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Network Health
Introduction
3
3-7
Interpreting Error Results
Collisions
A collision is the result of two or more nodes transmitting
at the same time on the segment. Collisions are not
necessarily bad. They are a normal part of Ethernet’s
operation. In general you need not worry about collisions
unless the AVERAGE collision rate is greater than 20%.
Excessive collisions are more often associated with too
much network traffic and less often a physical problem
with the network. Usually the best way to fix a “collision
problem” is to understand why there is excessive traffic.
You may find that the Network Assistant collision count
does not agree with that of some protocol analyzers that
under report collisions. Just like a Hub, the Network
Assistant identifies collisions that occur in the frame’s
preamble. These are the most common types of collisions
in a 10BASE-T network.
Late Collisions
A late collision is one that occurs after the first 64 bytes in
a frame. Consider late collisions a serious network error
to be resolved quickly. Late collisions may manifest
themselves as frames with a bad Frame Check Sequence
(FCS). Late collisions are caused by either a faulty NIC
or a network that is too long (i.e., end-to-end signal
propagation time is greater than the minimum legal frame
size of ~57.6 microseconds for 10BASE-T).
Short Frames
A short frame is a frame that is less than the minimum
legal size (less than 64 bytes) with a good frame check
sequence. In general, you should not see short frames.
The most likely cause of a short frame is a faulty card or
an improperly configured or corrupt NIC driver file.
Jabbers
A jabber is a frame greater than the maximum legal size
(greater than 1518 bytes) with a good or bad frame
sequence.
Consider jabbers a serious network error to be resolved
quickly. The most likely causes of Jabbers are a faulty
NIC or driver or perhaps a cabling problem.
Bad Frame Check Sequence (FCS)
A legal sized frame with a bad frame check sequence
(FCS) has been corrupted in some way. Bad FCSs can
be caused by late collisions, a faulty NIC/driver, cabling,
hub or induced noise.