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22.0 Testing Unshielded Twisted Pair Cables

     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.

Parent document is top of "Data Communications Cabling FAQ"
Previous document is "21.0 ISDN Cabling"
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