Category 5 (CAT5) Cable

Category 5, or as it is more commonly known, CAT5 is a twisted pair cabling standard developed by the TIA.

Physical Properties of CAT5

Four wire pairs are twisted around each other within a protective sheeth. Twisted pair cables have largely won out over thick or thin coaxial cable because twisted pair cable is cheaper & easier to install.

The CAT5 standard was developed for use with 100BASE-TX networks in mind. 100BASE-TX only uses two of the four wire pairs. CAT5 can also be used for 1000BASE-TX networks though it is not recommended due to increased signal reflection by the cable connectors & signal crosstalk. 1000BASE-TX uses all four of the wire pairs.

Cable runs should not exceed 100 meters (328 feet).

Typical Uses for CAT5

CAT 5 cable is usually used in structured cabling installations. Structured cabling was very large step up both in terms of reliability and bandwidth over the earlier shared media networks that went before.

Older shared media networks tended to be very unreliable because any fault on the cable itself, like a missing or faulty terminator for instance, would bring down the entire network. In addition, all of the network traffic was going through a single wire so bandwidth was limited. Security could also be a problem because all of the network traffic was being carried inside a single wire, any of the stations on the network could therortically sniff the network and see all of the traffic from all of the nodes.

Structured cabling systems are also useful because they help to make the cabling more flexible in how it can be used. Many indistries like audio/visual and telephony have converged upon structured cabling as a means of communication. In the past each industry used its own cable standards that had to be installed alongside all of the other cabling. You would need to know precisely how many outlets of each type to place in each location. An outlet indended for one technology couldn't be used for anything else.

Structured cabling systems mean that everything can use CAT5 as the physical cabling standard meaning that all outlets can be used for any technology.

Testing CAT5 Cabling

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
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.

CAT5 Cable Performance Requirements

Permanent Links
Frequency	NEXT
1 Mhz	60 dB
10 Mhz	45.5 dB
100 Mhz	29.3 dB
Frequency	PS_NEXT
1 Mhz	N/A
10 Mhz	N/A
100 Mhz	N/A
Frequency	ELFEXT
1 Mhz	57.7 dB
10 Mhz	37 dB
100 Mhz	17 dB
Frequency	PS-ELFEXT
1 Mhz	N/A
10 Mhz	N/A
100 Mhz	N/A
Channel Links
Frequency	NEXT
1 Mhz	60 dB
10 Mhz	44 dB
100 Mhz	27.1 dB
Frequency	PS_NEXT
1 Mhz	N/A
10 Mhz	N/A
100 Mhz	N/A
Frequency	ELFEXT
1 Mhz	57 dB
10 Mhz	37 dB
100 Mhz	17 dB
Frequency	PS-ELFEXT
1 Mhz	54.4 dB
10 Mhz	34.4 dB
100 Mhz	14.4 dB

CAT5 Testing Equipment

A wide range of CAT5 testing equipment is available, from high end cable certification testers, through qualification testers all the way down to simple continuity testers.