BORSCHT Functions
The seven key functions required for an analog subscriber line interface (Battery, Overvoltage, Ringing, etc.).
The Gateway Between Analog and Digital Worlds
To connect a traditional analog telephone to a modern digital telephone exchange, a specialized interface is required. This interface, known as a Subscriber Line Interface Card (SLIC), must perform a set of essential tasks. These tasks are collectively known by the acronym BORSCHT.
BORSCHT outlines the seven fundamental functions necessary to bridge the robust, unpredictable, and high-voltage analog world of the copper subscriber line with the delicate, precise, low-voltage digital world of the switching system. Each letter in the acronym represents a critical job the interface must perform.
Breaking Down the BORSCHT Functions
Let's explore each function in detail:
- B
Battery Feed
This function supplies the necessary DC power to the subscriber's telephone set over the copper wire pair. Traditional telephones don't have their own power cords; they draw power directly from the telephone exchange. This DC voltage, typically -48V in North America and much of the world, powers the phone's microphone and circuitry.
- O
Overvoltage Protection
The long copper wires running to a house can act as antennas, picking up dangerous high voltages from sources like lightning strikes or accidental contact with power lines. This function protects the sensitive, low-voltage electronics of the telephone exchange from being destroyed by such surges.
- R
Ringing
To signal an incoming call, the exchange must send a high-voltage, low-frequency AC signal down the line. This ringing signal, typically around 75-90V AC at 20-25 Hz, is what drives the electromechanical bell or electronic ringer in a standard telephone.
- S
Supervision
This function monitors the telephone line to detect its status. 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
This is the heart of the analog-to-digital conversion. The 'Coding' function handles the conversion of the continuous analog voice signal into a digital bitstream (using techniques like PCM) and vice-versa. It's often performed by a dedicated chip called a .
- H
Hybrid
The subscriber line is a 2-wire circuit, meaning both transmitted and received voice signals travel over the same pair of wires simultaneously. However, the internal circuitry of the digital exchange is 4-wire, with separate paths for sending and receiving. The is the crucial component that performs this 2-wire-to-4-wire conversion, separating the two directions of transmission and preventing the speaker from hearing an echo of their own voice.
- T
Testing
This function provides the necessary access points for the telephone exchange to perform remote testing of the subscriber line. It allows technicians to diagnose problems like shorts, open circuits, or grounding faults without having to physically visit the subscriber's premises.
Schematic of an Analog Line Card (SLIC)
The following diagram shows a comprehensive block diagram of a subscriber line equipment package (SLIC 1), which is the physical implementation of a SLIC containing all BORSCHT functions. The schematic illustrates the actual architecture used in telephone exchanges.
Analysis of Schematic Elements
Protection and Monitoring Section
On the left side of the diagram is the Test block, which enables remote testing of the subscriber line without disrupting normal operation. Adjacent to it is the Ringing Current generator - a high-voltage AC signal source (~75-90V, 20-25Hz) required to activate the telephone's bell or ringer.
Overvoltage Protection
The central element of the schematic is the "Overvoltage Protection" block with 48V Power Supply. This crucial module implements the Battery (B) and Overvoltage protection (O) functions from the BORSCHT acronym. It provides constant DC power for the subscriber's telephone while protecting the exchange's sensitive electronics from dangerous surges originating from external lines.
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 Splitter (Hybrid - H), which converts the 2-wire subscriber line circuit to the exchange's internal 4-wire system. Signals are then routed to the CODEC (Coding - C), which performs the crucial conversion of analog voice signals to digital PCM streams and vice versa.
Signal Filtering and Processing
Between the Splitter and CODEC are positioned Low-pass Filters, which prevent aliasing distortion during analog signal sampling. These anti-aliasing filters remove frequency components exceeding half the sampling frequency, in accordance with the Nyquist theorem.
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. This PCM trunk is a standard digital highway (typically 2.048 Mb/s for E1 systems), carrying 32 channels of 64 kb/s each, enabling multiplexing of multiple telephone conversations on a single link.
SLIC 8 Control Bus
At the bottom of the schematic is SLIC 8 - a control bus that connects multiple SLIC units (typically 8 or 16 per card). This bus carries control and synchronization signals between individual subscriber ports and the central control system. The "Testing" and "Ringing" connections from the control unit show how the exchange can selectively control functions of each subscriber port.
Key Significance of the Schematic
This diagram represents not just the theoretical BORSCHT functions, but the actual hardware implementation used in telephone exchanges. It shows how the raw analog signal from a home telephone is systematically processed, protected, and converted into a precise digital data stream, ready for switching in the modern switching fabric of the exchange.