1. VHT Frame Padding Date: 2010-03-16 Authors: Month Year
doc.: IEEE yy/xxxxr0
VHT Frame Padding
Date:
Authors:
Yong Liu, Robert Stacey et al.
John Doe, Some Company

2. Changes from r2
Use L-SIG length/rate to indicate PPDU duration (number of symbols)
The duration field (12-bit) is no longer needed in VHT-SIG A
Yong Liu, Robert Stacey et al.

3. Outlines 802.11n (HT) frame padding VHT frame padding
Month Year
doc.: IEEE yy/xxxxr0
Outlines
802.11n (HT) frame padding
VHT frame padding
802.11n (HT) A-MPDU
VHT A-MPDU
Early End-of-Frame indication
Conclusions
Yong Liu, Robert Stacey et al.
John Doe, Some Company

4. Month Year
doc.: IEEE yy/xxxxr0
802.11n (HT) frame padding
A length field (16 bits) is included in HT-SIGs to indicate the end of PSDU
Tail bits and PHY Pad are appended to fill in the last symbol
Yong Liu, Robert Stacey et al.
John Doe, Some Company

5. Month Year
doc.: IEEE yy/xxxxr0
VHT frame padding
L-SIG length and rate indicate PPDU duration (number of symbols)
No per user length (number of octets) indication in preamble
MAC provides an A-MPDU that fills the frame to the last byte for each per-user stream
PHY provides 0-7 bits of padding
Same preamble structure is used for both SU and MU VHT frames
Require that A-MPDU always be used with both SU and MU VHT frames
Yong Liu, Robert Stacey et al.
John Doe, Some Company

6. Month Year
doc.: IEEE yy/xxxxr0
802.11n (HT) A-MPDU
Except when it is the last A-MPDU subframe in an A-MPDU, padding octets are appended to make each A-MPDU subframe a multiple of 4 octets in length.
Any subframe may be preceded by one or more zero-length delimiters to meet the minimum MPDU Start Spacing requirement, for example
Yong Liu, Robert Stacey et al.
John Doe, Some Company

7. VHT A-MPDU
VHT A-MPDU subframes have the same structure as the subframes of an n A-MPDU
The QWord MAC Pad (0-3 octets) is added to reach the next 32-bit boundary of MAC octets, if such octets fit into the last PHY symbol (Note that normally, the LAST subframe is NOT padded)
Zero or more Null subframes and a Final MAC Pad (0-3 octets) are appended to extend the A-MPDU to the last octet of the last symbol of a VHT frame
Null subframes are absent if last PHY symbol is full with 0-3 octets of Final MAC Pad after the QWord MAC Pad
Yong Liu, Robert Stacey et al.

8. Early EOF indication
The Null subframes appended to the end of a VHT A-MPDU can be special Padding Delimiters each with an EOF flag
When RX MAC detects the EOF Padding Delimiter, it may inform RX PHY to stop receiving to conserve power
Yong Liu, Robert Stacey et al.

9. Conclusions
L-SIG length and rate indicate PPDU duration (number of symbols)
No per user length (number of octets) indication in preamble
Zero or more Null subframes and a Final MAC Pad can be appended to a VHT A-MPDU to extend the A-MPDU to the last byte of the VHT frame
In order to maintain the same preamble structure for both SU and MU frames, all VHT frames use A-MPDU structure
The Null subframes appended to the A-MPDU can include an EOF indication in the delimiter so that the receiver knows it can stop receive processing to conserve power
Yong Liu, Robert Stacey et al.

10. Reference 11-10-0070-02-00ac-802-11ac-preamble.ppt
Yong Liu, Robert Stacey et al.

11. Spec framework changes (1)
Add a section on A-MPDU format under Frame Formats:
6.3 Frame formats
6.3.1 A-MPDU format
The VHT A-MPDU format is an extension of the n A-MPDU as shown in Figure YY. The extension (shaded in the figure) consists of zero or more delimiters with MPDU length zero and a possible final MAC Pad of less than 4 octets. The A-MPDU of a VHT PPDU fills the available octets in the payload.
Figure YY – A-MPDU format for VHT PPHU
Yong Liu, Robert Stacey et al.

12. Spec framework changes (2)
Add a requirement to the power saving section:
6.1 Power saving
R6.1.A: The delimiters with MPDU length zero that pad through the end of the frame shall include an indication that no additional MPDUs are present in the frame. This would allow the receiver to stop receive processing to conserve power.
Yong Liu, Robert Stacey et al.

13. Straw Poll
Should the spec framework document (09/992) be updated to include the changes shown on slide 11 and 12 of this presentation?
Yes: 33
No: 14
Abstain: 4
Yong Liu, Robert Stacey et al.