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Max Frame Length Changes

Max Frame Length Changes. Authors:. Date: 2011-01-17. Contents. Max management frame length change Max MPDU length in VHT Capabilities element A-MPDU length limit. Max management frame length (1). In 802.11REVmb_D6, the max management frame body length is limited to 2324 octets

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Max Frame Length Changes

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  1. Max Frame Length Changes Authors: Date: 2011-01-17 Yong Liu, Marvell

  2. Contents • Max management frame length change • Max MPDU length in VHT Capabilities element • A-MPDU length limit Yong Liu, Marvell

  3. Max management frame length (1) • In 802.11REVmb_D6, the max management frame body length is limited to 2324 octets • Based on the max MSDU size 2304 octets • In 11ac, longer management frame is desired to carry VHT/MU beamforming reports, esp. when Nr > 4 and/or channel BW > 80MHz • Otherwise, beamforming report segmentation is required • The management frame body is not an MSDU, thus need not be limited by the max MSDU size Yong Liu, Marvell

  4. Max management frame length (2) Max MPDU = 11454 octets • Proposal: • If a management MPDU is sent using a VHT PPDU, the length of the MPDU is constrained by the maximum MPDU length supported by the recipient • The Max Frame Body Length can be up to: 11422 octets = 11454 – 28 (MAC header) – 4 (FCS) octets • VHT STAs supporting larger management frames need not segment beamforming reports for most cases • If a management MPDU is not sent using a VHT PPDU, the maximum management MPDU size is 2356 octets (still limited by the max Frame Body length 2324 octets) Yong Liu, Marvell

  5. Max MSDU/A-MSDU Length • In 802.11REVmb_D6: • 8.2.3 General frame format • The Frame Body field is of variable size. The maximum frame body size is determined by the maximum MSDU size (2304 octets) or the maximum A-MSDU size (3839 or 7935 octets, depending upon the STA’s capability),(11n) plus any overhead from security encapsulation.(#2211) • Frame body: 7955 = 7935 (max A-MSDU) + 20 (TKIP overhead) • Note that here max MSDU/A-MSDU lengths are fixed, while max MPDU length is variable Yong Liu, Marvell

  6. Max MPDU Length (1) • In 11ac, the max MPDU length is fixed (11454 octets), while the max A-MSDU size can be variable • Max A-MSDU length = Max MPDU length (11454) – MAC header length (36) – CCMP overhead (16) – FCS (4) = 11398 octets • 11414 octets Frame Body should count in the security overhead • It is clearer to define the max supported MPDU length capability • The max supported MPDU length can be used to calculated the max length of the management frame body Max MPDU = 11454 octets Yong Liu, Marvell

  7. Max MPDU Length (2) • Proposal • Replace the max A-MSDU length subfield in the VHT Capabilities element with a max MPDU length subfield • If a VHT STA does not support 11454 octet max MPDU length, it shall set the max VHT MPDU length in the VHT Capabilities element consistent with the max A-MSDU length in the HT capabilities element Yong Liu, Marvell

  8. MU-MIMO paired STAswith different A-MDPU length limit 802.11n A-MPDU length limit: If the A-MPDU includes any padding delimiters (i.e., delimiters with the Length field set to 0) in order to meet the MPDU start spacing requirement, this padding is included in this length limit. Case study: STA1’s A-MPDU length limit is shorter than STA2’s A-MDPU length limit. EOF is counted in A-MPDU length limit. AP has to limit A-MPDU for STA2. STAs with long A-MPDU length limit have to wait for the next A-MPDU transmission. Data field for STA#1 EOF EOF STA#1 Data field for STA#2 Remained data to transmit. STA#2 STA2’s A-MPDU length limit. MU-MIMO Tx to STA#1 and STA#2 STA1’s A-MPDU length limit. Because of STA1’s A-MPDU length limit, AP can transmit until the STA1’s A-MDPU length limit. STA1’s A-MPDU length limit. Slide 8 Byeongwoo Kang, LG Electronics Yong Liu, Marvell

  9. Proposal Adding the following text in the specification: “In case of null padding with EOF, the AP will not count the padding delimiters for the length limit.” Result STA1’s A-MPDU length limit is shorter than STA2’s A-MDPU length limit. EOF is not counted in A-MDPU length limit. AP does not have to limit STA2’s A-MDPU. EOF EOF Data field for STA#1 EOF EOF STA#1 Data field for STA#2 STA#2 STA2’s A-MPDU length limit. MU-MIMO Tx to STA#1 and STA#2 STA1’s A-MPDU length limit. Slide 9 Byeongwoo Kang, LG Electronics Yong Liu, Marvell

  10. Pre-motion 1 • Should the spec framework document be updated to define the Max VHT Management Frame Length as following? • If a management MPDU is sent using a VHT PPDU, the length of the MPDU is constrained by the maximum MPDU length supported by the recipient • The Max VHT Management Frame Body Length can be up to: 11422 octets = 11454 – 28 (MAC header) – 4 (FCS) octets • Yes • No • Abstain Yong Liu, Marvell

  11. Pre-motion 2 • Should the spec framework document be updated toreplace the Maximum A-MSDU Length subfield in the VHT Capabilities element with the max MPDU length subfield • Set to 0 for 3895 octets (Maximum A-MSDU Lenth in HT Capabilities set to 3839 octets) • Set to 1 for 7991 octets (Maximum A-MSDU Length in HT Capabilities set to 7935 octets) • Set to 2 for 11454 octets (Maximum A-MSDU Length in HT Capabilities set to 7935 octets) • The value 3 is reserved • Yes • No • Abstain Yong Liu, Marvell

  12. Pre-motion 3 • Should the spec framework document be updated with the following rule: • In case of null padding with EOF, the transmitter shall not count the padding delimiters for the A-MPDU length limit • Yes • No • Abstain Yong Liu, Marvell

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