QUALITY OF SERVICE AND TELECOMMUNICATION IMPAIRMENTS
Nonlinear envelope delay.
To summarize, IM noise results from either a nonlinearity or a malfunction that has the
effect of nonlinearity. The causes(s) of intermodulation noise is(are) different from that
of thermal noise. However, its detrimental effects and physical nature can be identical to
those of thermal noise, particularly in multichannel systems carrying complex signals.
. Impulse noise is noncontinuous, consisting of irregular pulses
or noise spikes of short duration and of relatively high amplitude. These spikes are often
called hits, and each spike has a broad spectral content (i.e., impulse noise smears a broad
frequency bandwidth.) Impulse noise degrades voice telephony usually only marginally,
if at all. However, it may seriously degrade error performance on data or other digital
circuits. The causes of impulse noise are lightning, car ignitions, mechanical switches
(even light switches), fluorescent lights, and so on. Impulse noise will be discussed in
more detail in Chapter 10, Data Communications.
. Crosstalk is the unwanted coupling between signal paths. There are
essentially three causes of crosstalk:
1. Electrical coupling between transmission media, such as between wire pairs on a
voice-frequency (VF) cable system and on digital (PCM) cable systems.
2. Poor control of frequency response (i.e., defective filters or poor filter design).
3. Nonlinear performance in analog (FDM) multiplex systems.
Excessive level may exacerbate crosstalk. By "excessive level" we mean that the level or
signal intensity has been adjusted to a point higher than it should be. In telephony and
data systems, levels are commonly measured in dBm. In cable television systems, levels
are measured as voltages over a common impedance (75 ohms). See the discussion of
level in Section 3.4.
There are two types of crosstalk:
1. Intelligible, where at least four words are intelligible to the listener from extraneous
conversation(s) in a 7-second period.
2. Unintelligible, with crosstalk resulting from any other form of disturbing effects of
one channel on another.
Intelligible crosstalk presents the greatest impairment because of its distraction to the
listener. Distraction is considered to be caused either by fear of loss of privacy or primarily
by the user of the primary line consciously or unconsciously trying to understand what is
being said on the secondary or interfering circuits; this would be true for any interference
that is syllabic in nature.
Received crosstalk varies with the volume of the disturbing talker, the loss from the
disturbing talker to the point of crosstalk, the coupling loss between the two circuits under
consideration, and the loss from the point of crosstalk to the listener. The most important
of these factors for this discussion is the coupling loss between the two circuits under
consideration. Also, we must not lose sight of the fact that the effects of crosstalk are
subjective, and other factors have to be considered when crosstalk impairments are to be