P1: IML
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648 VOICE OVERINTERNETPROTOCOL(IP)Table 1One Illustrative Encoding of LettersA 00000 I 01000 Q 10000 Y 10000
B 00001 J 01001 R 11001 Z 11001
C 00010 K 01010 S 11010 11010
D 00011 L 01011 T 11011 11011
E 00100 M 01100 U 11100 11100
F 00101 N 01101 V 11101 11101
G 00110 O 01110 W 11110 11110
H 00111 P 01111 X 11111 11111Transmission of Voice Signals
Almost all voice communications signals begin as analog
sound signals. An analog signal is one that is continuous
in shape (i.e., no sharp angles) and typically analogous
in shape to the original signal or physical phenomenon it
represents (Newton, 1998). The physical signals that our
bodies generate (e.g., sound and visual) and receive (i.e.,
sense) are analog (Truxal, 1990). This is very important
in the design of telephony systems, because almost all
information that we are concerned with in telephony is
ultimately either originated and/or received by a human
being.
When we speak or play an instrument, combinations
of one or more waves of alternating compression and ex-
pansion of air are generated at the sound’s source, e.g., the
vocal chords in our throats or the reeds of instruments.
Because sound energy in air does not travel well over
long distances, long-distance communications requires
that these sound waves be converted to alternate forms of
energy by a transducer, and then that signal energy is con-
veyed over a compatible transmission medium, such as
an electronic transmission channel. For voice communi-
cations, the microphone of a telephone device is typically
used to transform analog sound wave energy into ana-
log electrical wave energy to be transmitted over a caller’s
local telephone wires.
To transform the electrical signal back to a sound wave
that the human ear can detect, the analog electrical sig-
nal is applied to a “reverse microphone” transducer (such
as the an earpiece of a telephone). The “reverse micro-
phone,” or speaker, outputs physical patterns of com-
pressed air corresponding to the original sound wave. The
entire transformation from analog sound wave to analog
electrical signal and back to analog sound wave is illus-
trated in Figure 1.The Characteristics of an Analog Voice Signal
Communications networks and their elements are de-
signed to cost-effectively accommodate the signals they
are intended to carry, with the least degradation (e.g., dis-
tortion, delay, etc). Telephony networks were optimized
to carry analog voice signals over long distances using
the most cost-effective technology available. However, the
original designs were done from the late 1800s through
the early 1900s. Those voice-centric design templates con-
tinue to influence telephony network design today due to
the need for new networks to interoperate with older ones,
and the need for those older networks to be backwards
compatible with older equipment (which sometimes dates
back to the early 1900s).
A human voice signal is made up of alternating sig-
nal components of various frequencies, mostly within the
range from 100 to 3,400 alternating cycles per second.
Although this range may vary somewhat from person to
person, analog telephony networks were designed based
on this assumption. Virtually all of the components of the
telephone networks have been built to limit the frequen-
cies transmitted to those in this predefined voiceband.
Any voice frequencies outside this voiceband are filtered
out by the network equipment and, in turn, not transmit-
ted.
This voiceband range is broad enough to allow a tele-
phone listener both to recognize the person who is speak-
ing and to understand what he or she is saying. High fi-
delity devices, such as CDs and FM radio, transmit a wider
range of sound frequencies (up to 10,000 cycles per sec-
ond or more). With conventional telephony, any attempt
to transmit sound from such devices would be filtered to
only include the voiceband frequency components. How-
ever, with the flexibility of VOIP, it is at least theoretically
possible to cost-effectively deploy a customized high fi-
delity telephony service that could coexist with voiceband
VOIP services.Digitizing an Analog Voice Signal
Most long distance transmission systems in use today
(including those using VOIP) transmit voice signals dig-
itally. To transmit an analog signal digitally, its analog
version must be transformed from its electrical analog
form to a digital form, thus changing its representation
from a varying electrical voltage to an equivalent string of
discrete 0s and 1s. This transformation is done by a dig-
itizer, sometimes referred to as an “analog to digital con-
verter,” “A/D Converter,” or “CODEC” (COder-DECoder).Communications Channel: Electrical SignalOriginal Soundwave Reproduced SoundwaveFigure 1: Transmission of a sound wave and its reconstruction.