AMPDU
One of the fundamental features of 802.11n/ac etc is the Aggregation of Frames (packets) into a single larger frame. Block-ACK messages are sent to acknowledge the successful receipt of an aggregated frame. Aggregation works to decrease the amount of channel overhead by allowing a device to obtain access to the radio and then using that time to transmit multiple frames. Interestingly 802.11ac uses only AMPDU frames for an efficiency improvement.
The AMPDU setting is used to enable/disable transmit AMPDU aggregation for the entire interface. Receiving of aggregate frames will still be performed, but no aggregate frames will be transmitted if this is disabled. There really isn't a case anymore to disable AMPDU. When 802.11n first came out there were compatibility issues around how AMPDU was implemented amongst different vendors and disabling this setting was one way to work around it or to test compatibility.
AMPDU Frames
This command will set the maximum number of sub-frames to place into an AMPDU aggregate frame. Frames are added to an aggregate until either:
a) the transmit duration is exceeded,
b) the number of sub-frames is exceeded,
c) the maximum number of bytes is exceeded, or
d) the corresponding queue is empty.
The sub-frame that causes the excess conditions will not be included in the aggregate frame, but will be queued up to be transmitted with the next aggregate frame.
The default value is 32. On some long distance, or poorly performing links, performance can be improved if this number is reduced, thus reducing the aggregate frame size.
AMPDU Limit
This parameter will limit the number of bytes included in an AMPDU aggregate frame.
The default value of this parameter is 50000. As with the AMPDU Frames setting, on some links reducing this number can improve performance if the link suffers from e.g. packet loss, otherwise the default value should be used.
