Cisco SPA1001 Administration Guide - Page 24

Technology Background Chapter 1 Introducing Linksys Analog Telephone Adapters Session Initiation Protocol Linksys ATAs 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). Network Address Translation This section describes issues that arise when using a Linksys ATA on a network behind a network address translation (NAT) device. It includes the following topics: • NAT Overview, page 1-10 • NAT Types, page 1-11 • Simple Traversal of UDP Through NAT, page 1-12 • SIP-NAT Interoperation, page 1-12 NAT Overview Network Address Translation (NAT) allows multiple devices to share the same public, routable, IP address for establishing connections over the Internet. NAT is typically performed by a router that forwards packets between the Internet and the internal, private network. 1-10 Linksys ATA Administrator Guide Document Version 3.1