User manual

1-9

Linksys IP Phone Administrator Guide

Firmware Version 5.1

Chapter 1 Introducing Linksys IP Phones

Technology Background

• Using Linksys IP Phones with a Firewall or Router, page 1-9

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. Figure 1-2 illustrates a 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 SIP Requests and Responses

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 Linksys IP Phones with a Firewall or Router

When using a Linksys IP phone behind a firewall or router, make sure that the following ports are not

blocked:

• SIP ports—By default, UDP port 5060 and 5061

• RTP ports—16384 to 16482

If security is not a concern in your environment, you can consider disabling SPI, if this function exists

on your firewall.

SIP UA

SIP Proxy

RTP

SIP Proxy