Cisco Systems SPA9000 manual Technology Background, Session Initiation Protocol

Chapter 1 Using the Linksys Voice System

Technology Background

Technology Background

This section provides background information about the technology and protocols used by the SPA9000 system. It includes the following topics:

•Session Initiation Protocol, page 1-6

•SPA9000 Media Proxy, page 1-7

•Using the SPA9000 with a Firewall or Router, page 1-8

•SPA400 SIP-PSTN Gateway, page 1-8

•Network Address Translation (NAT), page 1-9

Session Initiation Protocol

The LVS is implemented using open standards, such as Session Initiation Protocol (SIP), allowing interoperation with all ITSPs supporting SIP. Figure 1-2illustrates 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 1-2 SIP Requests and Responses

SIP UA

2

4

SIP Proxy

RTP

SIP Proxy

3

SIP Proxy

1

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).

	Linksys SPA9000 Administrator Guide
1-6	Document Version 3.01