Understanding the network.pdf - Back to Home

NOTE
Another common network error not related to data transmission is duplicate Layer
3 (network layer) end-station addresses, particularly with IP. Under normal
operating conditions, IPX and AppleTalk are relatively immune to duplicate
addressing problems. However, because IP addresses can be statically assigned and,
in many cases, changed by users with ease, duplicate addressing with IP is common.
A packet sniffer can be used to track down duplicate network address users using
the system's MAC addresses. If you do not know the location of the devices or the
MAC addresses associated with the duplicate address, however, tracking down the
location of the mis-addressed station is difficult. This is why good network
documentation is so important.
Ethernet Errors
When looking at Ethernet performance, you will come across four types of errors:
collisions, errored or corrupted frames, illegal frame, and jabbers. Collisions are a
natural byproduct of the CSMA/CD medium-access method employed by Ethernet
to manage access to the transmission medium. Collisions result when two stations
are on the same segment attempt to transmit data on the medium simultaneously.
Under normal operation, a moderately populated segment (20 to 200 end-stations)
should have a collision rate of around 1 to 2 percent of the overall segment traffic.
Higher density segments (200 to 500 end-stations) should expect collision rates of
around 3 to 5 percent.
If the collision rate for the segment exceeds normal operational values, the endstations
involved should be identified and examined for possible faults. High
collision rates can be the result of a bad hub or transceiver, a faulty NIC or connector,
or a cable length violation.
When errored or corrupted frames are transmitted over the wire, they are recorded
as CRC errors by protocol analyzers and management probes. To ensure data
integrity, Ethernet employs Cyclical Redundancy Checking. Before an Ethernet
frame is transmitted, the NIC performs a CRC on the "header and data" portion of
the frame and appends the result, to the end of the Ethernet frame. When the
station receives a frame, it performs its own CRC check on the header and data
portion and compares it to the CRC transmitted along with the Ethernet frame. If
they match, the frame is processed further; if they do not, a CRC error results and
is sent to the network—the frame is discarded. Often, a CRC is due to an alignment
error in the bit placement of the frame. An occasional CRC error can be attributed to
environmental or corrupted ULP data. CRC error rates above 2 percent of network
traffic indicate a problem, often a bad NIC, transceiver or cable connection.