22.0 Testing Unshielded Twisted Pair Cables (Data Communications Cabling)
Description
This article is from the Data Communications Cabling FAQ, by Peter Macaulay pmac@fox.nstn.ca with numerous contributions by others.
22.0 Testing Unshielded Twisted Pair Cables (Data Communications Cabling)
22.1 Testing UTP Introduction
Many of the problems encountered in UTP cable plants are a result
of miswired patch cables, jacks and crossconnects.
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 crossconnects 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).
22.2 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.
22.3 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.
22.4 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.
22.5 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.
22.6 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.
Forthcoming updates to cabling standards call for a number of new
tests which will add to this list.
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