Why are low-speed modems (9600 bps) still used in railways?

Low-speed modems (600-9600 bps) are still used for critical legacy infrastructure like SCADA (power control) and signaling telemetry because they can operate reliably over long-distance analog copper lines where modern fiber optics or 4G/5G coverage may not exist.

What is the difference between 2-wire and 4-wire transmission in UIC 911-4?

A 2-wire circuit typically supports Half-Duplex communication (data flows one way at a time), while a 4-wire circuit allows Full-Duplex communication (simultaneous transmit and receive), offering higher reliability for critical data links.

What interface standard does UIC 911-4 use?

UIC 911-4 relies on the CCITT V.24/V.28 interface standards (functionally similar to RS-232) to connect the modem to the data terminal equipment (DTE).

UIC 911-4: Legacy Data Transmission & Modem Standards (600 – 9600 bps)

UIC 911-4 (Chapter 9) specifies the standards for legacy data links using analog modems with binary rates between 600 and 9600 bits/s. This guide covers the CCITT-compliant interface requirements (V.24/V.28), the distinction between 2-wire and 4-wire copper circuits, and the signal attenuation limits required to maintain reliable telemetry for remote railway infrastructure.

October 14, 2023 4:22 pm

UIC 911-4 Chapter 9 defines the technical standards for low-speed data transmission over analog lines. While modern networks run on Gigabits, critical railway infrastructure (SCADA systems, remote switch monitoring, and older signaling telemetry) often relies on robust, low-bandwidth copper links running between 600 and 9600 bits per second (bps).

This leaflet specifies the interface requirements for Modems and the quality parameters for Transmission Lines to ensure data integrity over long distances where digital fiber is unavailable.

1. Modem Interfaces and Standards

UIC 911-4 aligns railway telecommunications with the historic CCITT (now ITU-T) recommendations. It standardizes the physical and electrical characteristics of the connection to ensure equipment from different manufacturers can talk to each other:

Interface Type: Primarily based on the V.24 / V.28 standard (equivalent to RS-232), defining voltage levels for “0” and “1”.
Modulation: Specifies Frequency Shift Keying (FSK) for lower speeds (V.23) and Phase Shift Keying (PSK) for higher speeds (V.29) to resist line noise.
Synchronization: Distinguishes between Asynchronous transmission (start/stop bits) for simple telemetry and Synchronous transmission for continuous data streams.

2. Transmission Line Configurations

The standard dictates how the copper wires are utilized. The choice between 2-wire and 4-wire circuits affects the duplex capability (talking and listening at the same time).

Configuration	Duplex Mode	Typical Application
2-Wire Circuit	Half-Duplex	Simple “Request/Response” telemetry. Data can flow only one way at a time. Lower cost.
4-Wire Circuit	Full-Duplex	Continuous monitoring. Dedicated pairs for Transmit (TX) and Receive (RX). Higher reliability.
Point-to-Point	Dedicated Link	Direct connection between a Control Center and a single remote substation.
Multipoint	Polled Network	One master modem polling multiple slave modems along a railway line.

3. Line Quality Requirements

For a 9600 bps signal to travel kilometers without corruption, the physical line must meet strict quality metrics defined in Chapter 9:

Attenuation: The signal loss (measured in Decibels, dB) must not exceed specific limits at the carrier frequency (typically 1000 Hz or 1700 Hz).
Group Delay Distortion: Different frequencies travel at slightly different speeds in copper. The modem must compensate for this to prevent “Symbol Interference.”
Signal-to-Noise Ratio (SNR): The signal must be sufficiently stronger than the background static (crosstalk from other cables).

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