agreement is in place • After association, there is a management exchange to establish the BA session • Without BA agreement, initiator cannot send data using VHT frames • Initiator would need buffer traffic until BA session was established • Or initiator would need to use HT/11a format packets • Both options require change from 11n • In 11n you simple send non-aggregate packets until BA session is in place Robert Stacey, Intel

  10. Data transfer with BA agreement • Even with BA agreement in place, it is useful to do a Data/ACK exchange • As a collision detect mechanism at start of a long TXOP • When you only have a single MPDU to send and all previous MPDUs have been acknowledged Data Data/Data/Data/Data (A-MPDU) ACK BA Robert Stacey, Intel

  11. Why signal in A-MPDU delimiter? • Options for Single MPDU signaling: • VHT-SIG-A • SEVICE field • A-MPDU delimiter • We prefer signaling in A-MPDU delimiter: • Purely a MAC function: affects protocol only • Different from 11n where Aggregate bit had two functions: defined frame format (MPDU vs A-MPDU) and protocol PHY MAC Robert Stacey, Intel

  12. Use EOF bit for Single MPDU signaling EOF=0, length>0 EOF=0, length=0 EOF=1, length=0 A-MPDU MPDU MPDU Multi-MPDU frame Delimiter Alignment Pad Delimiter Delimiter Alignment Pad Delimiter Delimiter Delimiter MAC EOF Pad EOF=1, length>0 EOF=1, length=0 A-MPDU MPDU Single MPDU Delimiter Alignment Pad Delimiter Delimiter Delimiter MAC EOF Pad Delimiter with EOF=1, length>0  single MPDU frame Robert Stacey, Intel

  13. Management Frames • 802.11n currently disallows sending management frames in A-MPDU • This rule simple ensures that management frames are not aggregated (together or with other frames) • We should permit management frames in VHT A-MPDU provided only one management frame is sent Robert Stacey, Intel

  14. Single MPDU Protocol Rules • If an A-MPDU contains a single MPDU, the initiator may set the EOF field in the A-MPDU subframe containing the MPDU to 1 • A responder that receives a QoS Data MPDU with ack policy set to “normal ack/implicit block ack req” shall respond: • With ACK if the EOF field of the subframe containing the MPDU is set to 1 • With BA if the EOF field of the subframe containing the MPDU is set to 0 • An initiator may send a management frame requiring an ACK response in a VHT frame provided it is the only MPDU in the VHT frame and the EOF field on the subframe carrying the MPDU is set to 1 Robert Stacey, Intel

  15. Strawpoll #1 • Do you support updating the spec framework document to show the A-MPDU delimiter format extended from 802.11n as described below and including the figure? • An MPDU Length Extension field is added in B2-B3 and contains the high order bits of MPDU length • An EOF field is added in B0 Robert Stacey, Intel

  16. Strawpoll #2 • Do you support updating the spec framework document to include the Single MPDU Protocol rules on slide 13? Robert Stacey, Intel

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