802.11 Wireless frame Aggregation and A-MSDU and A-MPDU in 802.11 N/AC

802.11 Frame Aggregation 

The Frame aggregation was introduced in purpose of reducing the overhead on wireless space and by doing so increasing the efficiency and performance of wireless client. By introducing frame aggregation in the given amount of  Airtime/TXOP the wireless client could now send more data than before.

MSDU-MAC Service Data Unit 

The upper layer information for a MAC layer in 802.11 is called the  MSDU. i.e the stuff [IPheader|TCP-UDP header|Data].

MPDU-MAC Protocol Data Unit

Once there is a header and footer added to the MSDU its now called as MPDU. This is now send to the lower layer called physical layer.

MSDU

Aggregated-MSDU

This is new function added from 802.11N this make it possible to send more data  during the given Transmit opportunity. Collection of MSDU packets is called A-MSDU and then sending them in a single burst through the physical layer.

a-msdu

Aggregated-MPDU:

Again this is new function added from 802.11N and its collection on multiple MPDUs and send them in a single burst through physical layer. You will find multiple mac header and mac checksum inside a A-MPDU.

a-mpdu

Maximum 802.11 Frame Body Size(without encryption)

MSDU: 2304 bytes [standard frame]

MPDU: 2346

MSDU inside A-MPDU: 4065 bytes

A-MSDU: 7955  bytes

802.11AC @11,454

Key points on this discussion:

* 802.11AC the transmission is always aggregated (a-msdu or a-mpdu) even though its a small chunk of data to be transferred and its backward compatible.

*Since the maximum MTU size on a wired network is 1500bytes this is further getting fragmented on wired network (When Jumbo frames disabled on wired).

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