Last time, I mentioned that as the number of mobile LAN users rises, greater contention for shared wireless capacity could cause users to experience some response-time slowdowns. While the back end of a wireless access point connects to high-speed 10/100M bit/sec wired LANs, contention among many users for shared 11M bit/sec access to an 802.11b wireless access point could take a bite out of overall performance.

From a network design standpoint, the difficult part here is that, unlike wired Ethernet, you can’t necessarily just divvy up the LAN segments to support smaller and smaller numbers of users to boost performance. Fixed wired LAN connections stay put; but users roaming about the building and campus connect and reconnect transparently to different APs as they move, so predicting which ones will be saturated -- and when -- can be pretty difficult.

There are some options here. 3Com’s AirConnect enterprise-class 802.11b wireless LAN, for example, is one that will let you specifically associate a client with an AP, rather than having the software just match the AP with the strongest signal to the client as most products do. This can be valuable if you want to segregate your traffic to balance the network load.

Another strategy is to place clients on different frequencies (channels). The result is kind of like using an Ethernet switch instead of a shared Ethernet hub, so it is likely to improve performance. In 3Com’s case, 3Com’s AirConnect APs can overlap three channels for an aggregate of 33M bit/sec, though each user would be limited to the 11M bit/sec connection of his or her LAN card, says Paul Fulton, vice president and general manager in 3Com’s Wireless Connectivity Division.

Note, though, that the effective throughput of 802.11b LANs is somewhere between 4M and 5M bit/sec. So, in 3Com’s case, I’d expect your aggregate to be on the order of 12M to 15M bit/sec. This strategy of assigning users to different channels for higher aggregate throughput can be accomplished using many vendors’ wireless LAN equipment, including high-performing wireless LANs from Cisco, Enterasys and Lucent.

» posted by ITworld staff

Network World

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