Telex: The Global Teleprinter Network

Before internet-based email and long before fax machines, telex was the world’s primary method of sending written messages across international distances. At its peak it connected over a million subscribers in more than two hundred countries — banks, news agencies, shipping companies, governments, and hotels — all exchanging typed text in real time over a dedicated switched network. Like RTTY, it ran on ITA2 (Murray Code) and electromechanical teleprinters. Unlike RTTY, it was a public dial-up network, complete with subscriber numbers, answerback codes, and legal standing as a business document.

Telex systems originated in the United Kingdom and several other European countries during the early 1930s. The underlying technology was the same five-bit ITA2 (Murray Code) used in press RTTY, running at 50 baud — slow enough to be highly reliable over long-distance circuits that were still quite noisy by modern standards.

In the United States, development took a parallel but distinct path. In 1931 AT&T introduced its Teletypewriter Exchange Service, TWX. TWX used machines built by the Teletype Corporation — an AT&T subsidiary — including the heavy-duty Model 15 and the later Models 32, 33, and 35. These were the same machines familiar to American RTTY operators. In 1962 Western Union established its Telex system in the United States, and eight years later acquired TWX from AT&T.

The two American systems were not directly compatible. Telex and TWX could not communicate with one another because the keyboard coding schemes and transmission speeds differed; the amalgamated systems were connected by processing computers that translated between the two codes during transmission. TWX had evolved toward ASCII and higher speeds; Telex remained on ITA2 at 50 baud, consistent with the international standard.

European telex equipment came primarily from manufacturers such as Siemens, Lorenz, and Creed. The machines looked and behaved similarly to American teleprinters but were built to European electrical standards and wiring conventions.

Many surplus telex machines of earlier model generations found their way into amateur hands, forming the foundation of the RTTY scene in Europe. Because European commercial telex ran at 50 baud, and because most European machines used governed motors rather than synchronous motors, it was straightforward for amateurs to reduce speed from 50 baud to 45 baud — the North American amateur RTTY standard — by adjusting the governor. This gave European amateurs an easy entry point into RTTY using equipment that was plentiful and inexpensive as the commercial networks modernized.

Telex is a network of teleprinters connected by a system of switched exchanges. Subscribers exchange communications directly with one another; a connection is opened by entering the assigned call number of the destination subscriber. The destination responds with a code verifying its identity, and the communication line is opened.

That identity code — the answerback — was central to telex’s character. Each subscriber had a unique code combining a subscriber number, an abbreviated name, and a country code. For example, the answerback 39286 drwal a identifies subscriber number 39286, abbreviated name DRWAL, registered in Austria (country code a). When a connection was established, both machines exchanged answerbacks automatically, confirming that the right parties were connected and creating a record of the transaction.

This made telex legally authoritative in a way that fax and internet-based email never achieved: both sender and recipient were positively identified, and the exchange was on the record. Telex messages carried legal standing in many jurisdictions, and maritime law required ships to carry telex capability well into the modern era.

The physical interface was simple: a 40 mA current loop, two wires, mark and space — the same interface used by RTTY machines on landlines. Strip the network infrastructure away and a telex machine is, electrically, an RTTY machine, which is why so many of the same devices served both roles.

Beginning in the 1980s, the ability to conduct high-speed digital communication — particularly fax transmission — over dial-up telephone lines led to a decline in the use of telex. Western Union sold its Telex network to AT&T in 1990 before declaring bankruptcy in 1993. In 2008 both AT&T and British Telecom announced that they would no longer directly provide telex service. In Germany, the public telex network closed on 31 December 2007.

A hobby telex network existed as early as 2001 — a modem-based system called “telexphone,” with roughly twenty subscribers. The technology that transformed it into what i-Telex is today began to take shape in July 2007, when Henning Treumann visited a railway museum in Braunschweig and met Fred Sonnenrein.

Fred was a Diplom-Ingenieur in electrical engineering whose professional specialty was railway signalling technology. In his workshop, however, he had built something remarkable: a self-built TW39 switching system, already connected to teleprinters and fitted with connectors, designed for local operation — no modem, no Ethernet, but a working demonstration that the full telex switching experience could be recreated from scratch. What it lacked was a network.

Over the years that followed, Fred designed the components that made the network possible. In 2008 he designed the modem card for the redesigned system, then called Telexphone2. By the end of 2011, after intensive planning with Henning, Fred completed an Ethernet card. In early 2012 Henning assembled the first complete systems and coined the name: i-Telex. The hardware is open source, built on Eurocard PCBs, and configured through a web interface. An Ethernet connection links the teleprinter to the i-Telex network over ordinary broadband.

Henning later described Fred’s contribution: “All ideas were implemented promptly and purposefully by Fred. Fred has been immersed in i-Telex — for him it was a huge hobby project, always with the premise that TW39 and ED1000 are realized as authentically as possible with the i-Telex system. Fred has changed all our lives with his newly created system.”

Fred’s designs live on in several hundred i-Telex systems worldwide. The network has subscribers in Germany, Finland, Australia, Canada, the United States, Italy, Sweden, Switzerland, and the Netherlands, and the number continues to grow — today larger than many country-level commercial telex networks were at their peak. Telecommunications museums use i-Telex to give visitors the experience of exchanging live messages with a machine somewhere in the world, the same experience that made telex indispensable to business for half a century.

For technical details, subscriber information, and hardware sources, visit the i-Telex project at www.i-telex.net. The site is in German, but modern browsers translate it to English automatically and do an excellent job of it.