In this section, we will begin to discuss how various types of "switching" work. This applies to both voice and data. There are three types of switching used in networks today: circuit switching, used in voice networks; packet switching, used in data networks; and cell relay, now being deployed for both voice and data transmission.

Circuit Switching
The telephone network is a circuit-switched network (although there is also a packet-switched network used for signaling data). In a circuit-switched network, a dedicated circuit must first be connected. Once the circuit has been "nailed up", transmission can begin. When the transmission is complete, the circuit is released for the next transmission.

Let us look at a simple telephone call. When you remove the receive from the telephone and dial a telephone number, the telephone company searches its database to determine which circuit should be used to deliver the telephone call. If it is a long distance call, the switch knows it must connect to another telephone company office, where a switch called a tandem is located. The tandem switch will then use a circuit that connects it to another office, the toll office switch.

This process continues until there are circuits connected from the originator to the destination. These circuits cannot be used for any other telephone call; they are dedicated to this one call until the call is complete. Once the call is complete, the circuits can then be released and used for another call.

Circuit switching is not an efficient method for routing any kind of data, whether it is digital voice or user data. The circuit is wasted much of the time because no transmission is using the bandwidth of the circuit 100 percent of the time. Any time there are idle period on the circuit, the circuit is being wasted. It would be much more efficient to have a transmission facility capable of transmitting many different "conversations" over the same circuit at the same time.

This was achieved (sort of) through multiplexing. A circuit can be divided into channels, with each channel used for a transmission. Digitial telephone circuits are multiplexed and are capable of transmitting several different conversations at the same time on the same circuit.

There is one catch; each channel then becomes dedicated to the conversation until the caller disconnects. Then the channel can be released for another transmission. So in the case where there are 24 channels (the common denominator in today's digital facilities), there can be 24 different conversations going on at once over the same facility.

This is better than wasting the circuit for one transmission, but it could still be better. Imagine having no channels. Transmissions are sent over the same circuit as needed, but there are no limits to the number of conversations that can be sent over the same facility at the same time.

NEXT PAGE: Packet Switching

Copied with permission,McGraw-Hill Telecommunications from the book Telecommunications Protocols, by Travis Russell, 1997 McGraw-Hill Telecommunications, pages 45-48.