For the DFS method (Dynamic Frequency Selection) required for 5 GHz WLANs, an unused frequency is automatically selected, for example, to avoid interference from radar systems. Occasionally, however, signals from weather radar stations cannot be identified reliably.
For this reason the European Commission is extending the requirements of the standards ETSI EN 301 893 V1.3.1 and ETSI EN 301 893 V1.4.1 to additionally avoid the use of three channels (120, 124 and 128) in subband 2 of the 5 GHz band, and not to allow use of these bands for automatic channel selection until a process to auto-detect weather radar station signals is made available. The versions EN 301 893 V1.3 and EN 301 893 V1.4 are referred to as "DFS-2".
In the middle of 2010 the new version ETSI EN 301 893 V1.5.1 came into force, which was accompanied by changes in the usage of WLAN frequencies in the ranges 5.25 to 5.35 GHz and 5.47 to 5.725 GHz. The new Version 1.5.1 regulates the DFS (Dynamic Frequency Selection) method for the protection of radar stations from WLAN systems working in this frequency range. By using DFS to detect certain patterns in the radio signals received, it is now possible to detect active radar stations, and WLAN systems can automatically switch their operating channel. To differentiate from previous regulations, the new standard EN 301 893-V1.5 for the updated DFS is referred to as "DFS-3".
A pulse pattern can generally be described in terms of its pulse rate, pulse width and the number of pulses. Former DFS technology was only able to detect fixed radar patterns as defined by the various combinations of pulse rates and pulse widths which were stored in the WLAN device. According to DFS-3, the device is now able to recognize changing pulse rates and pulse widths as radar patterns. Furthermore, two or three different pulse rates may be used within a radar signal.
The version ETSI EN 301 893 V1.5.1 (DFS-3) expired on January 01, 2013. The new version ETSI EN 301 893 v1.6.1 (known as "DFS-4"), which applied thereafter, also detects shorter radar pulses.
The version ETSI EN 301 893 V1.5.1 (DFS-4) expired on December 31, 2014. The new version ETSI EN 301 893 V1.7.1 applies (known as "DFS-5"), which applied thereafter, brought some changes to signal strengths. Since then there have been several other revisions to this standard.
Operating principles
After switching on or booting, the device randomly selects one of the available channels (e.g. based on the country settings). It checks whether radar signals exist on this channel, and whether it is already in use by another WLAN. This scan is repeated until it finds a radar-free channel with as few other networks as possible. The selected channel is then monitored for radar signals for a further 60 seconds. For this reason, data traffic may be interrupted for a period of 60 seconds while the frequencies are scanned for a free channel.
- Was a radar signal detected on the channel?
- How many other networks were found on the channel?
- The "blacklist" in the database stores the channels to be blocked due to the detection of radar signals. This entries are removed from the list every 30 minutes in order to keep the information up to date.
- The "whitelist" in the database stores the channels where no radar was detected. These entries remain valid for 24 hours, although if radar signals be detected in the meantime, an entry is made to the blacklist.
By default, the AP permanently uses the channel that was selected as the operating channel during the first scan. Connections can now be operated for any length of time on the channel selected by the DFS algorithm until either a radar signal is detected or the radio cell is restarted (e.g. by changing the device configuration, firmware upload, or restart).
- The device is switched on or cold-started. In this case the database is empty, so the device cannot select the preferred channels from the whitelist. A scan has to be performed.
- Within the first 24 hours of scanning, it becomes necessary to switch channels because a radar signal is detected within range of the AP. In this case, the AP can refer to alternatives in the whitelist. It then informs associated WLAN clients and/or P2P partners of the new operating channel and switches to this channel. This process takes place within about a second, so the switch can be considered to be uninterrupted.
- The device is in operation for 24 hours already, so a new channel scan is required. Entries in the whitelist are "out of date" and thus discarded. The AP has no alternative channel that it can switch to directly. In this case the database requires new information from a scan and WLAN operation is interrupted for one minute.
