15
VOICE-OVER PACKETS IN A PACKET
NETWORK
15.1
AN OVERVIEW OF THE CONCEPT
On the surface the concept seems fairly simple. We digitize the voice and break up the
resulting serial bit stream into packets of some length. A header is added to each packet,
and the packet can now be sent over the data packet network. This type of operation
is most commonly called voice over IP (VoIP). "IP" is the Internet protocol, which we
discuss in Chapter 12. Others prefer the broader term voice over packet. This would also
encompass "voice over ATM (asynchronous transfer mode" (Chapter 20). Still another
approach is to place voice (in packets) over frame relay (Chapter 12). The concept of
VoIP is illustrated in Figure 15.1.
15.2
DATA TRANSMISSION VERSUS CONVENTIONAL DIGITAL TELEPHONY
Conventional voice telephony is transported in a full duplex mode on PSTN circuits
optimized for voice. By the full-duplex mode we mean that there are actually two circuits,
one for "send" and one for "receive" to support a normal telephone conversation between
two parties. With a few exceptions in the local area, all these circuits are digital. The
descriptive word digital may seem ambiguous to some.
When we say digital in this context, we mean that all circuits would carry 8-bit "words"
(timeslots), where each "word" represents an 8-bit voltage sample ostensibly of an analog
voice conversation in a PCM format. This is often characterized in the literature as G.711
service (i.e., ITU-T Rec. G.711). Data are also commonly transported in 8-bit sets called
bytes, but more properly called octets from our vantage point. It is comparatively easy to
replace 8-bit voltage samples of voice with 8-bit octets of data.
However, there remained essential philosophical differences between voice in 8-bit
octets and data transmission. A voice circuit is established when a subscriber desires to
converse by telephone with some other telephone subscriber. The circuit between the two
Voice
A/D conv.
Voice
V/A conv.
Packetizer
(IP)
NET
Depacketizer
(IP)
Figure 15.1
Rough conceptualization of VoIP.
Fundamentals of Telecommunications, Second Edition, by Roger L. Freeman
ISBN 0-471-71045-8
Copyright
2005 by Roger L. Freeman
387
Summary :
15.2 DATA TRANSMISSION VERSUS CONVENTIONAL DIGITAL TELEPHONY Conventional voice telephony is transported in a full duplex mode on PSTN circuits optimized for voice. When we say digital in this context, we mean that all circuits would carry 8-bit "words" (timeslots), where each "word" represents an 8-bit voltage sample ostensibly of an analog voice conversation in a PCM format.
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