1 / 16

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

max
Download Presentation

A-MPDU Delimiter Changes

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript


  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

More Related