802.11a and scalability

By Joanie Wexler, Network World |  Networking

Because wireless LANs use shared-bandwidth media access control mechanisms, scalability to larger volumes of users can be an issue. This is particularly true at current 802.11b speeds of a mere 11M bit/sec (effective throughput: 4M to 5M bit/sec). So will 802.11b’s faster successor, 802.11a, offer greater scalability?

It all depends on your point of view. On the one hand, 54M-bit/sec wireless 802.11a LANs, due to start shipping late this year, support many more channels than 802.11b LANs do. 802.11b, which runs in the 2.4-GHz frequency range, supports three channels, each with 11M-bit/sec capacity. By contrast, 802.11a networks will support eight to 12 channels, depending on vendor implementation, each with up to 54M-bit/sec capacity.

Generally, spreading user access across different channels works somewhat like switching in wired Ethernet networks, enabling more users to simultaneously communicate. So more channels, with more capacity per channel, bodes well for scalability. And vendors and standards body members anticipate that 802.11a products should be comparable to today’s 802.11b prices out of the chute. Such a deal, right?

The catch to all this, though, is that you will likely need to purchase a larger number of access points -- the more expensive component of the wireless LAN system -- to cover the same range as an 802.11b LAN does. The range supported from access point to client in an 11M-bit/sec network is about 300 feet. The shorter, wider radio waves in a 5GHz 802.11a LAN, while offering more capacity, transmit only about 90 feet. So for each access point you install today, you might need three or four in the future.

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