LESSON 2: SIP SYSTEM ARCHITECTURE

As a part of Learning SIP, in the last post I did demonstrate on a Basic Call Flow of SIP. Today, I am going to take you through the SIP System Architecture.

SIP is a revolution in this modern world of communications. Rather I would say that SIP alone did not bring this change. That's where I would like to introduce SIP SYSTEM ARCHITECTURE. Each component of this architecture has contributed and made this drastic change in Telecom World.

So, why wait! Lets go through the architecture and its components.

SIP System Architecture

SIP Endpoints:

SIP Phones, SIP Soft clients running on PCs, mobiles etc.

Proxy:

• This server accepts INVITE from any SIP endpoints and process the request.
• This is the server which decides whether the call need to terminated on to another SIP Endpoints (or) to another PBX (or) to PSTN.
• The routing decision would be taken by the logic written by Administration of this proxy server.
• It is purely for Session Management.

Registrar:

• Every SIP User-Agent should register to this server with its current location on the network.
• This server authenticates and authorizes by accepting every REGISTER request from User-Agents in the network.

Location Server:

• This server keeps a record of all the location ip-address information of User-Agent who registers with Registrar Server.
• Its a kind of database with Address of Records (AoR)

SIP-to-PSTN Gateway:

• SIP could not be understood by PSTN. This Gateway fills the gap of understanding SIP signaling to PSTN SS7 and vice-versa.
• Generally this would be provided as a service by SIP Service Provider.

As we have come through the functionality of each component in the architecture, now we will go through an example call i.e. an Incoming PSTN call to the SIP Endpoint. This call flow gives more clarity!

1. PSTN call enters through the SIP-to-PSTN Gateway.
2. This SIP-to-PSTN Gateway converts the SS7 signaling of PSTN to SIP and it forwards the request to the respective Proxy server based on the Domain/Sub-network.
3. Now, Proxy server accepts the INVITE request from Gateway and acknowledges immediately that the accepted INVITE is in process.
4. Proxy Server will now find the current location of the SIP Endpoint (User-Agent) by sending the request to Registrar Server
5. Registrar Server in turn verifies the current location (AoR) by querying the database of Location server.
6. Once Registrar Server responds Proxy Server with the current Address-Of-Records. Then Proxy will send the request to all those location IP-addresses.
7. Later, the call will be terminated on SIP Endpoint.

I have started a new naming convention for each post. Now, it makes more easy to navigate and be aligned.

Thanks,

Raj