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Manuale Utente (Inglese)

Zero Wait DFS

European dual-band Keenetic models, i.e. those designed for the European market and having the EU and TR country code, use the DFS (Dynamic Frequency Selection) mechanism on the 5 GHz Wi-Fi network. DFS comes in two types:

  • In-band;

  • Out-of-band (Zero Wait DFS).

Keenetic Titan (KN-1811) utilises Zero Wait DFS.

In turn, Zero Wait DFS is subdivided into Dedicated Zero Wait DFS and Adjustment Zero Wait DFS based on the operation mechanism..

Keenetic Titan (KN-1811) uses Dedicated Zero Wait DFS (channels 52–140).

Below, we'll look into the algorithms of In-band DFS and Out-of-band DFS modes in detail.

In-band DFS

In the In-band DFS mode, if a Wi-Fi access point switches to a manually set channel or as a result of the auto channel selection, the CAC (Channel Availability Check) mechanism activates on the access point:

  1. The access point disables any data sending, even beacons.

  2. The access point starts the radar detection mechanism.

  3. The radar detection mechanism lasts at least 600 seconds for channels 116–128 (weather radar channels) and at least 60 seconds for other channels. During this time, the access point will be unavailable, and you cannot connect to it.

  4. If no radar signal is detected during the above time, CAC operation is terminated, data transmission and beacons are switched on, but the detector continues to listen to signals in the background.

  5. If a radar signal is detected during CAC operation or during normal channel use, the IEEE 802.11h standard requires the AP to select another random channel from the list of available bandwidth-aware channels. If it happens to be channel 36–48, then DFS is completely disabled, and the AP goes into normal mode. If it turns out to be another DFS channel, everything is repeated from the first CAC checking step. In this case, the channel on which the radar was detected (more precisely, a group of channels considering the bandwidth) is marked as unusable during the NOP (Non-Occupancy Period) for 30 minutes. Such channels cannot be selected manually, either in the auto-selection process or in the DFS procedure, until the NOP expires. The user can manually select such a channel, but the AP will ignore this setting. For Central Europe (CE) regions, there is one nuance related to the use of the 160 MHz band and channels 36–48. If you select any channel from 36 to 48 on this band, channels 52–64 are also activated when sending data, so CAC will start tracking radars on channels 52–64 too.

Out-of-band DFS (Zero Wait DFS)

The Dedicated Out-of-band DFS (Dedicated Zero Wait DFS) mechanism requires a dedicated radio module that can listen for channels independently of the primary AP.

In this mode, when you attempt to use a DFS channel (for example, 52), or if DFS channels fall into a channel block according to the selected band, the Wi-Fi access point performs the following actions:

  1. Tunes the dedicated antenna to the specified channel (52) and band; the Spectrum Analyser function is temporarily suspended.

  2. Starts the radar detection mechanism on the dedicated out-of-band CAC channel. The operating time of the out-of-band CAC is 90 minutes for weather radar channels and 6 minutes for other channels.

  3. Switches the main radio module's channel temporarily to a non-DFS channel (e.g. 36 or 149) while waiting for the out-of-band CAC to complete.

  4. If the out-of-band CAC mechanism has not detected a radar, the main radio's channel is switched to the previously set and listened to channel (52). Next, a second backup DFS channel (from the DFS pool), e.g. 100, is randomly selected. The dedicated antenna is switched to listen (out-of-band CAC) to this spare channel. After listening to the spare channel, it is marked radar-free and the dedicated radio returns to the Spectrum Analyser function. The primary radio operating on the current channel (52) starts the radar detector on the main radio (in-band CAC) and continues listening to the current channel (52). If a radar is detected on the current channel, the channel is marked with NOP, and then an equal probability selection is made from the list of available channels. If it turns out to be channel 36–48, the DFS mechanism is completely switched off, and the access point switches to normal mode. If it is a different DFS channel, the previously listened to backup DFS channel is used instead.

  5. If a radar is detected during the out-of-band CAC, the channel is marked NOP, and a new DFS channel is selected with equal probability to listen for the radar on the dedicated module. After successful listening, it returns to step 4.

Usually, the maximum operating bandwidth of the dedicated antenna is limited to 80 MHz, so some devices disable Zero Wait DFS when selecting the 160 MHz bandwidth, and some implement a two-stage listening mechanism. In the second case in step 4, the same way the transition to a given channel takes place, but on the 80 MHz band, and the dedicated antenna listens for the CAC of the second half of the band. Only after successfully completing the two CACs, the channel and band become equal to the set channel, and the dedicated antenna proceeds to listen to the backup channel.

The Adjustment Zero Wait DFS has a channel block limit of 36–64, but does not require a dedicated radio module. If a channel from channel block 36–64 on the 160 MHz band, or a channel from channel block 52–56 on the 80 MHz band, is selected (automatically or by the user) on such a device, the following actions will be performed:

  1. If the selected channel falls in the 52–64 block, it will be temporarily changed to 36. The effective band (displayed in beacon frames; used for association and client management) will change to 80 MHz, with the effective band (on which the radio receives data from the air) becoming 160 MHz to guarantee coverage of the block's DFS channels.

  2. If no radar is detected during CAC, the channel and band are switched to the selected one.

  3. If a radar is detected, block 52–64 is marked NOP and the channel is changed according to standard DFS rules. If the transition results in a forced band reduction to 80 MHz and the selected channel falls into block 36–48, an attempt to extend the band to 160 MHz will be made after the NOP time expires on block 52–64 through CAC pre-listening (starting from step 1).

On Keenetic devices, the DFS activation is strictly linked to the region parameter, which is set at the factory. For example, on a device manufactured for the EAEU region, the DFS mechanism will be switched off, while on models for the EU and TR regions, it will be switched on.

Devices that work only as basic Wi-Fi access points (without client part, i.e. without using Wi-Fi Client, WISP, or WifiStation) act as DFS masters. This means that the access point is the master and switches channel in case of radar detection immediately, generating a CSA (Channel Switch Announcement) in the form of a packet and updating the beacon. Clients are led by and follow the AP by receiving a CSA or by losing the AP's beacons on the air.

If a device uses some client part (Wi-Fi Client, WISP, WifiStation) in addition to the Wi-Fi access point, the access point becomes a 'slave' and obeys the WifiStation. The DFS mechanism is temporarily disabled when the WifiStation connects to the remote point and during connection checks.

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