1. A-MPDU Delimiter Changes
May 2010
doc.: IEEE /0570r0
September 2010
A-MPDU Delimiter Changes
Date:
Authors:
Robert Stacey, Intel
Sameer Vermani, Qualcomm

2. A-MPDU Delimiter Changes
September 2010
A-MPDU Delimiter Changes
Increase Length field to accommodate longer MPDU
Support ACK response to QoS Data frame
Robert Stacey, Intel

3. September 2010
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)
Marvell
MAC Efficiency Calculations (1)
TXOP Limit = 3ms (w/ truncation), AIFS=34us, Avg CW = 67.5us, MSDU 1500B, BA buffer limit = 32

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

6. A-MPDU Delimiter format
September 2010
A-MPDU Delimiter format B0 B1
B B3
B B15
B16 B23
B B31
EOF
Reserved
MPDU Length Extension
MPDU Length
CRC
Delimiter Signature
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
September 2010
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
September 2010
MAC efficiency with ACK vs BA
440us
44us
68us
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%
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
Robert Stacey, Intel

9. Data transfer before BA agreement is in place
September 2010
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
September 2010
Data transfer with BA agreement
Data
Data/Data/Data/Data (A-MPDU)
ACK BA
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
Robert Stacey, Intel

11. Why signal in A-MPDU delimiter?
September 2010
Why signal in A-MPDU delimiter?
Options for Single MPDU signaling:
VHT-SIG-A
SIGNAL 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 and protocol
PHY
MAC
Robert Stacey, Intel

12. Use EOF bit for Single MPDU signaling
September 2010
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. Single MPDU Protocol Rules
September 2010
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” 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 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

14. September 2010
Strawpoll #1
Do you support updating the spec framework document to show the A-MPDU delimiter format extended from n 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 B0 B1
B B3
B B15
B16 B23
B B31
EOF
Reserved
MPDU Length Extension
MPDU Length
CRC
Delimiter Signature
Robert Stacey, Intel

15. September 2010
Strawpoll #2
Do you support updating the spec framework document to include the Single MPDU protocol rules on slide 13?
Robert Stacey, Intel