QUALITY OF SERVICE AND TELECOMMUNICATION IMPAIRMENTS
inputs were standardized with a level of either
-15 dBm or -16 dBm, and the out-
puts of demultiplexers were
+7 dBm. These levels, of course, were test-tone levels.
In industrialized and postindustrialized nations, in nearly every case, multiplexers are
digital. These multiplexers have an overload point at about
+3.17 dBm0. The digital
reference signal is 0 dBm on the analog side using a standard test tone between 1013
and 1022 Hz.
ECHO AND SINGING
Echo and singing are two important impairments that impact QoS. Echo is when a talker
hears her/his own voice delayed. The annoyance is a function of the delay time (i.e., the
time between the launching of a syllable by a talker and when the echo of that syllable
is heard by the same talker). It is also a function of the intensity (level) of the echo, but
to some lesser extent. Singing is audio feedback. It is an "ear-splitting" howl, much like
the howl one gets by placing a public address microphone in front of a loudspeaker. We
will discuss causes and cures of echo and singing in the next chapter.
Define signal-to-noise ratio.
Give signal-to-noise ratio guidelines at a receiving device for the following three
media: voice, video-TV, and data. Base the answer on where a typical customer
says the signal is very good or excellent.
Why do we use a sinusoidal test tone when we measure S/N on a speech channel
rather than just the speech signal itself?
The noise level of a certain voice channel is measured at
-39 dBm, and the test-tone
signal level is measured at
+3 dBm. What is the channel S/N?
If we know the loudness rating of a telephone subset earpiece and of the subset
mouthpiece, what additional data do we need to determine the overall loudness
rating (OLR) of a telephone connection?
The BER of an underlying digital circuit is 1
, for data riding on this circuit.
What is the best BER we can expect on the data?
What are the three basic impairments on a telecommunication transmission channel?
Of the three impairments, which one affects data error rate the most and thus limits
Explain the cause of phase distortion.
Name the four types of noise we are likely to encounter in a telecommunica-
What will be the thermal noise level of a receiver with a noise figure of 3 dB and
a bandwidth of 1 MHz?
Define third-order products based on the mixing of two frequencies
Give four causes of impulse noise.