Agents and Servers. These components can take the form of
an additional software program (on a laptop computer, for
example), or as an inherent part of a mobile device such as a
PDA or cell phone.
Using user agents
User agents (UAs) are applications in SIP endpoints (such
as a SIP phone, cell phone, PDA, or workstation, as shown
in Figure 2-1) that interface between the user and the SIP
network. A user agent can act as either a client or a server.
When sending SIP messages, the UA acts as a user agent client
(UAC), and when receiving messages, it acts as a user agent
server (UAS). A back-to-back user agent (B2BUA) is an applica-
tion that acts as an intermediary between two parties, but
appears as an endpoint to both parties — like a middleman.
It serves as both UAS and UAC simultaneously to process ses-
sion requests.
Figure 2-1: Some typical SIP user agents.
dude@dummies.com
Softphone
PDA
SIP Phone
Dual-Mode
Mobile Phone
Instant Messaging
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