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Rogue detection and counter measures 629
Nortel WLAN—Security Switch 2300 Series Configuration Guide
RF detection scans
All radios continually scan for other RF transmitters. Radios perform passive scans and active scans:
• Passive scans—The radio listens for beacons and probe responses.
• Active scans—The radio sends probe any requests (probe requests with a null SSID name) to solicit
probe responses from other access points.
Passive scans are always enabled and cannot be disabled. Active scans are enabled by default but can be
disabled on a radio-profile basis.
Radios perform both types of scans on all channels allowed for the country of operation. (This is the regulatory
domain set by the set system countrycode command.) 802.11b/g radios scan in the 2.4 GHz to 2.4835 GHz
spectrum. 802.11a radios scan in the 5.15 GHz to 5.85 GHz spectrum.
Both enabled radios and disabled radios perform these scans.
The active-scan algorithm is sensitive to high-priority (voice or video) traffic or heavy data traffic. Active-scan
scans for 30 msec once every second, unless either of the following conditions is true:
• High-priority traffic (voice or video) is present at 64 Kbps or higher. In this case, active-scan scans for
30 msec every 60 seconds.
• Heavy data traffic is present at 4 Mbps or higher. In this case, active-scan scans for 30 msec every
5 seconds.
On a disabled radio, the radio is dedicated to rogue detection and scans on each channel in round-robin
fashion.
Dynamic Frequency Selection (DFS)
Some regulatory domains require conformance to ETSI document EN 301 893. Section 4.6 of that document
specifies requirements for Dynamic Frequency Selection (DFS). These requirements apply to radios operating
in the 5 GHz band (802.11a radios).
In countries where Dynamic Frequency Selection (DFS) is required, WSS Software performs the appropriate
check for radar. If radar is detected on a channel, the AP radio stops performing active scans on that channel in
accordance with DFS. However, the radio continues to passively scan for beacons from rogue devices.
When an AP radio detects radar on a channel, the radio switches to another channel and does not attempt to
use the channel where the radar was detected for 30 minutes. WSS Software also generates a message.
Countermeasures
You can enable WSS Software to use countermeasures against rogues. Countermeasures consist of packets
that interfere with a client’s ability to use the rogue.
Note. The Auto-RF feature must be enabled. Otherwise WSS Software cannot change
the channel.