Off-channel scan is in an important operation, where the AP collects information about the specific area where it is mounted. It will report sources of interference, different SSIDs implemented on each channel, duty cycle etc. This information is useful for dynamic RF module developed by each vendor and wireless intrusion detection/prevention system (WIDS/WIPS).
The idea is that AP will operate on the static/dynamically assigned channel serving mobile clients for fixed amount of time. After that it will scan the next channel for very short period of time and jump back to the operating channel for mobile clients.
Example: AP is on channel 1 for 10 seconds serving clients. After 10 seconds, it will go to channel 2 and scan it for 30-150ms (depends on vendor implementation and settings). Once it completes the scan, it will go back for 10 seconds to channel 1. After another 10 seconds, it will scan channel 3 for 30-150ms. The process continues and all (configured/operating) channels are scanned. Based on that analysis and statistics, each dynamic RF algorithm assigns the best channel to the current RF conditions.
How about voice calls in wireless networks and off-channel scan?
How vendors took into consideration voice calls’ presence in off-channel scan?
Cisco (firmware version 8.3):
There is an Off Channel Scanning Defer feature available in WLAN settings -> Advanced
Cisco allows to categorize the traffic into QoS policy (bronze, silver, gold, platinum). Platinum marks all downlink traffic to UP=6.
As long as there is detected platinum traffic, there is implemented scan defer time (100ms-60sec). In this example, it means the scan will be delayed for configurable period of time of 100msecs.
Aruba (firmware version 220.127.116.11):
The configuration might be found in:
Mobility Master -> Configuration -> System -> Profiles -> RF Management -> ARM -> ARM Profile
Enabled by default. AP will not scan, if VoIP call is in progress.
Aruba IAP (firmware version 18.104.22.168):
Instant APs have less functionality than Campus APs connected to wireless controllers.
There is scanning option in VC -> RF -> Advanced options
Scanning is enabled, however client aware enabled option doesn’t mean the off channel scan will be delayed if voice or client traffic is present. It means the AP won’t change the operating channel, when the mobile clients are active, with an exception of radars or excessive noise.
Aruba TAC presented a workaround for IAPs. Applying a policy to disable scanning when any traffic is present. This might be changed to apply to only specific voice traffic generated by specific application, like Skype for Business or other.
Extreme Wireless WING (firmware version 22.214.171.124):
Configuration -> Wireless -> SMART RF Policy -> policy (edit) -> Scanning configuration
Voice aware scanning feature will disable off channel scan when voice calls are present.