When someone purchases Cat. 5, 6 or 7 cable and connectors, why should they have to test these parts after installing them? Isn't it enough to just buy quality material and install it by the rules?
We must test, because even if the cable and connectors meet the Cat. 5 specifications, the overall performance that the installed cable run can provide can be substantially below the minimum required by the Cat. 5 definition if they are not installed properly.
To make matters worse, the effects of poor installation work may not be immediately evident if the first user to be attached to the cable run is only operating at a relatively low speed such as 10 Mbps. Many problems will only become apparent when the user tries to switch up to 100 Mbps or higher — exactly the worst time for such a problem to emerge.
This is not an uncommon problem. Even when high quality cable and connectors are used, up to 20% of all installed cable runs can deliver sub-Cat. 5 performance if the installer is not using the proper techniques. Therefore, it is important to require all Cat. 5 cabling to be field certified after it is installed. It is also important to retest cabling after any changes are made as the network grows or as reconfigurations occur. Even patch cords, typically the shortest part of any cable run, can cause a big problem if not constructed and maintained properly.
Testing can also find interactions between the cable and connector, which are not detectable independently. Some brands of cable and connector can perform poorly when installed together in short (under 60 foot) runs. Field testing is the only way to find these marginal incompatibilities.
HOW TO START
As a cable installer, you should assure the quality of your materials and work by certifying all installed cable runs at the end of each job using equipment that is specifically designed for testing these data links. Digital multimeters alone are inadequate for this task. While a good multimeter may be able to confirm that signal is getting through from one end of the cable to another, they cannot tell you anything about the quality of the signal. Are crisp pulses coming through? What is the ratio of attenuation to crosstalk? Unless your tester can do these things well, it is inadequate for data testing.
In addition, it is helpful to have a tester that saves its results, and is set up to transfer them to a computer program for documentation. (This feature will save you a lot of time when completing the project.)
Data cabling should be tested each time you install, move or troubleshoot a LAN-attached workstation, so that you can prevent cabling problems from affecting the performance of your high-speed network.
UNDERSTANDING DECIBELS
In data cabling, most energy and power levels, losses or attenuations are expressed in decibels rather than in watts. The reason is simple: Transmission calculations and measurements are almost always made as comparisons against a reference: Received power compared to emitted power, energy in versus energy out (energy lost in a connection), etc.
Generally, energy levels (emission, reception, etc.) are expressed in dBm. This signifies that the reference level of 0dBm corresponds to 1mW of power.
Generally, power losses or gains (attenuation in a cable, loss in a connector, etc.) are expressed in dB.
The unit dB is used for very low levels. Decibel measurement works as follows — a difference of 3dB equals a doubling of halving of power. A 3dB gain in power means that the optical power has been doubled. A 6dB gain means that the power has been doubled, and doubled again, equaling four times the original power. A 3dB loss of power means that the power has been cut in half. A 6dB loss means that the power has been cut in half, then cut in half again, equaling one-fourth of the original power.
A loss of 3dB in power is equivalent to a 50% loss. For example, 1 milliwatt of power in, and 0.5 milliwatt of power out.
A 6dB loss would equal a 75% loss. (1 mW in, 0.25 mW out.)
WHEN TO TEST
The testing of network cables should be done both during the installation process and upon completion of the system. Testing during the installation process helps catch problems while they are still simple to fix. Testing the system upon completion is not only a good practice but is even required by law for communications systems.
COMMON CABLE TEST EQUIPMENT
The most common testing tools for copper data cabling are the following. (We will cover optical fiber later in this lesson.)
A DVM (Digital Voltmeter) measures volts.
A DMM (Digital Multimeter) measures volts, ohm, capacitance and some measure frequency.
A TDR (Time Domain Reflectometer) measures cable lengths and locates impedance mismatches. A TDR is capable of calculating about how far down the cable the fault lies. The formula for this feature uses a cable value known as nominal velocity of propagation (NVP), which is the rate at which a current can flow through the cable, expressed as a percentage of light speed. (0.8 C, for example, being eight-tenths of the speed of light, or 240,000 kilometers per second.) The cable tester multiplies the speed of light by the cable's NVP and by the total time it takes the pulse to reach the fault and reflect back to the tester, and divides it by two, for the one-way distance.
Such cable testers cannot check the first 20 feet or so of a cable. The reason for this blind spot is that a pulse transmitted by the tester will be reflected back to the device before it is entirely transmitted. Thus, the tester can't get an accurate reading.
