802.11n: A Survival Guide

Chapter 6. Advanced MAC Features for Interoperability

In the context of protocol design, coexistence can refer to many ways that devices are expected to show the equivalent of good citizenship and not cause undue harm to surrounding devices. 802.11n includes several capabilities that were completely new to 802.11 wireless LANs, and preserving the strong record of interoperability required that these new capabilities be designed in such a way that they could be used in the presence of older devices.

Radio Medium Coordination

802.11 derives a good portion of its total network speed from frequency (or spatial) reuse; that is, a single radio channel can be used multiple times in the same network. Each radio channel is subject to the rules of 802.11’s CSMA/CA. Neighboring APs that are operating on the same channel must share access to the radio medium and therefore must not transmit at the same time. Neighboring APs operating on different channels can transmit simultaneously, and therefore will typically have much higher total throughput.

802.11 defines the term overlapping BSS (OBSS) to refer to another network that uses available airtime. By definition, an OBSS must be both on the same channel and in the same space; if either of those conditions are not met, the two networks do not interfere with each other. Generally speaking, networks are designed to minimize OBSSes, typically through careful frequency assignment. Neighboring APs are assigned to different channels; although their coverage ...