Ref: 09930010
Title: Potential Problem Areas Using Type 3 Cabling
date: 3/7/90

Copyright 3Com Corporation, 1991.  All rights reserved.

Problems using Type 3 cabling include the following areas:

  *  Power and grounding
  *  Wire resistance and attenuation
  *  Extraneous voltage on the twisted-pair wires
  *  Wire characteristic impedance and impedance discontinuities
  *  Crosstalk
  *  Shorts, opens, and other wire faults
  *  Stubs, bridge taps, and extensions
  *  Electromagnetic interference

The following paragraphs elaborate on the potential problem areas to
assist you in determining the cause of a problem.


Power Receptacle

Improperly wired or grounded power receptacles can be a safety
hazard and can contribute unacceptable levels of noise onto the
network.  In some cases, errors may be highly intermittent.  If
the AC voltage is not within acceptable limits, the attached
device may introduce errors onto the network or may not work at
all.


AC or DC Voltage on Twisted-Pair Wires

AC voltage may be induced into the twisted-pair wires from nearby
power or signal cables.  AC or DC voltage may be caused by shorts
or unintentional connection to other circuits.  Such voltages
(depending on the magnitude) can be a safety hazard, introduce
errors into the network, or cause the network to be inoperative.


Wire Resistance

Wire resistance measurements indicate the presence of opens,
shorts, faults, high-resistance connections, or improper wire
size.  Opens, shorts, or faults usually will result in an
inoperative network.  High-resistance connections or improper
wire size may result in high error rates or an inoperative
network or attached device.  Small wire size may limit the drive
capability below the allowable distance specified.


Measured Wire Resistance Length vs. Electrical Length

Comparing the wire length determined by resistance measurement to
the length measured by a time domain reflectometer (TDR) may indicate
that wire of different sizes exists in the loop.  This indicates
that the wiring in the building is not uniform.   Such wire loops
should not be used for token-ring transmission.

Wire length exceeding specified limits can cause excessive
errors, intermittent errors, or result in an inoperative network
or attached device.


Characteristic Impedance

Wire whose characteristic impedance exceeds the specified limits
may not be suitable for high-speed data transmission.  Increased
error rates can result from use of such wire.


Impedance Discontinuities

Impedance discontinuities may be caused by opens, shorts, loose
connections, wiring stubs, bridge taps, loading circuits,
telephone jacks, connector blocks, and so forth, that exist along
the wire path.  Excessive impedance discontinuities cause
reflections that can increase the error rate, cause erratic
performance, or in extreme cases result in an inoperative network
or attached device.


Attenuation

Excessive attenuation (signal loss) may be caused by the cable's
attenuation characteristics, wire that is too small, or impedance
discontinuities.  Excessive attenuation will limit the drive
capability below the specified allowable distances.


Crosstalk

Excessive near-end crosstalk is a function of the design and
manufacture of the cable.  Excessive crosstalk may cause high
error rates or an inoperative network or attached device.  In
some cases it can cause diagnostic procedures to produce false
readings.


Excessive Token-Ring Error Rate

An excessive error rate indicated by the diagnostics program can
also be caused by one or more of the preceding items not within
specification, or from excessive electromagnetic interference.

