The A-MPDU Parameters define the size of an aggregated packet and also define the proper spacing between aggregated packets so that the receive side WLAN station is able to decode the packet properly.

The A-MPDU parameters are part of the HT Capability Information element and the VHT Capability information fields. The A-MPDU parameter set in the HT Capability IE is shown below.

Fig Courtesy: 802.11 Standard

The Maximum A-MPDU Length exponent indicates the largest A-MPDU size permissible. In 802.11n – the length exponent was an integer from 0 to 3. It is shown below

Fig Courtesy: 802.11 Standard

The maximum length of an A-MPDU is equal to 2¹⁶ – 1 (with Maximum A-MPDU Length Exponent set to 3) in 802.11n which equals 65535 bytes.

The next parameter – Minimum MPDU start spacing indicates the amount of time that needs to be provided for the receiver of the A-MPDU to decrypt the MPDUs sent within an A-MPDU properly. Its value is provided in time units of micro-seconds.

The transmitter of the A-MPDU needs to provide sufficient space (time) between two MPDUs of an A-MPDU by inserting MPDU de-limiters with Length equal to zero in between successive MPDUs to adhere to the timing constraint provided below.

Fig Courtesy: 802.11 Standard

The Timing constraint allows the receiver of the A-MPDU to decrypt the MPDUs in an A-MPDU without being over-whelmed.

The formula which defines the number of octets (bytes) to be inserted between two consecutive MPDUs of an A-MPDU according to the timing requirement in the A-MPDU Parameter set is given by

t_MMSS X r/8

wherein

• t_MMSS – Time in microseconds as present in Minimum MPDU start spacing (MMSS)
• r – PHY Data rate

AMPDU Parameters in 802.11ac

In the 802.11ac standard – the Maximum length exponent is part of the VHT Capability IE. It is an integer from 0 to 7. It is shown below

Fig Courtesy: 802.11 Standard

Hence, the maximum length of an A-MPDU is equal to 2²⁰ – 1 in 802.11ac which equals 1048575 bytes or 1 MB. The  Minimum MPDU start spacing is the same as in 802.11n.

HT Immediate Block ACK