How throughput enhancements dramatically boost 802.11n MAC efficiency--Part I

Probir Sarkar, ARM
EETimes (8/10/2010 1:40 PM EDT)

The ratification of the 802.11n amendment to the popular 802.11 WLAN standard represents a watershed event in the progress of wireless networking. The pent-up market demand for high speed wireless access, availability of more unlicensed spectrum, and advances in digital signal processing and semiconductor technology have created the perfect setting for the advent of very high wireless throughput. 802.11n improves the maximum raw data rate from 54 Mbps (in 802.11a,g) to 600 Mbps. This is achieved by doubling the channel width (from 20 MHz to 40 MHz) and capitalizing on this bandwidth efficiently, using multiple antennas to transmit several data streams simultaneously.

While these techniques bring up the PHY data rate dramatically, it was realized that the MAC protocol did not scale to these rates. At the highest rates proposed for the 802.11n PHY, the legacy MAC efficiency would fall to a dismal 15%--negating almost all of the gains for applications. Thus, unlike the previous amendments (802.11a, b and g), the 802.11n also needed to make significant improvements to the MAC protocol. Towards this end, new frame aggregation schemes, and improved cumulative acknowledgment and medium access rules were introduced. It also added some changes to improve the common usage scenario where primary data flow is in one direction.

These MAC enhancements are the primary focus of this article. We start with a description and analysis of the legacy MAC protocol and its overhead. We will then describe the PHY advances introduced in 802.11n, provide an overview of the MAC improvements in this amendment and describe these enhancements in some detail. Finally, we provide additional MAC tasks related to control and use of PHY features and present a summary of the enhancements and reiterate their impact.

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