More about Telephones

by Andy White

The Dial

Depending on the kind of dial on your telephone: If you have a rotary dial, dialling a number sends pulses to the exchange (sometimes herd as clicks). If you have a press button telephone, the dial sends out DTMF tones. The telephone exchange reads these dialling signals and tries to connect you  to your dialled number.
                                   

                                                Figure 3. A rotary dial. (In the diagram a
                                                'two' has just been dialled. A rotary cam
                                                causes spring contacts to open and close
                                                two times.)

Figure 4 An Edison transmitter from 1877

Speaking into the transmitter causes a diaphragm to vibrate. This intern causes carbon, sandwiched between the diaphragm and a backplate, to alternately compress and expand. This action varies the electric current flowing through it.

Figure 5. Receiver Bell No1: The first British Post Office telephone receiver

This varying current is passed along the phone line to the receiver at the other end. The current travels through a coil of wire, hence forming a varying electromagnetic field. This causes a metal diaphragm in the receiver to vibrate and thus reproduce the original sound.

Advances in electronics mean that modern telephones use electret microphones for transmitters and piezoelectric transducers for receivers, but the principles described above remain the same.

The Public Switched Telephone Network (PSTN)

The part of the network that runs between you and your exchange is known as the local loop.

Your telephone line runs underground or overhead on poles to the 'exchange'. For the majority of us this 'exchange' will actually be a line concentrator known as an RCU. For the remainder of us it is the Digital Local Exchange (DLE). (Between a third and a quarter of lines connect directly to a DLE.)

Figure 6. The local network.

If you are calling someone connected to the same RCU or DLE as yourself, then the a link between your phone and the phone of the person you called is created.

If it's a long-distance call, then your voice is digitized and multiplexed with millions of other voices and sent over the Tandem network.

Your voice normally travels over a fiber-optic line to the DLE at the receiving end, but it may also be transmitted by satellite or by microwave towers.

Figure 7. The Tandem network

The tandem network (shown in green in figure 7) consists of many tandem switching centres all networked together together. Other licensed providers (OLOs) may also be connected into the network at various points. (e.g. mobile phone operators).

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