What is a Telephone Exchange?

A telephone exchange is a collection of equipment designed to connect subscribers in a telecommunication network for the duration of information exchange. It consists of switching equipment responsible for setting up telecommunication connections and auxiliary equipment that guarantees the proper operation of the exchange. In analog exchanges, circuit switching is implemented, while in digital exchanges, packet switching is used. A telephone exchange is a switching node in telecommunication networks.

Think of it as the central post office or a major traffic intersection for phone calls. Its main purpose is to automate the process of connecting one subscriber to another, a task once performed manually by human operators.

General Architecture of a Digital Exchange

Modern digital telephone exchanges with stored program control are complex systems. Their overall architecture includes several key functional blocks.

Key Functional Blocks

• Subscriber Line Modules (Concentrators): These modules connect subscribers' lines to the exchange. They exist in both local and remote versions and contain their own switching fabric and subscriber line equipment.
• Trunk Modules: These modules allow the exchange to connect to trunks leading to other exchanges, typically in digital format (e.g., PCM trunks at 2 Mbit/s).
• Signaling Modules: These contain elements for sending and receiving signaling information, such as DTMF receivers and tone generators.
• Switching Fabric: This is the heart of the exchange, ensuring a connection path between the appropriate units.
• Control Unit: The "brain" of the exchange. It is a multiprocessor system that manages the entire operation, using standard processors connected via a local network.
• Auxiliary Units: These include power supply units (with battery backup), clock systems for synchronization, and testing units.

Subscriber Line Interface: The BORSCHT Functions

The analog line card (in Polish: AZL, Abonencki Zespół Liniowy) is the crucial interface between an old analog telephone and the modern digital exchange. It must perform a set of seven essential functions, known by the acronym BORSCHT.

• B - Battery feeding: Supplying DC power (e.g., -48V in Europe, -60V in the USA) over the subscriber line to power the analog telephone.
• O - Overvoltage protection: Protecting the sensitive exchange electronics from external voltage surges like lightning strikes or contact with power lines.
• R - Ringing: Sending a high-voltage AC signal (e.g., 75V ± 15V) down the line to ring the bell of the called telephone.
• S - Supervising: Monitoring the subscriber line for status changes (e.g., detecting when the handset is lifted - 'off-hook', or hung up - 'on-hook') and monitoring the dialing process.
• C - Coding: The actual process of analog-to-digital conversion, transforming the analog voice signal into a digital bitstream (PCM), and vice versa for the incoming signal.
• H - Hybrid: A two-wire to four-wire converter. The local subscriber loop is a two-wire circuit (sending and receiving on the same pair), while the internal workings of a digital exchange are four-wire (separate paths for sending and receiving). The hybrid circuit separates these paths.
• T - Testing: Providing access points for maintenance personnel to test the subscriber line from the exchange side.

Digital Subscriber Interface

For digital subscribers (e.g., using ISDN), a different type of line card is used. These digital subscriber line cards do not need all the BORSCHT functions, as the signal is already digital. Their primary roles are:

• Establishing Digital Lines: They provide the physical and logical connection for digital subscriber lines.
• Receiving Digital Signaling: They interpret the signaling information sent over the digital channel from the subscriber's terminal.
• Transmitting Digital Speech: They manage the flow of digital voice data to and from the switching fabric.