AMSDU and AMPDU

Data Link Layer Sub-Layers...

1. LLC (Logical link Control):- Name itself describes that it is responsible for handling layer-3 protocols (multiplexing/de-multiplexing), flow control and reliability. This is upper sub-layer of Data Link Layer.
2. MAC (Medium Access Control): It is responsible for controlling medium access, framing/de-framing, check-sum computation and verification.





Packets, Frames...etc, what is it called at each layer....

MSDU, MPDU, PSDU...

MSDU: The MSDU is the service data unit that is received from the logical link control (LLC)
MPDU: The MPDU is a message (Protocol data unit) exchanged between media access control (MAC) entities in a communication system.


AMSDU and AMPDU...

AMSDU........................

Why AMSDU:

1.  Ethernet is the native frame format
2.  Ethernet header is much smaller than the 802.11 header

Hence multiple Ethernet frames can be combined to form a single A-MSDU.  

A-MSDU minimizes the MAC header overheads when the channel conditions are good.

  

Issue with AMSDU:

Probability of error increases with frame size. Since each AMSDU has one MAC frame and one CRC, in case of error whole frame needs to be re-transmitted (in most cases at lower rates). So it is better to use it in good channel conditions.

AMSDU Beacon:

AMSDU Packet exchange:

AMPDU....................

Why AMPDU:

Multiple PDUs are aggregated, each with their own MAC header and CRCs. Hence, in the event of a failure, only those MPDUs can be retransmitted resulting in higher efficiency.

However, each MPDU in AMPDU has its own header and CRC, it adds to overhead but by using Block Ack mechanism this can be overridden.

Beacon of AMPDU enabled AP:

AMPDU Packet exchange: