Wi-Fi - Dynamic Frequency Selection (DFS)

Dynamic Frequency Selection

DFS is a spectrum-sharing mechanism that allows wireless LANs (WLANs) to coexist with radar systems. It automatically selects a frequency that does not interfere with certain radar systems while operating in the 5 GHz band. DFS is a feature of ETSI BRAN HIPERLAN/2 and IEEE Standard 802.11h.

Overview

In some regions (predominantly Europe), it is important to ensure that WLAN equipment does not interfere with certain radar systems that are the primary users in the 5 GHz band. The DFS feature of the AP software searches for radar pulses in the frequency channel where it is operating, or during the autochannel scan. It constantly monitors errors in the received frames and analyzes the timing patterns for periodicity. If the pattern matches a radar signal, it instructs the WLAN equipment to discontinue operation on that channel as quickly as possible. The WLAN equipment reboots, then begins to operate on another frequency after checking that the new frequency is free of radar signals.

Operation

DFS detects radar interference and moves the wireless network to another frequency with no interference. It maintains a list of channels where radar has been detected in the NOL (non-occupancy list). The AP avoids using these channels for at least 30 minutes after detecting radar on them. When DFS is enabled, the AP:

• Looks for radar detection before securing a frequency channel.
• Scans continuously for radar signal patterns during normal operation.

Radar Signal

A conventional radar signal is a burst of pulses of a high frequency signal. When one burst is over, it is repeated after some duration, called sweep time. ITU has defined several different types of radar based on different frequencies, the number of pulses in a burst, and pulse repetition frequency within the burst. The sweep time is a result of radars that slowly rotate through 360 degrees, so that in a 30-second sweep period, the AP may be hit by the radar signal only for fraction of a second. During this hit, the AP sees a few pulses separated by a time interval (called the period), which is anywhere from 250 microseconds to 20 milliseconds. Each pulse can be 2 microseconds or smaller in width.

fonte: http://wifi-insider.com/wlan/dfs.htm