Telephony Concepts: Networks, Devices, and Digital Services

Posted on Mar 30, 2025 in Electronics

Telephone Network Fundamentals

Subscriber Loop (Local Loop)

The loop is the connection enabling communication within our telephone network. It is commonly known as a two-wire twisted copper pair.

Loop Types

  • Multi-pair/Hybrid: A technology combining copper pairs and potentially coax, offering telephony, data return channels (e.g., for satellite TV), and value-added services.
  • Wireless Local Loop (WLL): Transmits the telephone signal using radio waves between the subscriber and a base station.

Telephone Exchange (Central Office)

Performs switching functions to connect calls.

Switching Types

  • Spatial or Space Division: Establishes a unique, physical path between the input and output for each connection. Used for both analog and digital signals.
  • Temporal or Time Division: Used only in digital transmission systems. It works by assigning a specific time interval (time slot) to each transmitted signal within a shared medium.

Call Types on a PBX/Exchange

  • Local: Connects two users within the same local exchange.
  • Outgoing: When a subscriber of one exchange wants to communicate with a subscriber of a different exchange.
  • Incoming: When a user outside the exchange wants to contact a user belonging to it.
  • Transit: Calls routed through an exchange between two users who are both outside that specific exchange.

Telephony Characteristics

Frequency Band

The standard voice frequency band for telephony covers 300 Hz to 3400 Hz.

Call Phases

There are three main phases in a typical call:

  1. Call Setup
  2. Communication / Data Transfer
  3. Call Release

The Telephone Device

Core Function

The telephone is the device allowing voice conversation. It acts as an energy transducer, converting acoustic energy (sound waves) to electrical energy and vice versa. When the handset is lifted (off-hook), the phone typically receives a dial tone (around 400 Hz), indicating the line is ready for dialing.

Key Elements

  • Phonic Circuit: Contains the microphone (transmitter) and speaker (receiver).
  • Local Suppressor Circuit (Anti-sidetone): Prevents the speaker from hearing their own voice too loudly through the earpiece.
  • Bell or Buzzer: Notifies the user of an incoming call.
  • Supply Circuit: Manages the DC voltage (typically around 48V) supplied by the telephone line.
  • Keyboard/Keypad: Used for dialing numbers using DTMF (Dual-Tone Multi-Frequency) tones, including the * and # keys.

Basic Installation Components

  • Base Unit: Houses the main circuitry and keypad.
  • Handset: Contains the microphone and speaker.
  • Coiled Cable: Connects the handset to the base unit.
  • Line Cord: Connects the base unit to the telephone wall jack (user network interface).

Common Services

Modern telephony offers various services, including:

  • Calendar
  • Call List / History
  • Redial
  • Time and Date Display
  • Handsfree / Speakerphone
  • Speed Dial / Shortcuts
  • Caller Identification (Caller ID)
  • Call Diversion / Forwarding
  • Voicemail
  • Call Waiting
  • SMS (on capable phones/networks)
  • Three-way Calling / Conference Call

Digital Telephony and Data Services

ISDN (Integrated Services Digital Network)

Emerged as a digital evolution of the basic telephone network.

Basic Rate Interface (BRI)

ISDN BRI consists of 3 channels:

  • Two ‘B’ channels (Bearer channels) at 64 kbps each for transferring information (voice and/or data).
  • One ‘D’ channel (Delta channel) at 16 kbps used for signaling (call setup, control).

This access is designed for small subscriber installations, allowing up to two simultaneous communications. Up to 8 ISDN-compatible terminals can typically be connected to the subscriber’s bus.

DSL (Digital Subscriber Line)

A family of technologies providing digital data transmission over copper telephone lines.

ADSL (Asymmetric DSL) Specifications

  • Asymmetry: Provides significantly higher download (downlink) speed than upload (uplink) speed.
  • Maximum Loop Length: Typically up to 5.5 km (distance affects speed).
  • Maximum Downlink Speed (ADSL1): Up to 8 Mbps.
  • Maximum Uplink Speed (ADSL1): Often around 0.9 Mbps.
  • Technology: Uses Frequency Division Multiplexing (FDM) to separate voice and data signals, utilizing bandwidth up to approximately 1.1 MHz for data.

ADSL2/2+

Later standards (ADSL2 and ADSL2+) offer improved performance, potentially doubling or further increasing transfer rates for both uplink and downlink compared to original ADSL, especially over shorter line distances.

Mobile Communications: GSM

GSM Overview

GSM (Global System for Mobile Communications) is a standard for digital cellular networks. It began operating widely in European countries in the 1990s and remains in use globally, often alongside newer technologies. It is considered a 2nd Generation (2G) mobile standard.

Key Requirements and Features

  • Increased service capacity compared to earlier analog systems.
  • Compatibility and roaming across participating countries (initially European).
  • Improved quality of service (digital voice).
  • Ability to connect with the Public Switched Telephone Network (PSTN) and ISDN.
  • Affordable cost for users.
  • A single telephone number associated with the subscriber (via SIM card).
  • Enhanced security, including terminal identification and PIN protection for the SIM card.
  • Introduction of new services like SMS (Short Message Service), voicemail, call forwarding, etc.