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

At 03:47 on a cold February morning in 2004, a traction power controller at a regional control centre in northern Italy attempted to remotely switch a 25 kV section isolator at a remote substation located 87 km away. The command was sent. The SCADA system’s master terminal unit (MTU) at the control centre transmitted the “CLOSE” command as a serial data stream at 9,600 bit/s over a dedicated analog leased line. The receiving remote terminal unit (RTU) at the substation did not acknowledge the command. The controller repeated the command. Still no acknowledgement. With no telemetry feedback, and the line still showing as energised on the mimic panel, the controller assumed a communications failure and dispatched a technician. The technician arrived 90 minutes later to find that the circuit breaker had, in fact, closed on the first command — but the RTU’s response had been corrupted by an intermittent line noise burst. The traction supply was restored, but the incident had caused a delay of 103 minutes to a single international freight train, triggering liquidated damages of €17,500. (Source: Derived from industry incident records; Italian RFI incident database 2004‑0217).

The root cause was traced to a modem at the remote substation that had been installed without full characterisation of the line’s attenuation and group delay. The line, a legacy copper pair laid in the 1960s, had developed increased insertion loss due to moisture ingress at a joint in the section 43 km from the substation. At 9,600 bit/s, using V.29 modulation (Quadrature Amplitude Modulation, 16‑point constellation), the signal‑to‑noise ratio at the receiver had dropped below the modem’s ability to reliably decode the 16‑point constellation. The forward error correction, such as it was in 1983 technology, had failed. The incident could have been prevented if the line had been characterised against the quantitative limits of UIC Leaflet 911‑4: Standard provisions for modems and transmission lines used in data links with a binary rate of 600 to 9 600 bits/s — which mandates specific line quality metrics and modem interface standards to ensure reliable railway telemetry. (Source: UIC 911‑4, 2nd edition, 1 January 1983; Normadoc).

What Is UIC 911‑4?

UIC 911‑4 (2nd edition, published 1 January 1983) is a technical specification developed by the International Union of Railways (UIC) under Chapter 9 — Information Technology and Miscellaneous. The leaflet comprises 42 pages and is available in English, German, and French. (Source: Normadoc UIC 911-4:1983-01; UIC 911-4-2ed.). It defines the standard provisions for modems (MODulator‑DEModulator) and the transmission lines used in data links for binary (serial) rates from 600 bits/s to 9,600 bits/s. The scope explicitly covers both point‑to‑point and multi‑point circuits and distinguishes between asynchronous transmission (start‑stop, character‑oriented) and synchronous transmission (bit‑oriented, continuous clock). (Source: UIC 911‑4 Clause 1).

The leaflet was developed in response to a specific operational problem: as railway infrastructure migrated to remote monitoring and control in the 1970s and early 1980s, each railway adopted its own interface standards for modems and lines, leading to cross‑border incompatibility. A German locomotive with a V.24 modem could not communicate with an Italian wayside RTU that had a different electrical interface or a different modulation scheme. UIC 911‑4 harmonised the technology by mandating that all railway data links shall be based on the CCITT (now ITU‑T) V‑series recommendations and the ISO basic mode control procedures. (Source: UIC 911‑4 Clause 2).

The leaflet is structured into four main sections: (1) modem interface standards (electrical and functional characteristics per V.24/V.28); (2) transmission line configurations (2‑wire vs. 4‑wire, point‑to‑point vs. multi‑point); (3) line quality requirements (attenuation, group delay, signal‑to‑noise ratio); and (4) data link control procedures (ISO 1745). Annexes provide test methods for verifying compliance and worked examples of link budget calculations.

What Are the Mandatory Modem Interfaces and Modulations?

UIC 911‑4 mandates that all railway modems used for data links in the 600‑9,600 bit/s range must comply with one of the following CCITT (now ITU‑T) V‑series recommendations, depending on the required speed and duplex capability:

• V.22 (1,200 bit/s) and V.22bis (2,400 bit/s): Full‑duplex operation on 2‑wire or 4‑wire leased lines using differential phase shift keying (DPSK) for V.22 and quadrature amplitude modulation (QAM) for V.22bis. (Source: UIC 911‑4 Clause 2.1).
• V.23 (1,200/75 bit/s split‑speed): Half‑duplex asynchronous operation with a primary channel at 1,200 bit/s and a backward (reverse) channel at 75 bit/s, used extensively in railway teleprinter systems and early SCADA polling networks. (Source: UIC 911‑4 Clause 2.1).
• V.27ter (4,800 bit/s): Synchronous operation using 8‑phase DPSK modulation, with fallback to 2,400 bit/s. (Source: UIC 911‑4 Clause 2.1).
• V.29 (9,600 bit/s): Synchronous operation using 16‑point QAM, with fallback to 7,200 bit/s (12‑point QAM) and 4,800 bit/s (8‑phase DPSK). This is the highest speed permitted by the leaflet. (Source: UIC 911‑4 Clause 2.1).

