An Introduction to Network Cable Testing

Network Cable Testing

Testing new cabling is an important consideration. During the installation process cables can become damaged or kinked, insulation can be broken, some connections may be poorly crimped, and so on. Electrical interference and other environmental factors may affect network performance. You will probably need to produce a report for your customer validating that everything conforms to the required standards.

It is wise to test the cables again at a later date to check for performance reductions or for damage to the cables that may have developed.

But what kind of testing is required? How do you decide which tests are important? What network testing tools do you need?

What do you need to test?

What you need to test depends on what you want to use the cabling for. A simple telephone cable will need only a few basic connectivity tests whereas a Gigabit data cable will need a more comprehensive set of tests to validate that it will handle the required data rate, that various technical aspects of the cabling are within specification and so on. The faster the network the more likelihood you will need a set of dedicated testing tools. The following table summarises the tests required for each cable type.

Cable Type	Test For
1000BaseTX	Pins connected correctly, Cable Length, Wire Map, Attenuation, NEXT, Skew, Propagation Delay, PS-NEXT, ELFEXT, PS-ELFEXT Return Loss
1000Base-T	Pins connected correctly, Cable Length, Wire Map, Attenuation, NEXT, Skew, Propagation Delay, PS-NEXT, ELFEXT, PS-ELFEXT
100Base-TX	Pins connected correctly, Cable Length, Wire Map, Attenuation, NEXT, Skew, Propagation Delay
10Base-T	Pins connected correctly, Cable Length, Wire Map, Attenuation, NEXT
Telephone (Voice) Cable	Pins connected correctly, Wire Map

Wire Map

A wire map is a basic test to check that the pairs are connected properly for the pinout system you are using. Your cabling may work if pairs are transposed, but it will be confusing to others and may make future troubleshooting more difficult. A network testing tool providing Wire Mapping is relatively inexpensive and will save hours of work. A Wire Mapping tool should test for the following.

Open Pairs

A wire not connected at either end will report as Open. Usually caused by a missing or faulty crimp or broken wire.

Shorts

Low resistance between two wires. Caused by two wires in contact somewhere in the cable.

Miswires/Reverse Pair

One or more wires are not connected to the correct pins at the other end of the cable. A Reverse Pair is when the correct pins are used but the connectors are reversed.

Split Pair

Pairs should be connected (T568A or B standard) in the order 1&2, 3&6, 4&5, 7&8. A cable would pass a connectivity test if both ends were incorrectly connected with the wrong pairings. If the same mistake is made at both ends than the cable will likely pass connectivity tests.

Cable Length Test

Specifications exist for all types of cabling and vary depending on the cable type you use and the type of network you plan to install. The maximum allowed cable lengths are an important consideration in your initial planning of the network and you should never exceed them. A network tester should allow length testing.

Although on paper you cable lengths may be fine it is worth checking them on the finished installation. Remember that due to the twist in the cable the actual length of the conductors will be more than the outside length of the cable. Not only will you know for certain that everything is within the specification, but you will also be able to detect shorts or faulty connections and so on by measuring the cable lengths.

Attenuation

All signals degrade when transmitted along cables. This degradation is called Attenuation. Too much attenuation can cause signals to be lost and is especially critical on high speed networks and where cable runs approach the maximum allowed for the network type.

Attenuation is measured in decibels (dB). Attenuation is measured by attaching a device to each end of the cable, sending a calibrated signal and measuring the loss. The worst case measurement indicates the performance of the cable.

Near End Crosstalk (NEXT) occurs when a signal on one pair is picked up by adjacent pairs in the cable. For completeness measurements are made for each pair against the other three pairs and from both ends of the cable. The worst case measurement indicates the performance of the cable.

For higher rate networks where signals are transmitted in two pairs at the same time (such as Gigabit Ethernet) a cumulative measure of crosstalk PS-NEXT (Power-Sum Next) is used. The worst case measurement indicates the performance of the cable.

Far End Crosstalk (FEXT) is a type of crosstalk that occurs at the other end of the cable. In order to average the crosstalk over the whole cable run you need to calculate the equal-level FEXT (ELFEXT). Subtracting the Attenuation value from the FEXT value gives the ELFEXT.

For higher rate networks where signals are transmitted in two pairs at the same time (such as Gigabit Ethernet) a cumulative measure of Far End crosstalk, PS-ELFEXT (Power-Sum ELFEXT) is used. The worst case measurement indicates the performance of the cable.

Propagation Delay

The time taken for the signal to reach the far end of the cable is the Propagation Delay. In twisted pair cables the length of the conductors are not the same, due to the different twists of each pair and so the propagation delay can also vary. On high speed networks that use all four pairs (such as Gigabit Ethernet) the signals need to arrive within a short time period, otherwise data will be lost.

Skew

The difference between the highest and lowest Propagation Delay in a cable is known as the Skew. On high speed networks that use all four pairs (such as Gigabit Ethernet) Skew should be as low as possible.

Return Loss

Impedance, measured in Ohms, is a measure of reflections due to variations in the cable construction and in crimps and connectors. The cumulative amount is known as the Return Loss. Too much return loss causes transmission errors and is worse on higher speed networks and at higher frequencies. The worst case measurement of a pair indicates the performance of the cable.

Network Testing Tools

It makes sense to invest in a set of network testing tools to help with validating your installations. Some of the more specialised tools are very expensive, but many tests can be performed by lower cost kits. Investing in a good set of network testing tools is wise, and even if you are not a full time installer, the time and money saved by having tools to hand is well worth while.

Summary

Testing your installation is vital, to check that everything is connected correctly, that the cabling meets the required standards, that the network will handle the data rates specified, and to produce a report for your customer that everything is in order.

You do not have to do all tests in all circumstances. Some cables only need a basic set of tests. Faster networks handling more data require more detailed testing. You should retest at a later date to check for deterioration in network performance. Some well chosen network testing tools will make the job much easier and save you a lot of time and money.