Details Of Operation

Contact: Phone Call

The landline contains a switchhook (A4) and an alerting device, usually a ringer (A7), that remains connected to the phone line whenever the phone is "on hook" (ie the switch ( A4) is open), and other components which are connected when the phone is "off hook". Components stall includes a transmitter (microphone, A2), a receiver (speaker, A1), and other circuits numbering, the filter (A3), and amplification.

An appellant who wishes to speak to another party will pick up the handset, and pressing a lever which closes the switchhook (A4), which powers the telephone by connecting the transmitter (microphone), receiver (speaker) audio components and the line. The circuit has a resistance drop low (less than 300 ohms) that causes a direct current (DC), which comes in (C) from the central office. The exchange detects this current, attaches a digit receiver circuit to the line, and sends a tone to indicate preparation. On a modern push-button telephone, the caller presses the number keys to send the telephone number of the person called. Keys control sound generator circuit (not shown) that DTMF tones that the exchange receives. A rotary phone uses pulse dialing, sending electrical pulses, that the exchange can count to get the telephone number (as of 2010 many exchanges were still equipped to handle pulse dialing). If the called line is available, the exchange sends an intermittent ringing signal (about 90 volts alternating current (AC) in North America and the United Kingdom and Germany 60 volts) to alert the called party to an incoming call. If the called line is in use, the exchange returns a busy signal to the caller. However, if the called line is in use but has call waiting installed, the exchange sends an intermittent audible tone to the called party to indicate an incoming call.

Telephone ring (A7) is connected to the line via a capacitor (A6), a device which blocks current into alternating current, but that passes. Thus, the phone does not know when it is on hook (a DC voltage is continually connected to the line), but exchange circuitry (D2) can send an AC voltage on the line to ring for an incoming call. (When there is no exchange, telephones often have hand-cranked magnetos to make the ringing voltage.) When a landline phone is inactive or "on hook", the circuitry at the telephone exchange detects the absence of current and therefore "knows" that the phone is on hook (therefore, only AC current will go through) with only the alerting device electrically connected to the line. When a party calls on this line, the exchange sends the ringing signal. When the called party picks up the handset, they actuate a double-circuit switchhook (not shown) which simultaneously disconnects the alerting device and connects the audio circuitry to the line. This, in turn, draws direct current through the line, confirming that the called phone is now active. The exchange circuit disables the ringer, and both phones are now active and connected through the exchange. The parties can now talk as long as both phones remain off hook. When a party "hangs up", placing the handset in the cradle or hook, direct current ceases in that line, signaling the exchange to disconnect the call.

Calls to beyond the local exchange are carried over "trunk" lines which establish connections between exchanges. In modern telephone networks, fiber-optic cable and digital technology are often employed in such connections. satellite technology can be used for communication over very long distances.

In most landline telephones, the transmitter and receiver (microphone and speaker) are located in the handset, although in a speakerphone these components may be located in the base or in a separate housing. Powered by the line, the microphone (A2) produces a modulated electrical current that varies its frequency and amplitude in response to sound waves arriving at the membrane. The resulting current is transmitted along the telephone line to the local exchange and the other telephone (via the local exchange or via a larger network) where it passes through the coil of the receiver (A3 ). The variable current in the coil produces a corresponding movement of the membrane receptor, reproduce the original sound waves present in the transmitter.

With microphone and speaker, additional circuitry is provided to prevent the loudspeaker signal into and from the microphone signal to interfere with each other. This is accomplished through a hybrid coil (A3). The incoming audio signal passes through a resistor (A8) and the primary winding of the coil (A3) which transmits it to the speaker (A1). Since the current path A8 - A3 has an impedance much lower than the microphone (A2), substantially all of the incoming signal through and around the microphone.

At the same time, the voltage across the line causes a current which is divided between the coil resistance (A8-A3) and the branch micro-coil (A2-A3) branch. The current through the resistance of the coil branch has no effect on the incoming audio signal. But the passage of current through the microphone is placed in alternating current (in response to speech sounds) which then passes through only the upper branch (A3) of the primary winding coil, which has much less of turns lower than the primary winding. This causes a small portion of the microphone output to be fed back to the speaker, while the rest of the AC out through the phone line.

A combination of linemen is a telephone designed for testing the telephone network, and may be attached directly to the airlines and other infrastructure components.