The tone generator and inductive amplifier trace cable pairs and follow cables hidden in walls or ceilings. The tone generator typically puts a 2 kHz audio tone on the cable under test. The inductive amp detects and plays this through a built-in speaker.
A wiremap tester checks a cable for open or short circuits, reversed pairs, crossed pairs and split pairs.
The Noise testers, 10Base-T standard sets limits for how often noise events can occur, and their size, in several frequency ranges. Various handheld cable testers are able to perform these tests.
Butt sets are telephone handsets that when placed in series with a battery (such as the one in a tone generator), allow voice communication over a copper cable pair. They can be used for temporary phone service in a wiring closet.
TESTING UTP CABLES
Many of the problems encountered in UTP cable plants are a result of mis-wired patch cables, jacks and cross-connects.
Horizontal and riser distribution cables and patch cables are wired straight through end-to-end; so that pin 1 at one end is connected to pin 1 at the other. (Crossover patch cables are an exception to this rule.) Normally, jacks and cross-connects are designed so that the installer always punches down the cable pairs in a standard order, from left to right: pair 1 (blue), pair 2 (orange), pair 3 (green) and pair 4 (brown). The white striped lead is usually punched down first, followed by the solid color. The jack's internal wiring connects each pair to the correct pins, according to the assignment scheme for which the jack is designed, such as EIA-568A, 568B or USOC.
One common source of problems is an installation in which USOC jacks are mixed with EIA-568A or 568B. When this is done, everything appears to be punched down correctly, but some cables will work and others will not.
WIREMAPPING
Wiremap tests will check all lines in the cable for all of the following errors:
An open is a lack of continuity between pins at both ends of the cable.
A short is two or more lines short-circuited together.
A crossed pair is a pair is connected to different pins at each end (example: pair 1 is connected to pins 4 and 5 at one end, and pins 1 and 2 at the other).
A reversed pair is two lines in a pair are connected to opposite pins at each end of the cable. For example: the line on pin 1 is connected to pin 2 at the other end, the line on pin 2 is connected to line 1. This is also called a polarity reversal or tip-and-ring reversal.
A split pair is one line from each of two pairs is connected as if it were a pair. For example, the blue and white-orange lines are connected to pins 4 and 5, white-blue and orange to pins 3 and 6. The result is excessive near end crosstalk (NEXT), which wastes 10Base-T bandwidth and usually prevents 16 Mb/s token-ring from working at all.
LENGTH TESTS
Checking cable length is usually done using a time domain reflectometer (TDR), which transmits a pulse down the cable and measures the elapsed time until it receives a reflection of the signal from the far end of the cable. Each type of cable transmits signals at something less than the speed of light. This factor is called the nominal velocity of propagation (NVP), expressed as a decimal fraction of the speed of light. (UTP has an NVP of about 0.59-0.65). From the elapsed time and the NVP, the TDR calculates the cable's length. A TDR may be a special-purpose, or may be built into a handheld cable tester.
TESTING FOR IMPULSE NOISE
The 10Base-T standard defines limits for the voltage and number of occurrences per minute of impulse noise occurring in several frequency ranges. Many of the handheld cable testers include the capability to test for this.
NEAR-END CROSSTALK (NEXT)
To understand NEXT, imagine yourself speaking into a telephone — you can hear the person on the other end and also hear yourself through the handset. Imagine how it would sound if your voice was amplified so it was louder than the other person's. Each time you spoke, you could barely hear any sound coming from the other end, due to the contrasting levels of volume. A cable with inadequate immunity to NEXT couples so much of the signal being transmitted back onto the receive pair (or pairs) that incoming signals are unintelligible.
Cable and connecting hardware installed using poor practices can have their NEXT performance reduced significantly.
ATTENUATION
A signal traveling on a cable becomes weaker the further it travels. Each interconnection also reduces its strength. At some point the signal becomes too weak for the network hardware to interpret reliably. Particularly at higher frequencies (10MHz and up) UTP cable attenuates signals much sooner than does coaxial or shielded twisted-pair cable. Knowing the attenuation (and NEXT) of a link allows you to determine whether it will function for a particular access method, and how much margin is available to accommodate increased losses due to factors such as temperature changes and aging.
COMPONENTS
Using Cat. 5, 6 or 7 cable alone doesn't guarantee high performance network cabling. Once the cable is properly installed, it is mandatory that the cable be terminated with appropriate connectors and procedures, then using only properly rated (Cat. 5, 6 or 7) punch-down blocks, patch panels and patchcords to complete the cable plant. Unless all the appropriate components are used, the system cannot properly be called Cat. 5, 6 or 7.