Band Select
Band select enables client radios that are capable of dual-band (2.4 and 5-GHz) operations to move to a less congested 5-GHz access point. The 2.4-GHz band is often congested. Clients on this band typically experience interference from Bluetooth devices, microwave ovens, and cordless phones as well as co-channel interference from other access points because of the 802.11b/g limit of 3 nonoverlapping channels. To prevent these sources of interference and improve overall network performance, configure band selection on the device.
Band select works by regulating probe responses to clients and it can be enabled on a per-WLAN basis. It makes 5-GHz channels more attractive to clients by delaying probe responses to clients on 2.4-GHz channels. In an access point, the band select table can be viewed by running the show dot11 band-select command. It can also be viewed by running the show cont d0/d1 | begin Lru command.
Band Select Algorithm
The band select algorithm affects clients that use 2.4-GHz band. Initially, when a client sends a probe request to an access point, the corresponding client probe’s Active and Count values (as seen from the band select table) become 1. The algorithm functions based on the following scenarios:
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Scenario1: Client RSSI (as seen from the show cont d0/d1 | begin RSSI command output) is greater than both Mid RSSI and Acceptable Client RSSI.
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Dual-band clients: No 2.4-GHz probe responses are seen at any time; 5-GHz probe responses are seen for all 5-GHz probe requests.
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Single-band (2.4-GHz) clients: 2.4-GHz probe responses are seen only after the probe suppression cycle.
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After the client’s probe count reaches the configured probe cycle count, the algorithm waits for the Age Out Suppression time and then marks the client probe’s Active value as 0. Then, the algorithm is restarted.
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Scenario2: Client RSSI (as seen from show cont d0/d1 | begin RSSI ) lies between Mid-RSSI and Acceptable Client RSSI.
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All 2.4-GHz and 5-GHz probe requests are responded to without any restrictions.
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This scenario is similar to the band select disabled.
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Note |
The client RSSI value (as seen in the sh cont d0 | begin RSSI command output) is the average of the client packets received, and the Mid RSSI feature is the instantaneous RSSI value of the probe packets. As a result, the client RSSI is seen as weaker than the configured Mid RSSI value (7-dB delta). The 802.11b probes from the client are suppressed to push the client to associate with the 802.11a band. |