Fragmentation with A-MPDU February 2008 doc.: IEEE 802.11-07/2376r0 Date: Jan, 2012 Fragmentation with A-MPDU Date: 2012-01-16 Authors: Liwen Chu Etc. Liwen Chu, STMicroelectronics

Fragmentation in the 802.11 Baseline Specification Date: Jan, 2012 Fragmentation in the 802.11 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

Why Narrower Channel needs Fragmentation with A-MPDU Date: Jan, 2012 Why Narrower Channel needs Fragmentation with A-MPDU Sub1GHz 802.11ah has comparably narrow operating channels (1Mhz, 2MHz, 4MHz, 8MHz, 16MHz) than legacy bands. 802.11af 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

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

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

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 128 +2 = 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