QUALITY OF SERVICE AND TELECOMMUNICATION IMPAIRMENTS
The IEEE defines attenuation distortion (amplitude distortion) as the change in attenuation
at any frequency with respect to that of a reference frequency. For the discussion in this
section, we'll narrow the subject to the (analog) voice channel. In most cases a user is
connected, through his/her metallic subscriber loop to the local serving exchange. This
circuit is analog. Based upon the CCITT definition, the voice channel occupies the band
from 300 to 3400 Hz. We call this the passband.
Attenuation distortion can be avoided if all frequencies within the passband are sub-
jected to the same loss (or gain). Whatever the transmission medium, however, some
frequencies are attenuated more than others. Filters are employed in most active circuits
(and in some passive circuits) and are major causes of attenuation distortion. Figure 3.3
is a response curve of a typical bandpass filter with voice channel application.
As stated in our definition, amplitude distortion across the voice channel is measured
against a reference frequency. CCITT recommends 800 Hz as the reference; in North
America the reference is 1000 Hz.
Let us look at some ways that attenuation distortion
may be stated. For example, one European requirement may state that between 600 and
2800 Hz the level will vary no more than
-1 to +2 dB, where the plus sign means more
loss and the minus sign means less loss. Thus if an 800-Hz signal at
-10 dBm is placed
at the input of the channel, we would expect
-10 dBm at the output (if there were no
overall loss or gain), but at other frequencies we can expect a variation at the output
-1 to +2 dB. For instance, we might measure the level at the output at 2500 Hz at
-11.9 dBm and at 1100 Hz at -9 dBm.
When filters or filter-like devices
are placed in tandem, attenuation distortion tends to
sum. Two identical filters degrade attenuation distortion twice as much as just one filter.
We can look at a voice channel as a bandpass filter. A signal takes a finite time to
pass through the telecommunication network. This time is a function of the velocity of
Typical attenuation distortion across a voice channel bandpass filter. Crosshatched areas
are response specifications, whereas the wavy line is the measured response.
Test frequencies of 800 and 1000 Hz are not recommended if the analog voice channel terminates into the
digital network. In this case, CCITT and Bellcore (now Telcordia) recommend 1020 Hz. The reason for this is
explained in Chapter 6.
Any signal-passing device, active or passive, can display filter-like properties. A good example is a subscriber
loop, particularly if it has load coils and bridged taps. Load coils and bridged taps are discussed in Chapter 5.