I’ve been busy lately working on a white paper entitled "Inside 802.11n Wireless LANs: Practical Insights and Analysis” now available for download. In the section ”Cutting the Overhead” I detail how 802.11n has improved the efficiency of the MAC protocol using a single block ACK (BA) for multiple frames as well as the ability to aggregate multiple frames into a single transmission. I’ve excerpted information on frame aggregation in this blog. For insight into the block ACK along with frame capture analysis, please consult the white paper.
802.11n can send multiple frames per single access to the medium by combining frames together into one larger frame. There are two forms of frame aggregation: Aggregated Mac Service Data Unit (A-MSDU) and Aggregated Mac Protocol Data Unit (A-MPDU).
A-MSDU increases the maximum frame transmission size from 2,304 bytes to almost 8k bytes (7935 to be exact) while A-MPDU allows up to 64k bytes.
A-MSDU creates the larger frame by combining smaller frames with the same physical source and destination end points and traffic class (i.e. QoS) into one large frame with a common MAC header. One way to visualize this is an access point receiving frames from the wired side at a rate faster than it can transmit them on the wireless side. Ethernet frames headed for the same wireless client can be queued then combined into one larger frame for single transmission, cutting down the overhead dramatically.
One caveat, and it’s a big one, is that like any wireless transmission, the larger the frame the less likely it will be received with no errors. I’ve observed that 802.11n senders tend to learn the maximum data rates possible for given frame sizes. Thus for instance, you may see a frame containing a TCP ACK send at 270 Mbps (or 300 Mbps if a short guard interval is in use) because it’s very small (typical 78 bytes including the 802.11 overhead). Frames containing FTP data may be sent at 121.5 Mbps simply because previous attempts at a higher data rate proved futile.
Thus it’s not clear how much we will gain in efficiency using the A-MSDU. With its frame size up to 7935 bytes, with one MAC header and payload protected by one CRC like any other single frame, it will most likely be transmitted at a lower data rate for reliable reception.
Contrast this to the A-MPDU which is essentially a chain of individual 802.11 frames sent back-to-back with one access to the medium (i.e. one preamble). The destination must still be one address and the traffic class (QoS) must be the same for each. Clearly, there is more overhead with the A-MPDU because we still have individual PDU frame headers vs. one in the A-MSDU. Unlike the A-MSDU however, individual PDU frames also have their own CRC; an error in one PDU will not affect the others in the group.
The bottom line is this: Reliability has its overhead. The A-MPDU allows a much larger "burst" of frame data to be sent compared the the A-MSDU. In fact, many 802.11n access points conservatively option for the 1/2 A-MSDU frame size (3839 bytes) by default to play it safe.