Building Telephony Systems With OpenSIPS---Introduction to SIP

Understanding the SIP Architecture

aliases

Another interesting feature used in SIP proxies are aliases;You can have multiple SIP addresses for a singel subscriber such as following:

• [email protected]
• [email protected]
• [email protected]

The SIP Architecture

1. In the traditional SIP model,a user agent,usually a SIP phone,will start communicating with its SIP proxy,seen here as the proxy(or its home proxy) to send the call using a message known as INVITE.

2. The outgoing proxy will see that the call is directed to an outside domain. According to RFC 3263,it will seek the DNS server for the address of the target domain and resolve the IP address.Then, the outgoing proxy will forward the call to the SIP proxy responsible for DomainB.

3. The incoming proxy will query its location table for the IP address of agentB if its address was inserted in the location table by a previous registration process. It will forward the call to agentB.

4. After receiving the SIP message,agentB will have all the information required to establish an RTP session(usually audio) whth agentA sending a 200 OK respense. Once agentA receives the respense from agentB,a two-way media can be established. A BYE request message can terminate the session.

In the preceding image,you can see the following componemts:

• UAC(User Agent Client):A client or terminal that starts the SIP signaling
• UAS(User Agent Server):A server that responds to the SIP signaling coming from a UAC
• UA(User Agent):A logical entity that can act as both UAC or UAS ,such as a SIP endpoint(IP phones,ATAs,softphones,and so on)
• Proxy Server:Receives requests from a UA and transfers to another SIP proxy if this specific terminal is not under its domain.
• Redirect Server :Receives requests and responds to the caller with a message containing data about the destination(302,Moved Temporarily)
• Registrar Server:Provides the callee’s contact addresses to the proxy and redirect Server

The Proxy,redirect,and registrar servers are usually avaliable physically in the same computer and software.