Telephone system Tutorial | Telephone system basics

A basic telephone system is an example of telecommunication network. Telecommunication refers to transmission of information over a distance using equipments. The information to be transported include voice, data and multimedia. The basic telephone system enables voice communication between two or more parties by converting sound waves into electrical signals during transmission at the transmitter end. At the receiver, electrical signals are converted back into sound waves. In Plain Old Telephone System (POTS), the path between transmitter and receiver was network of wires. Now the medium has changed to optical fiber, radio link, satellite link, cellular link and so on.

Telephone system Terminologies

Following terminologies are widely used in telephone system.
• Central office (CO) : The local exchange where telephone lines were interconnected and operators handled call routing.
• Local loop : The physical connection or circuit that links individual customers' premises to the nearest telephone exchange or central office of a telecommunications service provider. It is typically made of copper wires.
• Dial tone : A continuous tone heard when telephone handset is lifted off the cradle indicating that the phone is ready to receive dialing input.
• Busy tone or busy signal :
• Trunk line : High capacity telephone lines connecting one central office to another.
• Exchange : A central location where telephone lines were interconnected and operators handled call routing.
• Operator : A person who manually connected calls by plugging and unplugging wires on a switchboard.
• Automatic exchange : Automatic exchanges replaced manual switchboards operated by human operators, enabling more efficient call handling and reduced dependency on manual intervention.
• Subscriber : An individual or business with a telephone line connected to central office or mobile network operator (in modern days).
• Rotary dial : A circular dial with numbers (0 to 9) used to dial phone numbers.
• Pulse dialing : The method of dialing in which electrical pulses are generated corresponding to dialed numbers. These pulses are sent to CO.
• Touch tone dialing : The method of dialing in which Dual Tone Multi-Frequency (DTMF) tones are generated for each button press.
• Area code : The 3 digit code used to identify specific geographic regions for long distance calling.
• Telephone number : A unique series of digits used to identify a specific telephone line in earlier days. It is similar to mobile number used in modern days.
• Exchange code : The first three digits of a seven digit telephone number. This represents a specific exchange within an area code.
• PBX (Private Branch Exchange) : It is private telephone switching system used within an organization to manage internal communications using extension numbers. It also allows outside calls to be routed to appropriate telephone extension numbers based on appropriate programming.

How Telephone system works

As shown in the figure,all the landline telephones are connected with the use of twisted pair cables made of copper to the local exchange or central office(CO). Thousands of lines were connected to the central offices in POTS. The connectivity between COs is crucial to establish communication across various geographical locations to enable long distance calls. There are several methods of connecting COs to create a network that spans a broader area. These methods are trunk lines, interexchange cariers, long distance carriers, international gateway, satellite communication, microwave links, fiber optic networks etc.

Basic Telephone System

Let us understand how telephone system works with example of two telephones, one making call (calling telephone -A) and the other receiving the call (called telephone -B).
➨When telephone-A user lifts the handset, dial tone is provided from the exchange which indicates line is free and he/she can make a call. If line is busy he/she will get busy tone.
➨When telephone-A user dials the phone number of telephone-B, it sends series of electrical pulses or DTMF tones corresponds to the dialed number to the exchange.
➨The local exchange receives the dialed digits from telephone-A and interprets them to identify destination number of telephone-B. Based on location of the telephone-B, exchange will either directly connect it or pass the call to other exchange to provide connectivity.
➨When exchange sends the signal to telephone-B, it will ring indicating as incoming call.
➨When Telephone-B lifts the handset and answers the call, connection is established between two telephones. The call can be terminated by either of the parties.

Touch tone set - An example of Electronic telephone Set

The figure depicts block diagram of telephone system and its internal components. Let us understand functions of these modules of touch-tone telephone set.
• Keypad : It is the primary input interface of touch tone telephone. It consists of 12 buttons (digits 0 to 9 and symbols "*" and "#").
• DTMF generator : When any of the button is pressed on keypad, DMTF generator creates pair of specific frequency tones.
• Microphone : It converts user's voice into electrical audio signals during call which are transmitted over telephone line.
• Speaker : It produces sound from incoming audio signals received over telephone line which allows user to hear the voice of the opposite person.
• Hookswitch : It is a mechanical switch that detects when the handset is lifted off the cradle (off-hook) or placed back on the cradle (on-hook).
• CPU (Central Processing Unit) or MPU (Micro Processor Unit) : It manages various functions of telephone such as processing keypad input, controlling of DTMF generator/receiver, call setup handling, management of display and so on.
• Memory : Some telephones may have memory to store speed dial numbers, call history and other preferences.
• Line Interface : It connects the telephone set to the phone line which allows communication with the central telephone exchange.
• Display : Some modern telephones may have LED or LCD display screen to show caller ID information, call duration and other menu options.
• Ringer circuit : It generates ringing sound when there is an incoming call to the telephone set.
• Power Supply : Provides necessary electrical power to operate internal components. Modern telephones use battery for this purpose.

Electronic telephone block diagram

BORSCHT is an acronym used in telephony to describe the basic functions that a telephone line interface must perform. Each letter in the word "BORSCHT" represents a specific function or component related to the operation of a telephone line. These functions are crucial for proper communication between the telephone user and the telecommunications network. The BORSCHT functions are as follows:
B - Battery Feed, O - Overvoltage protection, R - Ring, S - Supervision, C - Codec, H - Hybrid, T -Test

Conclusion: In summary, the Plain Old Telephone System (POTS) offered stable and reliable voice communication but lacked mobility and advanced features. Modern mobile phones, on the other hand, provide greater flexibility, portability and a wide range of features, including text messages, mobile apps, internet access and so on. Refer GSM Mobile phone to understand its basics, various features and functions of its internal components including its block diagram.

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