SIP supports functionalities to establish and end multimedia sessions : location, availability, resource use, and negotiation features.
In order to implement these functionalities, there are different SIP components. There are two main elements, user agents (UA) and SIP servers
1. User Agent (UA): they have two different
parts, User Agent Client (UAC) and User Agent Server (UAS).
An UAC is a logical entity which sends SIP requests and receives
answers to those requests. An UAS is a logical entity that
sends answers to SIP requests.
Both entities are in every user agent, to allow the communication
between different user agents in a client-server communication.
2. There are 3 different SIP Servers:
- Proxy Server: they resend requests and
they decide the server they must send the messages to, altering
the request fields if necessary. It is an intermediate entity
that acts as a client and as a server in order to establish
calls between users. This server has a similar functionality
to an HTTP Proxy. It has the task of routing the requests
that receive from other entities. There are two types of Proxy
Server: Stateful Proxy and Stateless Proxy
Stateful Proxy: they keep the state of the transaction during
the request processing. It allows the division of a request
message in several ones (forking), with the purpose to find
in parallel the location of the called in order to obtain
the best path.
Stateless Proxy: they do not keep the state of the transaction during the requests processing, They only resend messages.
- Registrar Server: is a server which accepts
register requests of the users and keep the information from
these requests. It provides a location and address translation
service in its domain
- Redirect Server: is a server which generates
redirection answers to the received requests. This server routes
again the requests to the next server.
These categories are just conceptual, they can be all placed
in the same machine. They can be also in different machines
for scalability or processing matters. |
