Different Path
Vendors that have traveled the conventional RF-design route, with microcells and client-side control, have faced performance challenges as the density of clients per AP increases in tandem with traffic load. With some sophisticated site design, antennas and channel planning, most conventional vendors can tread water. But two, Xirrus and Meru, have taken a different tack, integrating multiple sectorized antennas into a single AP.
Xirrus offers four-, eight- and 16-radio APs in its Xirrus Wi-Fi Array line. All start with four 802.11a/b/g radios and add extra 802.11a radios as they scale up. Each radio serves a narrow, but slightly overlapping, arc of coverage (see diagram, in the gallery). Because it's difficult to create sufficient RF isolation among radios at such close range, even on nonoverlapping channels, coordination occurs on the MAC (Media Access Control) layer. As a result, radios take turns receiving and transmitting; that way, transmitted signals don't get distorted, and receivers aren't desensitized.
The APs sport directional antennas that deliver higher gain so they can transmit farther, which means fewer APs need to be placed. This, in turn, reduces deployment costs. With good planning, co-channel interference can be reduced because signals are not broadcasting omnidirectionally.
There are other benefits as well, including load balancing clients and redundancy between adjoining radios. Sure, the narrow coverage patterns afforded by directional antennas often mean more roaming events--a real problem for VoWLAN deployments--but having radios integrated in the same physical array lets roaming be highly optimized.
