Troubleshooting guide

Troubleshooting Guide
176
Collisions
Shared LANs
A collision occurs on a shared LAN when two or more network devices transmit data onto the
LAN at the same time. After detecting a collision the network device must abort transmitting
the packet and start transmitting a jam pattern to reinforce the collision, Then, the device must
wait a random period of time before attempting to re-transmit the packet. Excessive collisions
can affect LAN and network device performance.
Collisions can also cause alignment errors, FCS errors and fragmented packets. The number
of collisions that occur on a LAN can be related to traffic patterns on the LAN. Because of the
nature of collisions, it is difficult to state what is an acceptable number of collisions and what
is an unacceptable number of collisions. However, a high number of collisions on a LAN could
be an indication of faulty equipment on the LAN, or it could be an indication that the LAN is too
congested.
Switched LANs
On a half-duplex, twisted-pair interface, a collision occurs when receive and transmit are active
simultaneously.
A twisted-pair interface that is configured for full-duplex operation allows for simultaneous
transmission and reception of data. Collisions should be non-existent or minimal on LANs that
are running in full-duplex mode.
CRC/FCS & Alignment Errors
When a network device transmits a packet, it appends a Cyclical Redundancy Check (CRC)
to the end of the frame. The CRC value is unique for the particular packet since, like checksum
generation, the data in the packet is used by the CRC generation algorithm to generate the
CRC value.
If the data in the packet gets altered between the transmitting device and the receiving device,
then the receiving device will detect that the packet has been altered since the CRC will not
match the contents of the packet. The result is an CRC error.
A FCS error (Frame Check Sequence) is another name for a CRC error.
An alignment error occurs when a packet has an FCS error and the packet also fails to have
octet alignment. When a packet has octet alignment the packet has an even byte count.
Frame Length Errors
Legal length for ethernet packets is from 64 to 1518 bytes. Ethernet packets that are shorter
then 64 bytes or longer than 1518 bytes are illegal length packets and will cause the receiving
device to count an error.