Installation guide

Wireless Guide 147
Advanced Features for Wireless Analysis
 In an ad hoc wireless network (a wireless network with no access to a
distribution system), the Expert generates this alarm when it receives
beacon and/or probe response frames from a wireless station on a
channel other than the channel on which the station is operating.
In an 802.11 infrastructure wireless network, access points send beacon
frames at a regular interval. In addition, they send probe response frames in
response to probe request frames sent from wireless stations wanting to join
the network. In an ad hoc network, the stations themselves send beacon and
probe request frames.
Among other parameters, beacon frames and probe requests specify the
wireless channel on which the basic service set (BSS) is operating. The
wireless stations in a single BSS can only operate on one channel at a time —
the channel on which the BSS is operating. However, due to adjacent channel
interference, wireless stations can occasionally receive frames from stations
operating on a different channel. The Expert generates the Channel
Mismatch alarm when this happens.
PLCP Error
The Expert generates the PLCP Error alarm when it receives a Physical Layer
Convergence Protocol header with an invalid checksum.
Before frames are sent between wireless stations, the physical layer (PHY)
sends a PLCP header to a receiving station to negotiate the size of the frames
to be sent, the speed at which they should be sent, and so on. This PLCP
header includes a checksum that the receiving station uses to validate that the
received PLCP header is not corrupt. The Expert generates this alarm if it
receives a PLCP header in which the checksum is corrupt.
Wireless Layer Expert Alarms for Wireless Networks
ACK Frame Timeout
The Expert generates the ACK Frame Timeout alarm when it does not see an
acknowledgment to a unicast management or data frame within the time
specified in the Duration field of the original management or data frame. When
this happens, the sending station will resend the original frame and wait for
another ACK.
Unicast management and data frames include a Duration field indicating the
amount of time within which a receiving station should return an ACK frame.
The value of this field is typically equal to the amount of time required to send
an ACK frame plus one short interframe space (SIFS). The Duration field lets
other stations on the network know that during this period, the medium is
reserved for the response to the frame.