Session Initiation Protocol; Network Address Translation; Nat Overview - Linksys SPA1001 Administrator User Manual

Technology Background

Session Initiation Protocol

Linksys ATAs are implemented using open standards, such as Session Initiation Protocol (SIP),
allowing interoperation with all ITSPs supporting SIP.
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).

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:
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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.
Linksys ATA Administrator Guide
1-10
SIP Requests and Responses
SIP UA
2
4
RTP
3
1
SIP UA
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
Chapter 1 Introducing Linksys Analog Telephone Adapters
Figure 1-2 illustrates a SIP request for connection
SIP Proxy
SIP Proxy
SIP Proxy
Document Version 3.1