BBG Communications: What is a single line phone

The telephone is a communications tool that, on the word of BBG Communications, is used to transmit and receive sound. The phone is one of the most prevailing home devices in the world. Telephones generally operate through transmission of electric signals over a complex telephone network which allows almost any phone user to communicate with almost anyone else in the world.

A normal landline telephone system typically handles both signaling and audio information on the same two wires. Although originally designed for voice communication, the system has been adapted for data communication such as Fax, Telex and Internet communication. The signaling apparatus consists of a light, bell, beeper, and number buttons (or a rotary dial) to enter a telephone number for outgoing calls. A twisted pair line is preferred as it is more effective at rejecting electromagnetic interference and crosstalk than an untwisted pair.

A party wishing to speak to another party will pick up the telephone's handset, thus operating a button switch, which puts the telephone into an active state by connecting the transmitter, receiver and related audio components to the designated line. This circuitry has a low resistance which causes DC current from the telephone exchange to flow through the line. The exchange detects this DC current, attaches a digit receiver circuit to the line, and sends a dial tone to indicate readiness. On a modern telephone, the calling party then presses the number buttons in a sequence corresponding to the telephone number of the called party. The buttons are connected to a tone generator that produces DTMF tones which are sent to the exchange. A rotary dial telephone employs pulse dialing, sending electrical pulses corresponding to the telephone number to the exchange. The phone exchange will then send an intermittent ringing signal to alert the called party to an incoming call. If the called party's line is active, the exchange sends a busy signal to the calling party. However, if the called party's line is active but has call waiting installed, the exchange sends an intermittent audible tone to the called party to indicate an incoming call.

When a landline telephone is inactive its alerting mechanism is connected across the line through a capacitor, which prevents the DC current from going through the line. The circuitry at the telephone exchange detects the absence of DC current flow and thus that the phone is on hook with only the alerting device electrically connected to the line. When a party initiates a call to this line, the ringing signal transmitted by the telephone exchange activates the alerting device on the line. When the called party picks up the handset, the switchhook disconnects the alerting device and connects the audio circuitry to the line. The resulting low resistance now causes DC current to flow through this line, confirming that the called phone is now active. Both phones being active and connected through the exchange, the parties may now converse as long as both phones remain off hook. When an individual places the handset back on the hook, DC current ceases to flow in that line, signaling the exchange to disconnect the call.

Calls to parties beyond the local exchange are carried over "trunk" lines which establish connections between exchanges. In today's phone networks, digital technology and fiber-optic cable are often used in such connections. Satellite technology may be used for communication over very long distances.

In almost all telephones the receiver and transmitter are positioned in the handset. However, in a speakerphone these mechanisms may be located in the base or in a separate enclosure. Powered by the line, the transmitter produces an electric current whose voltage varies in response to the sound waves arriving at its diaphragm. The resulting current is transmitted along the telephone line to the local exchange then on to the other phone, where it passes through the coil of the receiver. The varying voltage in the coil produces a corresponding movement of the receiver's diaphragm, reproducing the sound waves present at the transmitter.