Technology Background; Session Initiation Protocol; Using 900 Series Phones With A Firewall Or Router - Linksys SPA921-NA Administrator's Manual

Technology Background

Technology Background
This section provides background information about the technology and protocols used by the
ATA. It includes the following topics:
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•
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Session Initiation Protocol

Linksys 900 Series IP phones are implemented using open standards, such as Session Initiation
Protocol (SIP), allowing interoperation with all ITSPs supporting SIP.
SIP request for connection to another subscriber in the network. The requestor is called the user
agent server (UAS), while the recipient is called the user agent client (UAC).
Figure 1-2
In a SIP VoIP network, when the SIP proxy receives a request from a 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).

Using 900 Series Phones with a Firewall or Router

When using a 900 Series phone behind a firewall or router, make sure that the following ports
are not blocked:
Linksys 900 Series IP Phone Administrator Guide
1-10
Session Initiation Protocol, page 1-12
Using 900 Series Phones with a Firewall or Router, page 1-12
Using 900 Series Phones with a Firewall or Router, page 1-12
SIP Requests and Responses
SIP UA
2
4
RTP
3
1
SIP UA
Chapter 1
SIP Proxy
SIP Proxy
Introducing Linksys 900 Series IP Phones
Figure 1-2 illustrates a
SIP Proxy
Document Version 3.0