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Channel operations in WLANs

Sep 29, 20032 mins
Cellular NetworksNetwork SecurityWi-Fi

* Back to basics with 802.11

Several of you have requested help with better understanding 802.11 wireless LAN channels and how users share them.  So let’s take time out for a quick refresher. Bear in mind that this is a discussion of WLAN basics and doesn’t account for deployment shortcuts being enabled by new WLAN architectures.

A single 802.11 radio can communicate across just one channel, or frequency, at a time. A single-band access point (AP) has just one radio, which operates in either the 2.4 GHz band (used by 802.11b and 802.11g networks) or the 5 GHz band (used by 802.11a networks). So a single-band AP can support a single communications channel – defined as a segment of the entire 2.4 GHz or 5 GHz band available to a radio – at a given point in time.

By contrast, dual-band 802.11 APs have two radios, one operating in the 2.4 GHz band and the other in the 5 GHz band. A dual-band AP, then, can connect to two channels simultaneously if both radios are active.

The 2.4 GHz radio band has three nonoverlapping frequencies available. The 5 GHz band has 11 to 24 nonoverlapping channels, depending on geographic location.

Radio frequency network designers generally associate each AP radio with a nonoverlapping channel to minimize interference generated between APs. And network implementers also usually abut or overlap radio cells – the three-dimensional transmission radius of a given AP of up to about 300 feet – to prevent coverage gaps. To achieve both goals, they often alternate channel assignments in adjacent cells.

For example, the 2.4 GHz range supports three nonoverlapping channels: 1, 6 and 11. So if radio cell No. 1 is configured to communicate across channel 1, APs that abut cell No. 1 could be configured to communicate across channel 6. And the APs in radio cells that abut channel-6 cells could be set using channel 11. Then the assignment process could begin again.

The greater the distance between same-channel cells, the lower the risk of co-channel interference. This is one reason that gaining channels from the 5 GHz space has been a boon; the extra frequencies preclude having to repeat channel assignments until APs are a hefty distance from one another.

Next time: How users share channels.