Cisco SPA901-NA Administration Guide - Page 20
Session Initiation Protocol, Using Linksys IP Phones with a Firewall or Router - voip
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Chapter 1 Introducing Linksys IP Phones • Using Linksys IP Phones with a Firewall or Router, page 1-9 • Using Linksys IP Phones with a Firewall or Router, page 1-9 Technology Background 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 SIP UA 2 4 SIP Proxy RTP 3 1 SIP Proxy SIP Proxy SIP UA 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. Firmware Version 5.1 Linksys IP Phone Administrator Guide 1-9