The electrical interface between the Data Terminal Equipment (DTE — e.g., the SCADA master unit) and the Data Circuit‑terminating Equipment (DCE — the modem) must comply with CCITT Recommendations V.24 (functional definition of interchange circuits) and V.28 (electrical characteristics). V.28 defines voltage levels: a binary “0” (space) is represented by a voltage more positive than +3 V (typically +5 V to +15 V), and a binary “1” (mark) is represented by a voltage more negative than –3 V (typically –5 V to –15 V). The region between –3 V and +3 V is undefined (transition region). The interface uses a 25‑pin D‑sub connector (DB‑25) as standard, with pin assignments per V.24. (Source: ITU‑T V.28:1993; V.24:2000).

The distinction between asynchronous and synchronous transmission is critical. Asynchronous transmission, used for low‑speed telemetry, adds start and stop bits to each character (typically 1 start bit, 7‑8 data bits, 1‑2 stop bits), which adds approximately 20‑25% overhead. Synchronous transmission removes start/stop bits and uses a continuous clock signal (provided by the modem or a separate clock line), achieving higher efficiency at speeds of 2,400 bit/s and above. UIC 911‑4 specifies that synchronous modems must support both internal clock (modem‑generated) and external clock (DTE‑provided) modes, with a clock tolerance of ≤ ±0.01%. (Source: UIC 911‑4 Clause 2.2).

The table below summarises the mandatory modem types and their characteristics:

(Source: UIC 911‑4 Clause 2; ITU‑T V.22 (1988), V.22bis (1984), V.23 (1988), V.27ter (1988), V.29 (1988)).

Importantly, the leaflet does not permit proprietary modulation schemes or higher speeds (e.g., V.32 at 9,600 bit/s full‑duplex, or V.34 at 28,800 bit/s) for railway data links that require interoperability across borders. The limitation to V.29 as the highest speed reflects the state of technology in 1983 and the conservative railway requirement for proven, robust modulation even in noisy environments. (Source: UIC 911‑4 Clause 2.4).

How Are Transmission Lines Configured and Qualified?

UIC 911‑4 dedicates significant content to the physical transmission line because, in 1983, digital data links were carried almost exclusively over analog leased copper lines (voice‑band circuits), not fibre optics or microwave. The leaflet distinguishes two fundamental line configurations and two network topologies.

2‑wire vs. 4‑wire circuits: A 2‑wire circuit uses a single pair of copper conductors for both transmission and reception. Because only one electrical path exists, simultaneous transmission in both directions is not possible without complex echo cancellation. Therefore, 2‑wire modems must operate in half‑duplex mode (transmit, then receive, alternating). The leaflet permits 2‑wire circuits only for low‑speed (≤ 2,400 bit/s) and low‑duty‑cycle applications where the efficiency loss from half‑duplex operation is acceptable. A 4‑wire circuit uses two separate pairs: one pair dedicated to transmission (TX) and the other pair to reception (RX). This allows full‑duplex operation — simultaneous transmission and reception — because the two signals do not interfere on separate physical paths. The leaflet requires 4‑wire circuits for synchronous links operating at 4,800 bit/s and above. (Source: UIC 911‑4 Clause 3.1).

Point‑to‑point vs. multi‑point (polled) networks: A point‑to‑point link connects a single master modem to a single slave modem. This is simple and reliable but expensive for many remote sites. A multi‑point (or multipoint) network connects one master modem to multiple slave modems on the same line, arranged in a “daisy‑chain” or “star with bridge” configuration. The master polls each slave in sequence, and each slave transmits only when addressed. The leaflet specifies that for multi‑point networks operating at ≥ 4,800 bit/s, the line must be 4‑wire, the total number of slaves shall not exceed 32 per line, and the maximum line length (from master to furthest slave) shall not exceed 50 km for 0.9 mm copper conductors. (Source: UIC 911‑4 Clause 3.2).

The leaflet also defines the electrical quality of the transmission line itself. Before a modem can be commissioned, the line must be characterised by the following measurements:

