The following is a suggested sequence for testing twisted pair cabling
installed under the proposed UCSB standards for communications cabling.
Correct Terminations
- Verify that the identification number (label) is identical for both the
jacks in the workstation wall plate and in the corresponding patch panel jacks
to be tested.
- Verify that a mechanically sound connection exists for each wire at each of
the attached terminations on an RJ-11 or RJ-45 jack.
- Verify correct straight-through correspondence of all four (4) wires at each
RJ-11 or RJ-45 termination of the cable.
- If a single-end test device is to be used, verify the pin-to-pin continuity
of each pair within the loopback plugs to be used at the opposite end.
Continuity
Check for the following.
- Crossed: One or both wires in a pair cross, or short, with one or both
wires in a second pair. Identify by use of third pair individual wire.
- Ground Fault: One or both wires in a pair read continuity with external
ground or shield.
- Intermittent: A break or "open" which appears and disappears as
the wire or connector is flexed. Identify by same method as "open"
pairs.
- Open Pairs: A break or "open" fault appears. Use a split-pair
loopback plug or individually use one wire in each of two pairs to determine
which individual wire is "open".
- Reversed: The termination of a wire pair is reversed, e.g. a telephone
circuit "tip" at one end appears as "ring" at the opposite
end.
- Short: Continuity between two wires without an external jumper or loopback
plug in place.
- Split: Between two pairs, one wire of one pair is reversed with one wire
of the second pair. Identify by use of third pair individual wire.
Signal Loss
This test measures attenuation of the wire by injecting a signal of known
strength at one end and measuring the strength of the signal at the opposite
end. This measurement over known distance is compared to the manufacturer's
design specification for loss over distance (typically xx dB per km).
Signal loss should be measured at multiple defined frequencies and compared
with manufacturer's specifications for loss at each of those frequencies.
NOTE: This test should be performed with the full length of cable off of the
shipping and/or package reel.
Crosstalk
Near-End Crosstalk (NEXT) is measured to determine a signal-to-noise ratio
for a cable run and normally will be a measurement of the noise level in a
receive pair generated by an adjacent transmit pair.
A common source of NEXT is the existence of a split pair in which one wire
of a transmit pair is reversed with one wire of a receive pair.
Interference
Interference is measured, or identified, as a source of noise external to
the twisted pair structure of the cable under evaluation. Typical interference
sources include electric motors and flourescent lighting, as well as any
high-voltage circuit running parallel to the signal cable path. Increasing the
distance between the signal and the electric cables is required. Shielding on
the cable, or for the termination may also be required.
Ground Loop
In a large wiring system, with multiple sub-terminals and backboards, a
ground loop may be formed by inadvertent use of two ground references for
shielding or equipment. Such a group loop may result in both a rise in crosstalk
signals or loss of attenuation.
A single grounding system should be established for each building, using a
single copper ground rod (8' earth driven) and running a bus circuit (#6 wire)
to each closet with a mounted ground bar. All equipment and cable shielding
should be grounded as required to this bar.
DCC
