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Morris WL040/Bidgoli-Vol III-Ch-53 September 15, 2003 12:23 Char Count= 0
Voice over Internet Protocol (IP)Voice over Internet Protocol (IP)
Roy Morris,Capitol CollegeIntroduction 647
Communications Theory: Bits of
Transmitted Information 647
Bits: The Most Basic Form of Information 647
Transmission of Voice Signals 648
The Characteristics of an Analog Voice Signal 648
Digitizing an Analog Voice Signal 648
The Telephony Network 650
The Communications Network Topology 650
Transmission Links 650
Transmission System Imperfections:
Noise, Loss, and Delay 651
Circuit-Switched Connections of a Call
in a Conventional Telephony Network 652
Voice Over Internet Protocol 653How VOIP Transmission Works 653
VOIP Signaling 653
H.323 vs. SIP 655
Integrating VOIP Into Conventional
Circuit-Switched Telephony Networks 656
ENUM, the Fully Interoperable
Numbering Plan 657
Quality of Service Issues 657
The Costs and Savings of Using VOIP 658
Security Issues for VOIP 658
Conclusion 659
Glossary 659
Cross References 659
References 659INTRODUCTION
The voice over Internet protocol (“VOIP”) is where con-
ventional telecommunications meets the technologies
used in the Internet. This chapter will begin by explain-
ing communications theory and conventional telephony,
which pre-date the popularized Internet. They serve as a
foundation for the VOIP, because almost all voice com-
munications networks must interconnect, and therefore
must be backwards compatible with conventional tele-
phony networks. The chapter will conclude with specifics
of VOIP design and deployment.COMMUNICATIONS THEORY: BITS
OF TRANSMITTED INFORMATION
Basic communications theory concepts form the build-
ing block of all communications applications, and their
understanding. VOIP is no exception.
“Communication” is simply the transmission of “infor-
mation” from one place (or device or person) to another,
over a “transmission network.” “Voice communications”
refers to transmitting the information contained in a voice
signal. The distance traveled by a transmitted signal can
be as short as from one chip to another chip on a circuit
board, to across the street, to around the world, or even
to the most distant galaxies in outer space.Bits: The Most Basic Form of Information
In digital parlance, a “bit” is the smallest measure of infor-
mation (Newton, 1998). It is the amount of information
required to distinguish between two equally likely possi-
bilities or choices. A bit can be used as a building block for
all information or messages and/or as a measure of infor-
mation. A bit is typically given one of two arbitrary logi-
cal values: e.g., 1 or 0. By stringing together bits, complexmessages can be formed representing familiar message
forms, such as letters, sentences, text documents, voice
signals, pictures, and movies. Table 1 illustrates one ar-
bitrary binary encoding scheme for the 26 letters of the
alphabet.
Over the years, one standardized binary encoding
scheme evolved for all letters of the alphabet, plus some
additional characters, which is universally understood
by almost all digital devices (Truxal, 1990). That text
encoding scheme is known as “ASCII.” ASCII has 128
standardized combinations of seven bits to represent
128 characters, including upper and lower case letters,
numbers, and symbols. With such a universally under-
stood binary text encoding scheme, we can transmit let-
ters strung together to form large text documents that
can be transmitted over digital transmission facilities,
received, and ultimately displayed in distant locations.
The key to the successful binary transmission of text
is that the transmitter and receiver of the binary trans-
mission must understand the same standardized vocab-
ulary used for that text encoding—which is primarily
ASCII.
Similarly, using standardized voice communication en-
coding schemes, voice communication signals can be rep-
resented as strings of 1s and 0s and then transmitted
over data networks, received, and ultimately heard in
distant locations. VOIP capitalizes on this ability, trans-
mitting those strings of bits over a data network using
the Internet protocol (IP). However, to the naked eye,
all binary strings look the same—like strings of 1s and
0s. What differentiates one kind of transmitted binary
string from another is a common understanding between
the encoder (at the transmitter) and decoder (at the re-
ceiver) of what that string represents (e.g., text vs. voice)
and how it was encoded (e.g., did it use ASCII, if it was
text? Did it use a particular voice coder/decoder, if it is
voice?)647