Table of Contents
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Chapter 6
Voice over IP
Definition
Components
SIP User Agents
SIP proxy server
SIP registrar
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6.1 Session Initiation Protocol
Session Initiation Protocol (SIP), defined in RFC 3261, is a signalling protocol used
for:
Initiating sessions in an IP network.
Modifying the sessions in an IP network.
Terminating sessions in an IP network.
A SIP-enabled IP network includes usual IP components and services such as
routers and DNS servers, as well as SIP User Agents (UA) and SIP network servers.
The SIP User Agents, addressed by SIP URLs, are effectively the end system
components for a session (e.g. IP phone, PC or PDA); they originate and terminate
SIP requests.
SIP User Agents can implement advanced services such as:
Call Waiting
Call Hold
Call Transfer for IP telephony
The SIP network servers are network devices that handle the signalling associated
with multiple sessions. In a proxy model, a SIP proxy server forwards signalling
messages between the called and calling parties; it is the only point of contact that
the clients have for signalling messages. After the session has been established, the
RTP audio or video packets are sent directly between the clients.
SIP signalling
messages
SIP UA1
john@rennes
Audio/Video
A SIP proxy server makes use of a location database, updated by a SIP registrar, in
order to locate the called user agent. SIP UAs periodically register their current
location with a SIP registrar that may be collocated with a SIP proxy server.
Registrars
(optional)
SIP proxy servers
(optional)
SIP-enabled IP network
RTP
IP network
SIP signalling
messages
SIP UA2
julie@paris
RTP
Audio/Video
E-DOC-CTC-20041126-0014 v3.0
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