Aggragation in 11n

Aggregation in 80211n

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Aggregation is must feature of 80211n - Send mutiple MPDU  in a single packet. MPDUs received in this single bundled packet are acked using single Block Ack.

Aggragation related fields In HT Capabilites IE

Capability Info - 1 bit for max A-MSDU length 0 - 3895 bytes 1 - 7990 bytes
AMPDU Parameters - Max AMPDU Exponent - Max length = 2^(13 + exp) -1 bytes


Paramters that are negotiated for Aggregation in each direction
- TID
- Block Ack Policy
- is AMSDU supported
- Numbers of buffers allocated(10 bits, Max value 1023)
- Starting Sequence Number

Each buffer represents packet of max size MPDU  or less.

Implementation:

1. After STA is associated, when first unicast packet is received from the stack for the peer station
AP will send ADD BA action frame to the peer device. First packet is sent in non-aggragated format.
2. At AP, for each connected client it will create 17 software queues for storing the packets corresponding that particulaer TID.
3.  aggragate_schd() can be implemented which picks the highest priority non-empty TID queue for processing the packets aggregation
4. In aggragated list if there are any  packets which are to be sent at legacy rate or minrate, that packet is seperated out
5. Packets till Block Ack window are considered for aggregation
5. A-Packets are given to firmware for transmission at best possible rate

A-MPDU format

| ---- 4 bytes of Delimiter---|------MPDU_1----|---EOD Padding----|------ same for MPDU_2

Delimiter has MPDU length.

Block Ack:
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