Access Point Channel Changed

WiFi Manager raises this alarm when any of the Access point in the wireless LAN changes its configured channel.

What is a channel ?

Channel is the frequency on which the transmission or reception of signals takes place. Wireless devices adhering to 802.11 b/g standard operates on 2.4 GHz ISM (Industrial, Scientific and Medical) band and uses the frequency between 2.402 GHz to 2.483 GHz. This band is further divided into 14 channels. The channels and their central frequency is highlighted in the table below:

If you see the above table you see that the difference between the central frequency of two adjacent channels is 5 MHz, also each 802.11b channels occupy 22 MHz. So, the adjacent channels overlap with each other. In order to avoid interference problems Access points using 802.11b should be 5 channel apart. Say for example channel 2, 7 and 12 are non-overlapping channels.

802.11b/g Channels and frequency overlaps

Similarly devices adhering to 802.11 a standard operates in the 5GHz UNII (Unlicensed National Information Infrastructure) band. and uses the frequency between 5.15 GHz to 5.35 GHz and 5.725 GHz to 5.825 GHz. This band is further divided into 16 channels. The channels and their central frequency is highlighted in the table below -

 

Channel Identifier	Frequency (in MHz)channel
36	5180
40	5200
44	5220
48	5240
52	5260
56	5280
60	5300
64	5320
149	5745
153	5765
157	5785
161	5805

 

As per the 802.11 standard the channel in which an Access point should operate is configurable and the wireless LAN Administrator can set the same on the Access point. Wireless client which needs to associate itself with the Access point, scans the entire band, identifies the channel in which the Access point is operating, adjusts its frequency to the same channel and then associates itself with the Access point.

What can be the probable reasons for the channel change ?

The change in operating channel can be because of any one of the following reasons:-

• Reconfiguration of the Access point as part of the network changes carried out by wireless LAN administrators.
• Access point automatically changing its channel to find a channel with least interference.
• WLAN attack, say for example if this alert is seen together with MAC address spoofing alerts then it may be because of an attacker masquerading as Access point in order to mount a man-in-the-middle attack or to capture legitimate clients.
• Attacker gaining access to the Access point configuration interface and making the changes.

Why is it a problem when Access points change their channel ?

If the change in channel is planned and legitimate then there should not be any problem, whereas a unplanned change can cause avhoc. Usually channels in the Access points are configured after a site survey taking into consideration the various RF parameters like signal, noise, inter Access point interference etc., Unplanned changes may lead to increased interference and can render the site survey useless.

What should the wireless LAN Administrator do ?

Wireless Administrator have to analyze the reason for such change in channel and take appropriate action. Most of the Access point have the capability to automatically change their channel and find the one with less interference. If this feature is enabled in the Access point then it is better to switch off this alarm in WiFi Manager.