Cisco Systems manual Session Initiation Protocol, Advanced Topics in SPA9000 Administration

Advanced Topics in SPA9000 Administration

Technology Background

Session Initiation Protocol

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The SPA9000 Voice System is implemented using open standards, such as Session Initiation Protocol (SIP), allowing interoperation with all ITSPs supporting SIP. The following figure illustrates a SIP request for connection to another subscriber in the network. In the SIP protocol, the requestor of the session is called the user agent server (UAS), while the receiver of the request is called the user agent client (UAC).

Figure 1 SIP Requests and Responses

SIP UA

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SIP Proxy

RTP

SIP Proxy

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SIP Proxy

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SIP UA

NOTE In this manual, the term client station is used to describe any SIP UA (including IP phones) that registers with the SPA9000.

In a SIP VoIP network, when the SIP proxy receives a request from a client station (UAS) for a connection and it does not know the location of the UAC, it forwards the message to another SIP proxy in the network. Once the UAC is located and the response is routed back to the UAS, a direct peer-to-peer session is established between the two UAs. The actual voice traffic is transmitted between UAs over dynamically assigned ports using the Real-time Protocol (RTP).

In the following figure, UserA and UserB are client stations (UAs) that register over the local area network to which the SPA9000 PBX is connected. When UserA calls UserB, the SPA9000 acts as a SIP proxy and establishes a session between the two UAs. After the session is established, RTP traffic flows directly between the two client stations.

SPA9000 Voice System Administration Guide 166