What is a telephone exchange?
A telephone exchange is a group of devices that connects subscribers in a telecommunication network for the duration of a call. It includes switching equipment that sets up connections and auxiliary equipment that supports exchange operation. Traditional analog and digital public telephone exchanges generally use circuit switching for voice calls. Packet switching appears in IP and hybrid exchanges. A telephone exchange is a switching node in the network.
The exchange analyzes signaling, allocates resources, and connects a subscriber line to another line or to an inter-exchange trunk. It automates tasks that were once performed manually by operators.
General architecture of a digital exchange
A digital exchange with stored program control uses processors and software for signaling, call setup, resource management, and release. Its architecture includes several functional blocks, with the exact arrangement depending on the application.
Functional blocks
- Subscriber line modules (concentrators): These modules connect subscriber lines to the exchange. They exist in local and remote versions and can include concentration and line equipment. A remote concentrator forwards traffic to the exchange over a digital trunk.
- Trunk modules: These modules connect the exchange to trunks leading to other exchanges, usually in digital form, such as PCM trunks with a rate of 2.048 Mbit/s.
- Signaling modules: These contain elements for sending and receiving signaling information, such as DTMF receivers and tone generators.
- : It provides a connection between the appropriate modules.
- Control unit: This system runs software for signaling, route selection, call setup and release, and resource supervision. It can use a multiprocessor structure with processors communicating over a local network.
- Auxiliary units: These include power supply units with battery backup, clock systems for synchronization, and testing units.
Analog subscriber line interface: BORSCHT functions
The analog subscriber line interface connects a subscriber loop to a digital exchange. In Polish it is called AZL, Abonencki Zespół Liniowy. The interface performs seven functions represented by the acronym BORSCHT.
- B - Battery feed: Supplying DC power over the subscriber line to the analog telephone. A common line-feed value is -48 V DC. The exact voltage depends on the network and line circuit.
- 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, low-frequency AC signal down the line to ring the called telephone. A common example is about 75 V RMS at 25 Hz. Voltage, frequency, and cadence depend on the network.
- S - Supervision: Monitoring the subscriber line for status changes, such as lifting the handset (off-hook) or placing it down (on-hook), and monitoring the dialing process.
- C - Coding: Converting the analog voice signal to a digital PCM stream and performing the reverse conversion. The circuit can include sampling, quantization, encoding, and filtering.
- H - Hybrid: A circuit that connects a bidirectional two-wire subscriber loop to separate four-wire transmit and receive paths. It separates the transmission directions and reduces echo caused by line mismatch.
- T - Testing: Providing access points for maintenance personnel to test the subscriber line from the exchange side.
Digital subscriber line interface
Digital subscriber access, such as ISDN, uses a digital line interface represented here by an AZLC line card rather than a full analog BORSCHT line circuit. ISDN B-channels carry user information, commonly voice at 64 kbit/s, while the D-channel carries signaling. Its roles include:
- Establishing digital lines: The line card provides the physical and logical connection for digital subscriber lines.
- Receiving digital signaling: The line card interprets signaling information sent over the D channel from the subscriber's terminal, as in an ISDN interface.
- Transmitting digital speech: The line card manages the flow of digital voice data to and from the switching fabric.