Installation guide

Chapter 10
152 Sniffer Technologies
Possible Cause
1 A relatively small number of these alarms is no cause for concern.
802.11 guarantees the sequential arrival of fragments at a receiving
station, but occasionally fragments may be missing due to interference
or other network problems. This is why the fragment number exists —
so that receiving stations can reassemble data units in the intended
order regardless of the sequence in which they arrive.
Because each fragment must be positively acknowledged by the
receiving station, 802.11 provides a mechanism to ensure that all
fragments eventually do arrive. If a sending station does not receive the
ACK for a fragment, it simply resends the fragment after an internal
timer expires. If the receiving station receives multiple copies of the
same fragment, it discards the excess copies of the fragment.
With this in mind, you can see that a large number of Missing
Fragment Number alarms may indicate significant interference on the
network. You should check the Dashboard to see if there are also a
large number of CRC errors on the network. If this is true, you may want
to adjust the fragment size used by the MAC to use smaller fragments
and see if this reduces the number of CRC errors on the network (and,
correspondingly, the amount of Missing Fragment Number alarms
generated).
Oversized WLAN Frame
The Expert generates the Oversized WLAN Frame alarm when it detects an
802.11 MAC frame longer than the maximum acceptable length dictated by the
802.11 specification.
The maximum acceptable length for an 802.11 MAC frame is 2346 bytes.
Reassociation Failure
The Expert generates the Reassociation Failure alarm when it detects an
802.11 Reassociation Response frame with a value other than zero in the
Status Code field. A non-zero value in the Status Code field indicates that the
access point sending the Reassociation Response is denying the requested
association.
Wireless stations send Reassociation Request frames to become associated
with a different access point within the same network as its current access
point (for example, because the station has moved and is now out of range of
its old access point and within range of another). In turn, access points reply
to Reassociation Requests with Reassociation Responses indicating the