PSTN and SIP open standard

I have tried to convince Anirban to continue writing about this, he is too busy. I am not that busy, but I start to feel a little hard in explain all these. I will try my best. Before we describe the complicated system, we still have some basic ideas to clarify. This time, it is the difference between the PSTN and SIP phone.

PSTN (public switched telephone network) is the world's collection of interconnected voice-oriented public telephone networks, both commercial and government-owned. The feature of this phone comparing to VoIP phone is the inefficiency of the use of wires. When a phone call is connected, the wire transfer of the voice between two users at either ends will occupy the entire wire connecting them. On the contrary, VoIP involves sending voice information in digital form in discrete packets which allows lots of packets to share parts of their routes along the same wire.

SIP is an open standard for VoIP technology, which is like SMTP for email communication. We all know about SKYPE, but in fact SKYPE built their system on a proprietary protocol instead of SIP open standard. As a result, SKYPE can only work within its individual client. And SIP based network can be connected with users out of the box, since most of the hardware development are following the open standard, and they are able to exchange in the future. This is how the SIP based VoIP network can also be connected with the PSTN network. (Skype can also do this at the moment, but it involves a fee for the calls. For SIP based phones, you can easily find free services.)

So, what the VoIP technology does, is to let the internet carrying voice signals, and also possible to connect to the existing PSTN network. Every PSTN has a number, so VoIP also needs a number to communicate with it. If you want others to reach you, you should have a number that everyone can dial using their home regular PSTN phone. Where and why can you get a free PSTN number, let's keep it for the next time.