VoIP Technology

Traditional Phone Networks - Circuit Switching

The traditional phone networks rely on 100 year old technology known as "circuit switching". When a call is made, a connection between the two parties in maintained for the duration of the call. Since you are connecting two points in both directions, it makes a circuit. This is the foundation of the Public Switched Telephone Network (PSTN).

A typical Phone call

You pick up the reciever and hear a dial tone, letting you know you have a connection to the local office of your telephone carrier.

You dial a phone number.

The call is routed through the switch at your local carrier office to the person you are calling.

A connection is made between your phone line and the other person's line, making a circuit.

You talk and then hang up and the circuit closes, freeing your line.


During your entire conversation, no matter how long, the circuit between the two lines is open. Telephone conversations over the traditional PSTN are transmitted at a fixed rate of about 64 kilobits per second (Kbps), or 1,024 bits per second (bps), in each direction, for a total transmission rate of 128 Kbps. That means that in a 10 minute conversation, roughly 9.4 megabytes of data are transfered.

If you think about your typical phone conversation, much of the data that is transmitted is wasted. When you're having a phone conversation, usually only one person talks at a time, which means the other half of the data transmition is wasted. Based on that, we could cut the data transmition in half. In addition, there are many times when both parties are silent. If we could remove these instances, the data transmition would be even less.

Data networks, the networks used for VoIP, do not use circuit switching. Your internet connection would be really slow if it maintained a constant connection with the web page you were accessing. Instead of maintaining a constant connection, data networks use packet switching.



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