This chapter refers to many legacy telephony services.
Readers may consult Newton’s Telecommunications Dictionary[1] for defini-
tions of the telephony and telecom services mentioned here. Chapter 11, “SIP
Telephony,” also discusses in detail many enhanced telephone services.
The overview of SIP services provided here reflects current thinking in the
community of SIP service and technology developers. Most (but not all) of
them have been actually tested and implemented. Some proposed Internet
drafts on SIP will make it to the level of IETF standards; some will not. It also
is likely that new technologies and services will emerge that have not been
made public or envisaged as of this writing.Internet Multimedia Protocols
Networks are defined by their protocols. The global telephone network uses
its own signaling and communication protocols, as do other telecom networks
such as the Public Switched Telephone Network (PSTN), X.25, Integrated Ser-
vices Digital Network (ISDN), Switched Multimegabit Data Services (SMDS),
frame relay, Asynchronous Transfer Mode (ATM), mobile circuit-switched net-
works, and the (seemingly always) proposed ITU-T Next Generation public
Networks (NGN). Besides legacy network protocols, there are also application-
level protocols, such as those used between fax machines.
Though started with much smaller resources than the previously dominant
telecom and non-IP data networks (SNA, DECnet, Novell), the Internet’s suc-
cess is solely due to its well-designed architecture and protocols. The architec-
tural principles of the Internet (covered in Chapter 3, “Architectural Principles
of the Internet”) have made it the most effective network for any type of appli-
cation, including real-time communications.
Internet telephony and the wider family of Internet communications are
defined by several key application level protocols. The list of Internet proto-
cols used for interactive communications is shown in Table 2.1.Table 2.1 Key Standard Internet Multimedia Protocols
FUNCTION OF
THE PROTOCOL STANDARD [2] DESCRIPTION
Real Time Transfer (RTP) RFC 3550 End-to-end real-time transport for
audio, video, and data, without
quality of service (QoS).
Audio/Video Profiles RTP/ RFC 3551 Defines protocol fields for audio and
AVP video and lists some basic standard
encodings.12 Chapter 2