Subscriber Line Interfaces

Subscriber line interfaces and network termination

Subscriber line interfaces form the boundary between subscriber equipment and the access network. The exchange-side interface applies the electrical and signalling requirements of the local loop, while the terminal provides the complementary line termination at the customer premises.

Analogue POTS uses the two-wire Z interface. Digital services such as ISDN use their specified digital interface and termination, so their electrical requirements must not be conflated with those of POTS.

The exchange-side SLIC

The exchange-side termination is the SLIC (Subscriber Line Interface Circuit), called AZL (Abonencki Zespół Liniowy)in Polish terminology. It terminates the two-wire analogue subscriber loop and connects it to the exchange's internal four-wire or digital path. ITU-T Q.551 assigns the analogue line functions to the Z interface.

BORSCHT functions of a SLIC or AZL

• B - Battery feed: Supplies DC power to the subscriber line. A nominal exchange feed near 48 V is common, but the voltage and current limits follow the national requirements and the line-card design. The feed polarity is selected by the network equipment.
• O - Overvoltage protection: Limits hazardous voltages reaching the line card, including lightning-induced surges and accidental contact with power circuits. The protection design is coordinated with the applicable terminal and network safety requirements.
• R - Ringing: Applies the AC ringing signal that alerts the called party. ETSI EN 300 001 specifies a terminal detection profile of 32 to 75 V AC at 23 to 28 Hz with 0 to 85 V DC superimposed. National implementations include 25 Hz and 50 Hz variants, and the SLIC controls the timing.
• S - Supervision:Monitors the DC state of the loop to detect events such as going "off-hook" or "on-hook". An off-hook terminal closes the loop and draws DC current, while an on-hook terminal leaves the speech path open.
• C - Coding: The coding function uses a codec (coder-decoder) that converts speech between the analogue loop and internal PCM (Pulse Code Modulation). ITU-T G.711 specifies 8,000 samples per second and 8 bits per sample, with A-law and μ-law companding variants.
• H - Hybrid: The hybrid circuit converts the bidirectional two-wire subscriber loop into separate transmit and receive paths inside the exchange. Its balance and return loss affect echo and sidetone, and an anti-local function may be integrated with it.
• T - Testing: Supports measurements of loop resistance, capacitance, and insulation so that faults can be detected and localized. The line card can connect the loop to dedicated test equipment.

The subscriber terminal and analogue telephone

A traditional analogue telephone terminates the two-wire loop and responds to the electrical and signalling conditions provided by the exchange-side interface.

Terminal components and functions

• Hookswitch: Connects or disconnects the telephone from the loop. In the on-hook state (handset down), it forms an open circuit. In the off-hook state (handset lifted), it closes the circuit, drawing DC current and signaling the exchange.
• Ringer and calling unit: Detects the AC ringing signal and produces an audible indication of an incoming call. Modern telephones generally use electronic ringers, while older sets used electromechanical bells.
• Speech circuit and anti-local function: This circuit processes the microphone and receiver signals. It uses an anti-local circuitto control sidetone, the part of the caller's own speech returned to the receiver, by balancing the line and the telephone impedance.
• Dial pad: Converts dialled digits into signalling information.
  • Pulse dialing: Older phones generated a series of make-break contacts (interruptions in the DC loop current) corresponding to the digit.
  • DTMF (Dual Tone Multi-Frequency): The telephone sends one tone from the low group, 697, 770, 852, or 941 Hz, and one from the high group, 1209, 1336, 1477, or 1633 Hz, as defined by ITU-T Q.23.