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Global Call IP Technology Guide — November 2007 29
Dialogic Corporation
1
1.IP Overview
This chapter provides overview information about the following topics:
• Introduction to VoIP . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
• H.323 Overview. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
• SIP Overview. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
1.1 Introduction to VoIP
Voice over IP (VoIP) can be described as the ability to make telephone calls and send faxes over IP-
based data networks with a suitable Quality of Service (QoS). The voice information is sent in
digital form using discrete packets rather than via dedicated connections as in the circuit-switched
Public Switched Telephone Network (PSTN).
Currently, there are two major international groups defining standards for VoIP:
• International Telecommunications Union, Telecommunications Standardization Sector
(ITU-T), which has defined the following:
– Recommendation H.323, covering Packet-based Multimedia Communications Systems
(including VoIP)
• Internet Engineering Task Force (IETF), which has defined drafts of the several RFC (Request
for Comment) documents, including the following central document:
– RFC 3261, the Session Initiation Protocol (SIP)
The H.323 recommendation was developed in the mid 1990s and is a mature protocol.
SIP (Session Initiation Protocol) is an emerging protocol for setting up telephony, conferencing,
multimedia, and other types of communication sessions on the Internet.
1.2 H.323 Overview
The H.323 specification is an umbrella specification for the implementation of packet-based
multimedia over IP networks that cannot guarantee Quality of Service (QoS). This section
discusses the following topics about H.323:
• H.323 Entities
• H.323 Protocol Stack
• Codecs
• Basic H.323 Call Scenario
• Registration with a Gatekeeper
• H.323 Call Scenario via a Gateway