Network Management Strategies for the CIO
Is it possible in a multi-radio wireless network system to assign each radio to a specific channel for transmission? Will each radio have a unique SSID or can all the radios be configured for the same SSID (but with different channels)?
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Most wireless LANs use precisely the approach you asked about to reduce co-channel interference between adjacent radios. When multiple APs are deployed inside a building, they should be assigned channels far enough apart to minimize interference. 802.11b/g APs often use channels 1, 6, and 11, while 802.11a APs use Channels 36, 40, 44, 48, 52, 56, 60, 64, 149, 153, 157, and 161. The larger set of available "non overlapping" channels is why more dense AP deployments are possible for 802.11a.
For example, if you were to put three 802.11g APs on floor one, you might assign channels 1 (left) 6 (middle) and 11 (right), all using the same SSID so that clients can connect to the AP nearest to them and roam freely between those APs. If you were to put three more APs on floor two, you might assign channels 11 (left) 1 (middle) and 6 (right) so that no pair of APs above/below were using the same channel. Now, all six APs would use the same SSID, because they serve as members of the same extended service set, providing wireless access to the same internal distribution network (company Intranet or public Internet). By the way, the same principal applies even if each AP has two or even three b/g radios inside the same enclosure.
Why not assign each AP its own SSID? That would force users to choose which AP to connect to, and cause decline in speed and disruption as the user moved away from the AP, even if moving towards another AP that could offer better service. You should only assign different SSIDs if you really want to segment the WLAN, causing some users to connect to one SSID (for example, "guest") while other users connect to a different SSID (for example, "company").
Why not assign all APs the same channel? That would cause all users to compete for exactly the same frequency band, spreading the bandwidth available in a single 22 MHz channel across everyone instead while wasting the rest of the spectrum. Furthermore, collisions would occur between users of adjacent APs when everyone tries to transmit on the same frequency at once. Those collisions result in errors that require retransmissions, further reducing effective throughput. When this happens, some APs can be configured to automatically retune themselves to a less noisy channel.
It is possible for all APs to operate on the same channel(s), but doing so effectively would require additional coordination. For an example of that approach, see Extricom.
This was first published in March 2007
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