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Cisco ATA 190 Analog Telephone Adaptor Administration Guide for SIP (Version 1.0)
OL-31821-01
Chapter 1 Cisco ATA 190 Analog Telephone Adapter Overview
Session Initiation Protocol Overview
• User agent server (UAS)—A server application that contacts the user when a SIP request is received
and returns a response on behalf of the user.
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 architectural standpoint, the physical components of a SIP network can also be grouped into
two categories—Clients and servers.
Figure 1-2 illustrates the architecture of a SIP network.
Note SIP servers can interact with other application services, such as Lightweight Directory 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 billable services.
Figure 1-2 SIP Architecture
SIP Clients
SIP clients include:
• Gateways—Provide call control. Gateways provide many services, the most common being a
translation function between SIP conferencing endpoints and other terminal types. This function
includes translation between transmission formats and between communications procedures. In
addition, the gateway also translates between audio and video codecs and performs call setup and
clearing on both the LAN side and the switched-circuit network side.
• Phones—Can act as either a UAS or UAC. The ATA 190 can initiate SIP requests and respond to
requests.
SIP Servers
SIP servers include:
SIP user agents
RTP
SIP
SIP proxy and
redirect servers
SIP gateway
PSTN
Legacy PBX
SIP SIP
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