Since we are mostly interested about what happens on the endpoint side of the VoIP picture, we thought it would be interesting to summarize the components of an Internet Telephony Service Provider (ITSP). So here is a brief of list of main components:
- User Agent Client (a.k.a SIP client)
- Registrar Server (provides network access)
- SIP Proxy Server (routes calls)
- User Agent Media Gateway (converts VoIP calls to PSTN calls)
- PSTN access (T1 line connected to a CO (Phone Central Office)
- PSTN telephone (the good ol’ telephone)
When we start Kapanga (or any other UAC), it registers with the Registrar Server to gain access to the network. Thus, Kapanga will send a “SIP REGISTER” request with its credentials and the Registrar Server will validate and grant it access to the network. Once we place a call to a regular number (a.k.a. PSTN number in tech jargon) the call will traverse the network and eventually will get routed to a User Agent Media Gateway at the ITSP site.
The Media Gateway will take care of translating SIP calls to ISDN or SS7 format (and viceversa), routing calls back to the SIP Proxy Server as needed. The SIP Proxy Server will check its routing list to make sure the destination device is available (in this case Kapanga) and whether to forward the call to a Voice Mail system or to Kapanga itself. The media content (voice or video) will then traverse the IP network from the Gateway to Kapanga (the User Agent Client) directly without going through any SIP Proxy Server.
And voilá! That’s how it all works. Of course, if you are offering this as a service, you’ll want to add a billing server that keeps tracks of calls/minutes/rates. We’ll get to that in upcoming posts. Cheers!