A-MPDU vs. A-MSDU

The 802.11n amendment addresses new enhancements to the MAC sublayer of the Data-Link layer to increase throughput and improve power management.

Frame aggregation is a method of combining multiple frames into a single frame transmission.

Medium contention overhead is addressed by using two new methods of frame aggregation:

A-MSDU (Aggregate MAC Service Data Unit)

  • Every time a unicast 802.11 frame is transmitted, a certain amount of fixed overhead exists as a result of the PHY header, MAC header, MAC trailer, interframe spacing, and acknowledgment frame.
  • Medium contention overhead also exists because of the time required when each frame must contend for the medium.

80211 unicast frame overhead.png

  • With the ratification of the 802.11n-2009 amendment, aggregate MSDU (A-MSDU) was introduced.
  • With A-MSDU, the maximum frame body size is determined by the maximum A-MSDU size of 3,839 or 7,935 octets, depending upon the STA’s capability, plus any overhead from encryption.
  • The fixed MAC layer overhead is reduced, and overhead caused by the random backoff timer during medium contention is also minimized.
  • Multiple MSDUs can be aggregated into a single frame transmission.

a-msdu.png

  • An 802.11n access point using A-MSDU aggregation would receive multiple 802.3 frames, remove the 802.3 headers and trailers, and then wrap the multiple MSDU payloads into a single 802.11 frame for transmission.
  • The aggregated MSDUs will have a single wireless receiver when wrapped together in a single frame.
  • The entire aggregated frame is encrypted with the Counter Mode with Cipher Block Chaining Message Authentication Code Protocol (CCMP) cipher.
  • It should be noted, however, that the individual MSDUs must all be of the same 802.11e QoS access category.
  • Voice MSDUs cannot be mixed with Best Effort or Video MSDUs inside the same aggregated frame.

 

A-MPDU (Aggregate MAC Protocol Data Unit)

  • The MPDU is an entire 802.11 frame including the MAC header, body, and trailer.
  • As pictured in Figure 18.16, multiple MPDUs can be aggregated into a single frame transmission.

 

a-mpdu.png

  • The individual MPDUs within an A-MPDU must all have the same receiver address.
  • The data payload of each MPDU is encrypted separately using the CCMP cipher.
  • Much like MSDU aggregation, individual MPDUs must all be of the same 802.11e QoS access category.
  • Voice MPDUs cannot be mixed with Best Effort or Video MPDUs inside the same aggregated frame.
  • Please note that MPDU aggregation has more overhead than MSDU aggregation because each MPDU has an individual MAC header and trailer.
  • CRC errors can be detected in the individual MPDU frames and therefore an entire A-MPDU does not need to be retransmitted, only the individual MPDU that is corrupted. Therefore, A-MPDU is less susceptible to noise than A-MSDU.
  • The majority of WLAN vendors use A-MPDU.
  • All 802.11ac frames are transmitted using the Aggregate MAC Protocol Data Unit (A-MPDU) frame format, even if only a single  frame is being transmitted.
  • A-MPDU reduces some of the overhead involved with transmitting multiple frames.
  • Aggregation also shifts some of the frame information from the Physical Layer Convergence Protocol (PLCP) header to the MPDU header.
  • Since PLCP information is transmitted at the lowest supported data rate, and the MPDU information is transmitted at the higher data rates, this will improve performance.
  • The higher transmission speeds of 802.11ac make Reduced Interframe Space (RIFS) obsolete. 802.11n allowed the use of RIFS (2μs) instead of SIFS (10 μs), which decreased the amount of time needed to transmit multiple frames.
  • By transmitting A-MPDU frames, 802.11ac removes the need for multiple transmissions of individual frames with individual headers and individual ACKs.
  • An AMPDU frame reduces the per-frame overhead and only requires a single block ACK. Because of this, the 802.11ac amendment states that the use of RIFS is nots supported and is obsolete.
  • Since every 802.11ac frame is an AMPDU frame, all acknowledgments are performed as block acknowledgments.