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378
C
17: R
HAPTER
OGUE
Ad Hoc Users
RF Detection Scans
D
C
ETECTION AND
OUNTERMEASURES
Rogue access points and users can also interfere with the operation of
your enterprise network.
Another type of rogue is a private WLAN user group. The group might be
using an access point or software that provides access point functionality
to a wireless laptop. Users can configure a wireless network interface
card (NIC) in "ad hoc" mode and easily set up a simple peer-to-peer
network. Although this WLAN might be isolated from the corporate
WLAN, the users are stealing the air from legitimate WLAN users. The
private WLAN can also cause RF interference with an authorized WLAN in
other parts of the company.
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.
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.
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, MSS
performs the appropriate check for radar. If radar is detected on a
channel, the MAP radio stops performing active scans on that channel in
accordance with DFS. However, the radio continues to passively scan for
beacons from rogue devices.
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