Subscriber line interface and BORSCHT functions
A traditional analog telephone connects to a digital exchange through a subscriber line interface. The interface is commonly implemented with a Subscriber Line Interface Circuit (SLIC) and associated line-card circuitry. Together they provide the functions represented by the acronym BORSCHT.
BORSCHT covers seven functions of a copper subscriber line interface: battery feed, overvoltage protection, ringing, supervision, coding, hybrid conversion, and testing. Each letter denotes a function provided by the line interface.
A practical line card can distribute these functions across a SLIC, a CODEC, and external protection or ringing components. Some SLIC devices integrate several of them in one chip.
BORSCHT functions
The functions cover line power, signaling, signal conversion, and line testing.
- B
Battery feed
This function supplies DC power to the subscriber's telephone set over the copper wire pair. Traditional telephones draw power from the telephone exchange through this pair. A common line-feed value is -48 V DC. The exact voltage depends on the national network and the line circuit, and it powers the telephone's microphone and other circuitry.
- O
Overvoltage protection
Long copper subscriber lines can carry transients caused by lightning, surges, or contact with power lines. Protection components limit these voltages and protect the exchange and line-interface electronics.
- R
Ringing
To signal an incoming call, the exchange sends a high-voltage, low-frequency AC signal down the line. A common example is about 75 V RMS at 25 Hz, although voltage, frequency, and cadence depend on the network and the connected load. The signal drives an electromechanical bell or an electronic ringer.
- S
Supervision
This function monitors the telephone line and detects its state. It determines whether the phone is (idle) or (busy, requesting service). When you lift the handset, a switch inside the phone closes the circuit, allowing the DC battery current to flow. The exchange supervises the line by detecting this current flow, which signals a request for a dial tone or answering a call.
- C
Coding
The 'Coding' function converts the analog voice signal into a digital bitstream, commonly using PCM, and performs the reverse conversion for the incoming signal. A dedicated chip called a .
- H
Hybrid
The subscriber line is a 2-wire circuit, so transmitted and received voice signals use the same pair of wires. The internal voice path of a digital exchange is 4-wire, with separate transmit and receive paths. The performs this 2-wire-to-4-wire conversion. Its balance network separates the transmission directions and reduces echo caused by line mismatch.
- T
Testing
This function provides the telephone exchange with access points for remote testing of the subscriber line. It allows technicians to diagnose shorts, open circuits, and grounding faults without visiting the subscriber's premises.
Schematic of a subscriber line interface (SLIC)
The diagram shows a block diagram of a subscriber line equipment package (SLIC 1) and the blocks that implement the BORSCHT functions of a SLIC. The connections show signal flow between the subscriber line, the analog section, and the exchange's digital interface.
Analysis of schematic elements
Protection and monitoring section
On the left side of the diagram is the Test block, which supports testing from the exchange side. Technicians can diagnose shorts, open circuits, and grounding faults without visiting the subscriber's premises. A test may temporarily change the line state during measurement. Adjacent to it is the Ringing Current generator, a high-voltage AC signal source. A signal around 75 V RMS at 25 Hz is one example of a ringing parameter.
Overvoltage protection
The central element of the schematic is the "Overvoltage Protection" block with a -48 V DC power supply. The block implements the Battery (B) and Overvoltage protection (O) functions from the BORSCHT acronym. It supplies DC power to the subscriber's telephone and limits surges arriving from the external line before they reach exchange electronics.
Supervision and monitoring functions
The Supervision block in the center of the schematic implements the Supervision (S) function, monitoring line status and detecting whether the subscriber has lifted the handset (off-hook) or placed it down (on-hook). This block generates "Line Status Information"that is passed to the exchange's control system.
Analog-to-digital conversion
On the right side of the schematic is the Hybrid circuit (H), which connects the 2-wire subscriber line circuit to the exchange's internal 4-wire path. Signals are then routed to the CODEC (Coding, C), which converts analog voice signals to digital PCM streams and performs the reverse conversion.
Signal filtering and processing
Between the hybrid circuit and CODEC are low-pass filters, which limit components that could cause aliasing during analog signal sampling. Their passband covers frequencies below the Nyquist limit set by half the sampling frequency.
Connection to switching field
At the right end of the schematic, digital signals are passed to the "PCM trunk to the switching field" and Decoder. An E1 trunk has a rate of 2.048 Mbit/s and a frame of 32 8-bit time slots at 8 kHz. In PCM-30, time slot 0 carries frame alignment and time slot 16 carries signaling. The other 30 time slots carry 64 kbit/s voice channels.
Repeated SLIC line unit
At the bottom of the schematic, the red block labeled SLIC 8denotes another subscriber-line unit, numbered 8 in the drawing. The arrows labeled "From the control unit" and "to the control unit" indicate control connections for that line unit. The "Testing" and "Ringing" paths show how the exchange can control those functions for each port.
Significance of the schematic
The diagram connects the BORSCHT functions with protection, ringing, testing, supervision, hybrid, codec, filtering, and PCM interface blocks. It shows the signal path from the subscriber line through analog processing and conversion to the digital stream delivered to the switching field.