1. Fragmentation with A-MPDU
February 2008
doc.: IEEE /2376r0
Date: Jan, 2012
Fragmentation with A-MPDU
Date:
Authors:
Liwen Chu Etc.
Liwen Chu, STMicroelectronics

2. Fragmentation in the 802.11 Baseline Specification
Date: Jan, 2012
Fragmentation in the Baseline Specification
Fragmentation creates MPDUs smaller than the original MSDU or MMPDU length to increase reliability, by increasing the probability of successful transmission of the MSDU or MMPDU in cases where channel characteristics limit reception reliability for longer frames.
An MSDU transmitted under an HT-immediate or HT-delayed Block Ack agreement shall not be fragmented even if its length exceeds dot11FragmentationThreshold.
An MSDU transmitted within an A-MPDU shall not be fragmented even if its length exceeds dot11FragmentationThreshold.
Group addressed MSDUs or MMPDUs shall not be fragmented even if their length exceeds dot11FragmentationThreshold. 2

3. Why Narrower Channel needs Fragmentation with A-MPDU
Date: Jan, 2012
Why Narrower Channel needs Fragmentation with A-MPDU
Sub1GHz ah has comparably narrow operating channels (1Mhz, 2MHz, 4MHz, 8MHz, 16MHz) than legacy bands af will also have narrow channels (e.g. 6MHz).
Fragmentation without A-MPDU increases the reliability:
When collision occurs, the unsuccessful transmission is shorter.
The CRC can provide higher protection to the shorter frame.
Fragmentation without A-MPDU creates more overhead.
Each fragmentation needs an acknowledge frame.
More IFSs and PHY headers are required for a MSDU/MMPDU.
MSDU
Fragment0
Fragment1
Fragment2
Fragment3
Fragment4
PHY Header
MPDU
Fragment0
Fragment1
Fragment2
Fragment3
Fragment4
PHY Header
ACK
SIFS 3

4. Why Narrower Channel needs Fragmentation with A-MPDU (Cont’d)
Date: Jan, 2012
Why Narrower Channel needs Fragmentation with A-MPDU (Cont’d)
Fragmentation with A-MPDU increases the reliability without increasing overhead.
When a collision occurs, the unsuccessful transmission is shorter.
Each A-MPDU subframe is identified by an MPDU Delimiter.
The CRC can provide higher protection to the shorter frame.
Aggregated fragments plus a BlockAck decrease the IFS and PHY overhead.
MSDU
Fragment0
Fragment1
Fragment2
Fragment3
Fragment4
4 bytes MPDU Delimiter
Pad (0 - 3bytes)
MPDU
A-MPDU Subframe
PHY Header
A-MPDU Subframe
Fragment0
Fragment1
Fragment2
Fragment3
Fragment4
BlockAck
SIFS 4

5. Solution: Allow Fragmentation with A-MPDU
Date: Jan, 2012
Solution: Allow Fragmentation with A-MPDU
In a narrow-channel band (e.g. Sub 1GHz TGah), a STA or an AP can combine Fragmentation with A-MPDU:
An MSDU transmitted within an A-MPDU may be fragmented if its length exceeds dot11FragmentationThreshold.
MSDU
Fragment0
Fragment1
Fragment2
Fragment3
Fragment4
4 bytes MPDU Delimiter
Pad (0 - 3bytes)
MPDU
A-MPDU Subframe
PHY Header
A-MPDU Subframe
Fragment0
Fragment1
Fragment2
Fragment3
Fragment4
BlockAck
SIFS 5

6. Issue and Solution with Fragmentation BlockAck
Date: Jan, 2012
Issue and Solution with Fragmentation BlockAck
Issue with fragmentation blockAck:
The BA Information field is a long field = 130 bytes. This may not be good to low PHY rate.
Solution:
Compress the fragmentation BA Information field to shorter length (e.g. 10 Octets, 4 MSDUs can be acknowledged by this shorter fragmentation BA Information field ).
A reserved bit in BA Control field can used as shorter fragmentation BA Information indication. Or A compressed basic blockAck Req can be defined for the shorter fragmentation BA .
Block Ack Starting Sequence Control
Block Ack Bitmap
Octets 2 8 6