Cisco Linksys SPA9000 - Manual (Page 20)

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Linksys SPA9000 Administrator Guide
Document Version 3.01
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 1-2 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 1-2 SIP Requests and Responses
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).
SIP UA
SIP UA
SIP Proxy
SIP Proxy
RTP
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SIP Proxy