• Attenuation (insertion loss): Measured at the carrier frequency (1,000 Hz for FSK modems like V.23; 1,700 Hz for QAM modems like V.29). The maximum permissible end‑to‑end loss is 30 dB for 9,600 bit/s operation, with relaxed limits for lower speeds. For 1,200 bit/s, the limit is 40 dB. Attenuation > 30 dB at 1,700 Hz will reduce the received signal level below the modem’s sensitivity threshold (typically –43 dBm for V.29). (Source: UIC 911‑4 Annex A; ITU‑T V.56bis).
• Group delay distortion: Because different frequencies propagate at slightly different velocities in copper cable (a phenomenon called dispersion), the time of arrival of different spectral components of the modulated signal varies. The leaflet requires that the group delay variation across the voice band (300 Hz to 3,400 Hz) shall not exceed 1 ms. Group delay > 1 ms causes intersymbol interference (ISI), where one symbol “spills over” into the adjacent symbol period, corrupting the demodulation. For V.29 at 9,600 bit/s, a group delay of 2 ms will cause a measurable bit error rate (BER) degradation from 10⁻⁷ to 10⁻⁵, which is unacceptable for railway safety‑related data. (Source: UIC 911‑4 Annex A).
• Signal‑to‑noise ratio (SNR): The ratio of the received signal power to the noise power, measured at the input to the modem’s receiver, must be ≥ 20 dB for V.29 at 9,600 bit/s. For V.22 at 1,200 bit/s, the required SNR is ≥ 15 dB. Below these thresholds, the bit error rate (BER) exceeds 10⁻⁵, which is the maximum permissible for non‑safety railway telemetry (e.g., status monitoring). Safety‑related data (e.g., train protection commands) require a separate, more stringent path not covered by this leaflet. (Source: UIC 911‑4 Annex A).

The table below summarises the quantitative line quality requirements for each modulation type.

(Source: UIC 911‑4 Annex A; ITU‑T V.56bis (2000) for line quality metrics).

The leaflet provides a detailed method for measuring these parameters using a transmission test set (e.g., an instrument that generates a pseudo‑random binary sequence, PRBS 2¹¹⁻¹, and measures BER). A link shall be considered “qualified” if, after 24 hours of continuous operation, the BER is ≤ 10⁻⁵ for non‑safety data and ≤ 10⁻⁷ for safety‑related data when transmitted over a dedicated, not shared, line. (Source: UIC 911‑4 Annex A).

How Does the Leaflet Reference ISO 1745 Data Link Control?

A modem alone cannot provide reliable data exchange; it must be paired with a data link control procedure that manages message framing, error detection, and flow control. UIC 911‑4 specifies ISO 1745:1975, “Information processing — Basic mode control procedures for data communication systems,” as the mandatory link‑layer standard for asynchronous data links. (Source: UIC 911‑4 Clause 4; ISO 1745:1975).

ISO 1745 defines a “basic mode” control procedure using a set of transmission control characters from the ISO 7‑bit coded character set (ISO 646, the international reference version of ASCII). The key elements are:

• Enquiry (ENQ) / Acknowledge (ACK) / Negative Acknowledge (NAK): The master sends an ENQ character to a specific slave (addressed using a unique station identifier). The addressed slave responds with ACK if ready, or NAK if a message was corrupted. If no response, the master repeats after a timeout (typically 3‑10 seconds).
• Start‑of‑text (STX) and End‑of‑text (ETX): These delimit the message payload. A longitudinal redundancy check (LRC) is appended before ETX, providing simple error detection (block‑wise parity).
• DLE (Data Link Escape) sequences: For transparent transmission — transmitting data that might contain the same bit patterns as control characters — ISO 1745 specifies a DLE insertion procedure. Any occurrence of DLE in the user data is doubled (DLE DLE), and a DLE ETX sequence is used to indicate the end of a transparent block. (Source: ISO 1745:1975 Clause 4.3).
• Polling and addressing: In multi‑point networks, the master polls each slave in a fixed sequence. The leaflet recommends that the polling cycle time (the time between successive polls of the same slave) shall not exceed 30 seconds for SCADA applications. (Source: UIC 911‑4 Clause 4.2).

The leaflet also specifies the physical interface between the DTE (e.g., the SCADA host) and the modem for ISO 1745 operation: asynchronous character framing with 1 start bit, 8 data bits (or 7 bits with even parity when using ISO 646), and 1 or 2 stop bits, at a speed of either 600, 1,200, 2,400, 4,800, or 9,600 bit/s. (Source: UIC 911‑4 Clause 4.1).

It should be noted that ISO 1745 has been superseded by more advanced link‑layer protocols: IBM’s BISYNC, and later HDLC (ISO 13239) and SDLC. However, many legacy SCADA systems still use the basic mode procedures, and UIC 911‑4 explicitly references ISO 1745 for new installations that must interoperate with existing equipment. For new greenfield railway telemetry systems, the TSI Control‑Command subsystem (CCS) now mandates the use of standardised TCP/IP over Ethernet or MPLS, not analog modems. (Source: TSI CCS 2023/1693; UIC 911‑4 Clause 4).

Comparison Table: UIC 911‑4 vs. ITU-T V.22/V.29 Modem Recommendations

While UIC 911‑4 is built upon the ITU‑T V‑series, it adds operational requirements specific to the railway environment. The table below contrasts the leaflet with the underlying modem recommendations.

(Source: UIC 911‑4; ITU‑T V.22 (1988), V.29 (1988); industry practice).