Special regulations for the 5 GHz band

Dynamic Frequency Selection – DFS

Certain requirements must be observed for the outdoor operation of 5-GHz WLANs if you wish to utilize the maximum permitted performance of 1 or 4 watts. It is vital to avoid interference with radar systems that are active in this spectrum (e.g. meteorological, military). For this reason the European regulatory authority ETSI requires WLAN devices operating at 5 GHz to employ the dynamic frequency selection (DFS) mechanism.

This ensures that radar and WLAN systems can co-exist without interfering with one another and that capacity utilization is spread evenly across available frequencies. When starting a WLAN wireless cell, the access point must check all channels for the presence of radar systems. The check requires an inactive period of one minute, during which the wireless cell cannot be used. As a result, the access point generates a list of radar-free channels which is valid for 24 hours. The best possible channel for operation is selected from this list. During operation, the current channel is continuously checked for radar activity.

If a radar system subsequently starts operation, the channel must be released immediately. In this case, the access point selects the next best available channel, informs the participants in the wireless cell of the impending change, and switches the channel.

Note: The currently selected channel can be used for any length of time, unless radar signals are detected or if the radio cell is restarted (e.g. due to device reconfiguration, firmware upload or reboot).

Note: If the system is able to respond to a channel switch instantaneously, the check must be repeated within 24 hours following a one-minute period of inactivity. The parameter "DFS Rescan Hours" (LCOS menu tree under "Setup/Interfaces/WLAN/Radio settings") allows a time to be set for conducting the channel check (assuming that the time is available, for example via NTP).

DFS is stipulated for the frequency ranges from 5250 - 5350 MHz, 5470 - 5725 MHz and from 5775 – 5875 (BFWA). It is optional for the frequency range of 5150 - 5250 MHz.
Transmission Power Control – TPC Automatic adjustment of the transmission power reduces radio interference. Without DFS and TPC, a maximum of only 200 mW EIRP is permitted. When operating DFS and TPC, a maximum of 200 mW (5150 to 5350 MHz) and 1000 mW EIRP (5470 to 5725 MHz) is permitted as transmitting power (compare 100 mW for 802.11b/g, 2.4 GHz, where DFS and TPC are unnecessary). The higher maximum transmission power not only compensates for the higher attenuation of 5 GHz radio waves in air, it also makes significantly longer ranges possible than in the 2.4 GHz range.
BFWA (broadband fixed wireless access) In Germany in July, 2007, the Federal Network Agency released additional frequencies for broadband fixed wireless bridges in the 5 GHz band. These additional frequencies located in the range between 5755 MHz - 5875 MHz are also referred to as BFWA (Broadband Fixed Wireless Access). The additional frequencies are intended for long-distance point-to-point (P2P) or point-to-multipoint (P2M) links used for providing high-speed Internet access to other users from a central node. This method is intended to provide rural areas with high-speed Internet access. The operation of BFWA is restricted to commercial providers only. There are no charges for using these frequencies, but registration is required by the Federal Network Agency. This band covers 120 MHz and offers 6 channels with a bandwidth of 20 MHz each. Maximum transmission power is 36 dBm or 4000 mW. TPC and DFS have to be used when operating BFWA links.