At the point of transmission, all signals are binary. That is, they are on or off. Depending on the encoding scheme of your network they are transmitted as positive or negative electronic pulses. These changes in current signify individual bits of the transfer. A transmitting piece of electronics must transfer these electrical or optical signals in an encoding fashion that the receiving device can understand. Further, the transmission must be free from external interference or EMI (Electro-Mechanical Interference) in order for all bits and electronic signal changes to traverse the cabling properly. We now see that the cabling becomes an important part of the network. What happens when a packet is moved to the cabling, and there is some interference or malfunction that causes the packet not to be received by the end device? Very simply, it will be discarded either at the switch or the receiving station and then will need to be retransmitted.
This is one of the hardest to diagnose problems causing an erosion of bandwidth. It is much like taking your car to a mechanic and it quits making the noise that caused the visit. Frequent retransmissions are a bandit of bandwidth. Bearing in mind that all data at this point is binary, one bit being dropped will cause the error checking to fail, and the entire packet must be transmitted again. This can be caused by poor cabling, poor installation of cabling, noise interference, poor grounding, etc. Cabling that is terminated outside
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of specification guidelines can cause a significant increase in these problems. All cabling is manufactured to tight standards. It is designed to have the appropriate amount of twists per foot, sheathed to lessen susceptibility, etc. Untwisting the pairs by violating bend radius requirements, cinching wire ties too tightly and not supporting the cable properly will cause errors. Stripping the sheath back too far and untwisting too much of the cable are common problems both at the wall, at the patch panel and in the patch cables. Fiber can also suffer from installation issues at the points of termination, bend radius issues, too many splices, etc.
SNMP (Simple Network Management Protocol) provides statistics for errors and discards. Using a management tool, one would need to examine these statistics on a per port basis. These errors can be evident in the following statistics,
Discards (In and Out)
Retransmissions
Errors (In and Out)
If any of these statistics is evident on one port and not other, a good place to begin investigating is the cabling. It is recommended that a certified cable installer test the cabling with an appropriate tester. The benefit of this type of testing is that there is a means to identify some of the interference and other factors that will cause cabling to function substandard to certified expectations. While you may not see errors at 10/100 operations, moving to higher frequencies, the problems may begin or worsen.
Errors in network electronics, buffers too small in the electronics, etc. will also cause retransmissions. While active components are the most visible components, and usually the first to be remediated, if the errors are caused from the physical channel, replacing the electronics will not stop the errors. It is far less expensive to test the cable and assure its stability first. Remember, when testing, it is imperative that the tester is recently calibrated and contains the latest firmware and software. Likewise, when testing electronics, these components should also have the latest hardware and firmware.
Some network errors may be corrected by increasing the buffer size in your equipment. Network devices can discard packets either because they are errant, runt, giant or jumbo packets or because the buffer is full. Understanding the reason for the discards is important. If the retransmissions are causing the issues, begin with the reason for retransmissions. If there are no retransmissions, but still a lot of discards, look at your buffer utilization to determine if you are overworking your equipment, it may be time for an upgrade to parts or all of it. It is in your best interest to learn what capabilities exist for upgrades prior to scratching the equipment and starting over. If you have an end of life notice on the equipment, it is wise to upgrade as soon as the budget allows. Newer equipment is far better at handling traffic than older versions and the wire speed interfaces have also increased in capability over time.
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This was first published in February 2009