User Agent

There are two types of User Agents as follows. But these are not strictly separate physical entities. One UA (User Agent) can work as either UAC or UAS depending on situations.

• UAC (User Agent Client) : UAC is the party that generates requests and send those to servers
• UAS (User Agent Server) : UAS is the party that gets requests, processes those requests and generate responses

Client

It normally mean the end users i.e. the applications running on the systems used by people. It may be a softphone application running on your PC or a messaging device in your IP phone. It generates a request when you try to call another person over the network and sends the request to a server (generally a proxy server).

Server

Server is an entity that handles the request coming from the clients in a predefined rule. There are several different types of servers described as below.

Proxy Server: These are the most common type of server in a SIP environment. When a request is generated, the exact address of the recipient is not know in advance. So the client sends the request to a proxy server. The proxy forwards the request to another proxy server or the recipient itself on behalf of the client

Redirect Server: A redirect server redirects the request back to the client indicating that the client needs to try a different route to get to the recipient. It generally happens when a recipient has moved from its original position either temporarily or permanently.

Registrar: This is the server that performs registration process for clients. During this registration, Registra stores the location information of users into Location Server.

Location Server: The addresses registered to a Registrar are stored in a Location Server.