LESSON 5: SIP REGISTRAR SERVER AND LOCATION SERVER

If you hate long posts as I always suggest go through this video lesson (click here)

Now, we already know that in real SIP world, any UA when it wants to communicate with another UA should take the help of “SIP Proxy Server” - which finds the actual location. Because it is not practically possible to remember all other UA’s addresses.

All good till now. The next question which comes to mind is How a Proxy Server able to find contact  address of an User Agent(Called Party).

Here, Proxy Server takes help of “Location Service”.

Location Server

• Location Service helps in providing the exact location of UA.
• Location server is a server which records and maintain the contact information of each and every UA with in a typical enterprise.
• Location Server has its own database, which keeps up-to-date info of each UA by another Service called “Registration Service”. The Server which provides this service  called as a “Registrar Service”.

Registrar Server

• This Server accepts the registration request from UA and responds with 2000k.
• Once the UA is successfully registered, then it is authorized to participate/communicate to any UA with in the enterprise.
• If the registrar Server wants NOT to accept the request, then it responds with “401 Unauthorized”.

---

When a SIP device starts up it sends its current address and location details to its Registrar Server. Registrar Server then updates that information in the database which is maintained by Location Server.

---

Now Let me align all the above scrap in an order:-

• Whenever a UA-1 wants to communicate with UA-2, then UA-1 sends an INVITE

         request to SIP PROXY SERVER

• The SIP Proxy Server then first checks with Registrar Server whether UA-1 is an authorized to participate in the session or not.
• Let's assume,  UA-1 is not registered. Then SIP Proxy Server responds with “407 Authentication Required” to UA-1
• Upon receiving 407 from SIP Proxy Server, now UA-1 sends a REGISTER request to Registrar Server to get authorized and authenticated. Once UA-1 is successfully registered, then as usual UA-1 sends an INVITE request to SIP PROXY SERVER.
• Now, SIP PROXY SERVER will accept the INVITE and interprets for Called Party’s information. SIP Proxy Server then forwards that info to Location Server to find the exact Location of the UA-2.
• Location Server maintains its own database which contains the Location information of an UA i.e; Address of Record(AOR).
• This AOR is getting updated by Registrar Server
• After finding AOR of UA-2, then SIP Proxy Server forwards the INVITE request to UA-2

Precisely that's how SIP PROXY SERVER finds the exact Location of UA-2 (called Party) with the help of Registrar and Location services. Generally, you don’t find a SIP PROXY Server without inbuilt services like Registration/Location.

Lot of people already has posted some comments and requests. Some suggested with different set of topics. I will definitely cover them but it takes time.

Hope this helps.

Thanks,

Rajesh Talluri,

http://telecomtechsource.blogspot.in