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Max Frame Sizes

Max Frame Sizes. Authors:. Date: 2010-09-13. Contents. 802.11n max frame sizes Larger MPDU/A-MPDU support Motivations CRC-32 characteristics MAC efficiency calculations 802.11ac max frame sizes Conclusions. 802.11n max frame sizes. Max MSDU: 2,304 B Max A-MSDU: 3,839 B or 7,935 B

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Max Frame Sizes

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  1. Max Frame Sizes Authors: Date: 2010-09-13 Yong Liu, Marvell

  2. Contents • 802.11n max frame sizes • Larger MPDU/A-MPDU support • Motivations • CRC-32 characteristics • MAC efficiency calculations • 802.11ac max frame sizes • Conclusions Yong Liu, Marvell

  3. 802.11n max frame sizes • Max MSDU: 2,304 B • Max A-MSDU: 3,839 B or 7,935 B • HT Capabilities element → HT Capabilities Info field → Maximum A-MSDU Length subfield (0: 3,839 B, 1: 7,935 B) • Max Frame body of CCMP MPDU:7919 B • 7919 B = 7935 B – 8B MIC – 8B CCMP header • Max MPDU in A-MPDU: 4,095 B • Limited by 12-bit MPDU Length field in MPDU delimiter • Max A-MPDU in HT-PPDU: 8,191B ~ 65,535 B • Limited by 16-bit HT Length field in HT-SIG • HT Capabilities element → A-MPDU Parameters field → Maximum A-MPDU Length Exponent: (213 -1) B ~ (216 -1) B Yong Liu, Marvell

  4. Larger MPDU/A-MPDU support • Motivations • Wider bandwidth (80/160MHz) => higher PHY data rate => shorter PHY payload duration => higher preamble overhead • The max VHT PPDU/A-MPDU duration is limited by the max number of symbols indicated by L-SIG length, which is 1365 * 4us = 5.46 ms (around 666KB @ 1Gbps, and 1.3MB @ 2Gbps) • 3 reserved bits remain in MPDU delimiter, which can be used to extend the max MPDU length Yong Liu, Marvell

  5. CRC-32 error checking characteristics • The error checking mechanism of CRC-32 was characterized by R. Jain, "Error Characteristics of Fiber Distributed Data Interface (FDDI)," IEEE Transactions on Communications, Vol. 38, No. 8, August 1990, pp. 1244-1252 • FDDI, Ethernet, and 802.11 use the same CRC-32 error checking mechanism • The probability of undetected errors remains constant for frame sizes between 3007 and 91639 bits • 91639b = 91607b data + 32b CRC Yong Liu, Marvell

  6. MAC Efficiency Calculations (1) • Parameters • AIFS: 34us • Average CW: 67.5us • MSDU: 1500B • Max MPDU (~ Max A-MSDU) • 1540B (w/o A-MSDU) • 4095B (w/ A-MSDU) • 8197B (w/ A-MSDU) • 12287B (w/ A-MSDU) • 16383B (w/ A-MSDU) • Max BA window: 32 • BA: legacy rate • TXOP duration: 3ms • TXOP truncation enabled • Note: actual A-MSDU/MPDU and A-MPDU sizes depend on TXOP duration and residual TXOP duration Yong Liu, Marvell

  7. MAC Efficiency Calculations (2) Yong Liu, Marvell

  8. MAC Efficiency Calculations (3) Yong Liu, Marvell

  9. MAC Efficiency Calculations (4) Yong Liu, Marvell

  10. MAC Efficiency Calculations (5) Yong Liu, Marvell

  11. Max MPDU size • Max MPDU size is increased to 11454B • 11454B = 11450B (~91607b) + 4B FCS • Maintain the same CRC-32 error checking characteristics • Improve MAC efficiency for very high data rate • MPDU Length in delimiter is extended to 14 bits Yong Liu, Marvell

  12. Max A-MSDU and A-MPDU sizes • Max A-MSDU • Max A-MSDU = Max MPDU – Max MAC header – FCS • Extend Maximum A-MSDU Length subfield by 1 bit (0: 3,839 B, 1: 7,935 B, 2: {11454 B – Max MAC header – FCS}, 3: reserved) • Max Frame body of CCMP MPDU = Max A-MSDU – 8B MIC – 8B CCMP header • Max A-MPDU • If the max BA window is maintained to 64, the max A-MPDU can be up to 733,056B • Extend Maximum A-MPDU Length Exponent subfield by 1 bit: 0-7 => (213 -1) B (=8191B) ~ (213+7 -1) B (=1MB -1) Yong Liu, Marvell

  13. Conclusions • Propose to extend max MPDU, max A-MSDU, and max A-MPDU sizes to improve MAC efficiency • Define max MPDU, max A-MSDU, and max A-MPDU sizes for 11ac • Extend the 802.11n max frame size negotiation/signaling to support new max frame sizes Yong Liu, Marvell

  14. Reference • 11-10-yyyy-00-00ac-a-mpdu-delimiter-changes.ppt Yong Liu, Marvell

  15. Straw Poll 1 • Should the spec framework document be updated to include the following limits: • Max MPDU = 11454B • Max A-MSDU = 11454B – Max MAC Header - FCS • Max A-MPDU = 1MB • Yes: • No: • Abstain: Yong Liu, Marvell

  16. Straw Poll 2 • Should the spec framework document be updated to include the following capability signaling: • Max A-MPDU length supported as exponent n where 0 <= n <=7 and indicates a max A-MPDU length (2^(13+n)-1)B • Max A-MSDU length supported as 3839B, 7935B or {11454B-Max MAC Header-FCS} • Yes: • No: • Abstain: Yong Liu, Marvell

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