Cisco 7940G Administrator's Manual - Page 2

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Cisco 7940G Administrator's Manual
What Is Session Initiation Protocol?
Conferences can consist of two or more users and can be established using multicast or multiple unicast
sessions.
The term conference means an established session (or call) between two or more endpoints. In this
Note
document, the terms conference and call are used interchangeably.

Components of SIP

SIP is a peer-to-peer protocol. The peers in a session are called user agents (UAs). A user agent can
function in one of the following roles:
•
•
Typically, a SIP endpoint is capable of functioning as both a UAC and a UAS, but functions only as one
or the other per transaction. Whether the endpoint functions as a UAC or a UAS depends on the UA that
initiated the request.
From an architecture standpoint, the physical components of a SIP network can also be grouped into two
categories: clients and servers.
In addition, the SIP servers can interact with other application services, such as Lightweght Directory
Note
Access Protocol (LDAP) servers, a database application, or an eXtensible Markup Language (XML)
application. These application services provide back-end services such as directory, authentication, and
billing services.
Figure 1-1
SIP user
agents (UAs)
Cisco SIP IP Phone Administrator Guide Version 3.0
1-2
User agent client (UAC)—A client application that initiates the SIP request.
User agent server (UAS)—A server application that contacts the user when a SIP request is received
and that returns a response on behalf of the user.
SIP Architecture
SIP
IP
Figure 1-1
illustrates the architecture of a SIP network.
SIP proxy and
redirect servers
SIP
SIP
SIP gateway
RTP
Chapter 1
Product Overview
PSTN
Legacy PBX
OL-2206-01