RealTime IT News

802.11a: First Glimpse - part 2

Earlier this week, we took a look at distance/performance and power-consumption issues for the just-out-of-the-starting-gate WLAN technology, IEEE 802.11a. Preliminary findings based on testing of early prototype components using Atheros Communications's AR5000 chipset suggest that the five-fold speed increase comes at a smaller penalty in effective radio coverage than theoretical predictions suggested.

Atheros product manager James Chen demonstrated the company's "reference design" components—access point and PCMCIA network interface card—in our offices. In addition to discussing the performance data, Chen touched on other important facets of the 802.11a specification—most importantly those relating to the broadcast spectrum allocated to this technology.

Here's the crux: Whereas the spectrum allotted to 802.11b in the 2.4GHz range affords only three clear transmission channels, the portion of the 5GHz spectrum set aside for 802.11a "indoor" applications has room for eight clear channels. (In addition there is a further allocation set aside for higher power, "outdoor" applications.) This has implications not only for dense deployments in corporate, campus, and other public venues, but for raw performance as well.

Beehive space
Wireless LANs designed to provide blanket coverage within institutional buildings—corporate offices, hospitals, factories, airport waiting rooms, and the like—are typically arranged in a grid of overlapping "cells," consisting of access points serving network nodes within about a 65-foot radius. Adjacent cells avoid interference from their neighbors by using different channels within the technology's overall spectrum allocation.

With 802.11b (Wi-Fi), which, again, is limited to three clear channels, deployments of more than three contiguous cells are likely to be subject to some performance degradation (up to as much as 50 percent) due to "co-channel interference" (CCI) between cells operating on a given channel. This is because with Wi-Fi, there's no way to avoid duplication of channel usage more than one cell diameter away. And the closer together the cells, the more interference.

With 802.11a's eight channels, however, it's a relatively simple matter to arrange a cell grid so that access points using the same channel are at least twice as far apart—and the overall density of cells using any given channel is roughly one fourth as great. This should greatly reduce the effect of CCI, if not eliminate it altogether.

Other spaces
In a setting—like the home—in which competition for broadcast channels isn't an issue, 802.11a has another potential advantage: It's possible to use multiple channels simultaneously in a coordinated manner. One interesting application of this concept cited by Chen would be a home gateway capable of distributing multiple video streams independently of each other, using discreet 802.11a channels.

Another application that Atheros has already implemented is a "turbo" networking mode that, theoretically doubles the basic 802.11a data-link speed to a whopping 108 Mbps, or roughly the equivalent of Fast Ethernet. Atheros doesn't claim this speed for turbo mode, rather splitting the difference and specifying 78 Mbps. Any way you slice it, however, this is great throughput for wireless LAN.