Session Initiation Protocol; Sip Capabilities; Sip Components - Cisco ATA 191 Administration Manual
Analog telephone adapter
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Session Initiation Protocol
Session Initiation Protocol
Session Initiation Protocol (SIP) is the Internet Engineering Task Force (IETF) standard for real-time calls
and conferencing over Internet Protocol (IP). SIP is an ASCII-based, application-layer control protocol (defined
in RFC3261). It is used to establish, maintain, and terminate multimedia sessions or calls between two or
more endpoints.
Like other Voice over IP (VoIP) protocols, SIP is designed to address the functions of signaling and session
management within a packet telephony network. Signaling allows call information to be carried across network
boundaries. Session management is used to control the attributes of an end-to-end call.
Note
SIP for the ATA 191 is compliant with RFC2543.
SIP Capabilities
Session Initiation Protocol (SIP) provides these capabilities:
• Determines the availability of the target endpoint. If the target endpoint is unavailable, SIP determines
• Determines the location of the target endpoint. SIP supports address resolution, name mapping, and call
• Determines the media capabilities of the target endpoint. Using the Session Description Protocol (SDP),
• Establishes a session between the originating and target endpoint. If the call can be completed, SIP
• Handles the transfer and termination of calls. SIP supports the transfer of calls from one endpoint to
SIP Components
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 these roles:
• 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
Cisco ATA 191 Analog Telephone Adapter Administration Guide for Cisco Unified Communications Manager
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whether the called party is already on the phone or didn't answer in the allotted number of rings. SIP
then returns a message indicating why the target endpoint was unavailable.
redirection.
SIP determines the lowest level of common services between endpoints. Conferences are established
using only the media capabilities that all endpoints support.
establishes a session between the endpoints. SIP also supports midcall changes, such as adding another
endpoint to the conference or changing the media characteristic or codec.
another. During a call transfer, SIP establishes a session between the transferee and a new endpoint
(specified by the transferring party). SIP also terminates the session between the transferee and the
transferring party. At the end of a call, SIP terminates the sessions between all parties. Conferences can
consist of two or more users and can be established using multicast or multiple unicast sessions.
and returns a response on behalf of the user.
Cisco ATA 191 Analog Telephone Adapter Overview
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