Telephone System Tutorial: Basics Explained

A basic telephone system exemplifies a telecommunication network. Telecommunication involves transmitting information over a distance using equipment. This information can be voice, data, or multimedia. The core function of a telephone system is enabling voice communication between two or more people. This is achieved by converting sound waves into electrical signals for transmission and then back into sound waves at the receiving end.

In the traditional Plain Old Telephone System (POTS), communication relied on a network of wires. Modern systems now use optical fiber, radio links, satellite links, cellular links, and more.

Telephone System Terminology

Here are some common terms you’ll encounter when learning about telephone systems:

• Central Office (CO): The local exchange where telephone lines were connected, and operators routed calls.
• Local Loop: The physical connection (usually copper wires) linking a customer’s location to the nearest telephone exchange or CO.
• Dial Tone: A continuous tone indicating that the phone is ready to receive dialing input.
• Busy Tone (or Busy Signal): A signal indicating that the dialed number is currently in use.
• Trunk Line: High-capacity telephone lines connecting different COs.
• 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: Automated systems that replaced manual switchboards for more efficient call handling.
• Subscriber: An individual or business with a telephone line connected to a CO or mobile network operator.
• Rotary Dial: A circular dial with numbers (0 to 9) used to dial phone numbers.
• Pulse Dialing: A dialing method where electrical pulses, corresponding to the dialed number, are sent to the CO.
• Touch-Tone Dialing: A method where Dual-Tone Multi-Frequency (DTMF) tones are generated for each button press.
• Area Code: A 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.
• Exchange Code: The first three digits of a seven-digit telephone number, representing a specific exchange within an area code.
• PBX (Private Branch Exchange): A private telephone switching system within an organization for managing internal communications using extension numbers.

How a Telephone System Works

The figure below illustrates a basic telephone system:

In a traditional landline setup, telephones are connected to the local exchange (CO) via twisted pair copper cables. Thousands of lines would connect to these COs in the older POTS systems.

Connecting COs is essential for long-distance communication. This is achieved through:

• Trunk lines
• Interexchange carriers
• Long-distance carriers
• International gateways
• Satellite communication
• Microwave links
• Fiber optic networks

Let’s walk through the process of a telephone call between two phones, A (calling) and B (receiving):

1. When user A lifts the handset, a dial tone from the exchange indicates the line is free. A busy tone is heard if the line is in use.
2. User A dials the phone number of telephone B, sending electrical pulses or DTMF tones to the exchange.
3. The local exchange receives and interprets these digits to identify the destination number of telephone B. The exchange then either connects directly or passes the call to another exchange.
4. The exchange sends a signal to telephone B, causing it to ring.
5. When user B lifts the handset, the connection is established, and the call can begin. Either party can end the call.

Touch-Tone Telephone Set

Here’s a block diagram of a touch-tone telephone and its components:

• Keypad: The main input, with 12 buttons (0-9, * and #).
• DTMF Generator: Creates specific frequency tone pairs when a button is pressed.
• Microphone: Converts voice to electrical audio signals.
• Speaker: Produces sound from incoming audio signals.
• Hookswitch: Detects when the handset is lifted (off-hook) or placed back (on-hook).
• CPU/MPU: Manages functions like keypad input, DTMF generator, call setup, and display.
• Memory: Stores speed dial numbers, call history, and preferences.
• Line Interface: Connects the phone to the phone line for communication with the exchange.
• Display: Shows caller ID, call duration, and other menu options.
• Ringer Circuit: Generates the ringing sound for incoming calls.
• Power Supply: Provides power, often using a battery in modern phones.

BORSCHT Functions

BORSCHT is an acronym used in telephony to describe the basic functions a telephone line interface must perform:

• B - Battery Feed
• O - Overvoltage Protection
• R - Ring
• S - Supervision
• C - Codec
• H - Hybrid
• T - Test

Conclusion

The Plain Old Telephone System (POTS) offered stable voice communication but lacked mobility and advanced features. Modern mobile phones offer greater flexibility, portability, and features like texting, apps, and internet access.