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Dynamic Frequency Selection (DFS) is a mandate for radio systems operating in the 5 GHz band to identify and avoid interference with Radar systems. IEEE 802.11h is the standard that covers the usage of DFS channels. To make use of DFS channels, wireless access points need to be certified to ensure that WiFi services on AP do follow the rule, e.g., stop immediately when a Radar signal is detected.
Since only a few business or enterprise APs are certified to use DFS channels, DFS channels are generally not so crowded in comparing with non-DFS channels. Network providers like to use DFS channels because of less interference. However, for the area with occasional Radar signals, service providers tend to avoid DFS channels.
Based on the 802.11h standard, AP needs to change its channel immediately when the Radar signal is detected in the current DFS channel. In general, AP can only hop back to the non-DFS channel. The main reason is:
According to 802.11h, an AP, when moving to a new DFS channel, has to listen silently to the medium for one minute before it is allowed to transmit anything (like a beacon) in order to make sure that no radar is currently operating on that channel.
AP cannot wait for one minute without serving its wireless clients. That's why falling back to regular non-DFS channels is the only way for conventional APs to comply with standards.
Instead of falling back to the non-DFS channel, zero-wait DFS is a technique providing the seamless change of channels. That is, stations do not lose their connectivity when an AP moves to another DFS channel. With the dedicated scanning radio, EnGenius S-series APs can always guarantee the one-minute clearance of the target DFS channel hence hop to that channel without any waiting.
When an AP changes its channel, Channel Switch Announcement (CSA), as defined by IEEE 802.11h, is adopted to announce that AP is switching to a new channel before it begins transmitting on that channel. This allows the clients, who support CSA, to transition to the new channel with minimal downtime.
All S-series APs support zero-wait DFS. Relative options are available in radio settings.
Good to know:
- Enable Spectrum Analyzer or Channel Scan in Live Diag would suspend zero-wait DFS temporarily because these Live Diag Tools need the scanning radio to scan all channels.
- AirGuard may work with zero-wait DFS smoothly except the containment feature is enabled. The containment feature requires the scanning radio to do the containment in advance.