Circuit-Switched Networks (Windows Embedded CE 6.0)
1/5/2010
The traditional phone network - known as the Public Switched Telephone Network (PSTN) - has a long history. Although the underlying technologies used by the PSTN have changed with the times, it is still useful to understand how the traditional phone network operates.
Note
This information is an ideal generalization of how the PSTN operates. There are many different ways to perform signaling and media transport. For example, rotary phones operate differently than touch-tone phones. However, for the purposes of the information provided here — which explains the difference between the PSTN and VoIP — differences in PSTN implementation are not relevant.
The original PSTN was an entirely analog network that transmitted sounds. Simplified, two phones on the PSTN functioned as microphone/speaker combinations connected by a wire.
On an analog network, when sounds strike a microphone in a telephone handset, the handset translates the sounds into analog electrical signals. These signals are then transmitted through the wire connected to the other phone, where the speaker translates them back into sound.
To transmit such signals, two primary operations are performed (by both PSTN and VoIP networks):
- Signaling (also called call control), which is the method of creating and ending a connection
- Media transport, which is the transmission of information between parties
Following is an overview of how the analog PSTN implements signaling and media transport operations.
Signaling (Call Control)
PSTN signaling uses tones generated by the phone to begin or end a call.
A PSTN phone is connected by wire to a physical location, often called a local exchange, operated by the phone company. When you answer a phone, you hear a dial tone generated by the local exchange. At this point, the only connection that exists is between your phone and the local exchange, over a single contiguous wire.
(There are many different kinds of PSTN signaling, including signaling between local exchanges. This documentation only discusses the basic ideas of signaling between a user and the local exchange.)
When you dial, the local exchange listens to the tones generated by your phone.
When you have entered enough information for the local exchange to identify the party you are calling, it connects your phone to the other phone.
If the party you're calling is connected to the same local exchange - for example, if they live in the house next to yours - the phone call might never leave the local exchange.
If the party you're calling is farther away - in another part of your city, or in another country/region - the local exchange makes a connection with other another local exchange, often through intermediaries, using other signaling methods.
Regardless of the number of intermediaries and local exchanges, you can think of the connection as a single wire from your phone to the other phone, although the connection is made through any number of physical devices, called switches, and may not even use wires.
A switch is a machine (usually a computer) that connects the wires coming from phones or from other switches. This connection is an electrical circuit, which is why the PSTN is known as a circuit-switched network.
When one of the parties hangs up the phone, the local exchange and any other involved switches use signaling to disconnect the circuit so the wires and switches can be used for other calls.
Media Transport
When the connection is made and the person you're calling picks up the phone, the media transport part of the network is used.
Because the PSTN is an analog network (at least in this simplified discussion), media transport consists of electrical waves generated by a microphone that are transported using a wire. When electrical waves reach the speaker on the called party's phone, that speaker translates them back into sound that the called party can hear.