The telephone is a telecommunications tool that, as mentioned in the BBG communications, is used to transmit and receive sound. It is one of the most prevalent home gadgets in the world. The typical telephone by and large operates through transmission of electric signals over a complex system of telephones which effectively allows nearly any telephone customer to speak with almost any other person in the world. A traditional landline telephone system commonly handles both signaling and audio information on the same pair of insulated wires. Although originally designed for voice communication, the system has been adapted for data communication such as Fax, Telex and Internet communication.
The signaling equipment consists of a bell, beeper, light or other device to alert the user to incoming calls, 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. An individual wishing to talk to another person will pick up the telephone handset, which puts the telephone into an active state by connecting the transmitter, receiver and related audio components to the required 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. If the called party’s line is not already active, the phone exchange will try and send intermittent ringing signals to alert the called party of 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 phone is on the hook its notification instrument is connected across the line through a capacitor, which prevents any of the DC current from traveling 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 a person places the handset back on the cradle, Direct Current ceases to flow in that phone line, telling the exchange to disconnect the call. Calls to individuals beyond the local exchange are passed 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.
