Testing Unshielded Twisted Pair Introduction

Many of the problems encountered in UTP cable plants are a result of miswired patch cables, jacks and cross connects.

Horizontal and riser distribution cables and patch cables are wired straight through end-to-end -- pin 1 at one end should be connected to pin 1 at the other. (Crossover patch cables are an exception, as described later). 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: EIA-568A, 568B, USOC or whatever. (One source of problems is an installation in which USOC jacks are mixed with EIA-568A or 568B. Everything appears to be punched down correctly, but some cables work and others do not).

Wiremap Tests

Wiremap tests will check all lines in the cable for all of the following errors:

• Open: Lack of continuity between pins at both ends of the cable.

• Short: Two or more lines short-circuited together.

• Crossed pair: A pair is connected to different pins at each end (example: pair 1 is connected to pins 4&5 at one end, and pins 1&2 at the other).

• Reversed pair: The two lines in a pair are connected to opposite pins at each end of the cable (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). Also called a polarity reversal or tip-and-ring reversal.

• Split pair: One line from each of two pairs is connected as if it were a pair (example: the Blue and White-Orange lines are connected to pins 4&5, White-Blue and Orange to pins 3&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 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 approximately 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 unit such as the Tektronix 1503, 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/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)

What's NEXT, you ask? Imagine yourself speaking into a telephone. Normally, as you speak 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'd be deaf to anything coming from the other end. 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 by as much as a whole Category.

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 co-axial 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 temperature changes, aging, etc.

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