telex

Siemens T-100, typical Telex terminal

Historical telex from the Gestapo

Telex (for TEL ePrinter EX change ) also telex called, is a system for transmitting text messages over a with the phone network equivalent, mediating telecommunications network , the telex network in which Telegraph be used. It was an important means of communication for companies and authorities in the 20th century. With the advent of the forms of communication fax and e-mail , it lost its importance and is now only used in a few niche areas.

Telex systems that are still in use today have performance features such as speed dialing , direct calling , circulars , attribution of fees, subscriber operating class, etc. There was a corresponding subscriber directory analogous to the telephone book .

Technical standard

The 32-character (5-bit) Baudot-Murray code CCITT-2 is used as the character set, although some characters are (may) not be used.
For a long time, the transmission speed in Europe was 50 baud (= 50 steps per second). Since every character transmitted consists of a start step, 5 character steps and a 1.5-fold stop step, i.e. 7.5 steps, a maximum of 6.67 characters per second can be transmitted. Later teleprinters had interchangeable gears and could be converted to 75 and 100 volumes.
Only half-duplex operation is possible (alternating sending / receiving).
Each individual connection of a subscriber in the telex network has its own telex number and can be dialed via this.
A teletype operator on the telex network must always be available. Local operation is only allowed if it can be interrupted due to an incoming call. Thanks to this regulation, the telex network has long retained its great importance despite its technological backwardness: messages can be delivered securely.
Every teleprinter must have an identifier that can be accessed remotely, the content of which (telex number as well as an abbreviation of name and a country code, etc.) is fixed by the network operator and may not be changed without authorization. By calling up the remote identifier, the sender of a message can reliably determine that he is connected to the correct remote station. By calling up the identifier again after the end of a transmission, it can also be ensured that the message has been completely transmitted. The recipient data then appears on the sender's document above and below the actual text.

Ribbon, red and black, identifier

Many teleprinters could print the text sent out in red and the text received in black. The keypad transmitter normally controls the two-color device. With attached or independent punched tape transmitters, an additional switching magnet takes over this control function.

Subscriber connection

A teleprinter in the original sense only consists of a transmitter and a receiver, which are driven by a common motor. In order to connect the telex to the telex network, a remote switching device is used which takes over the call signaling, switches the machine on and off and, if necessary, physically converts the signals between the telex and the telex network.

There are various approaches for connecting the subscriber to the exchange, which usually require a two-wire line, but in some cases also a four-wire line to the subscriber.

The classic connection between telex and remote switching device is the 40 mA current loop . The remote switching device was often built directly into later teleprinters. In the area of electronic machines, other connections are used at this point. The logical distinction between remote switchgear and teleprinter was largely retained.

Hand placement (HV)

In hand-switched telex networks, the subscriber is connected via a two-wire line.

When the line is idle, the exchange feeds the connection with negative polarity, which is also referred to as "sweeping position". Only a very small current of 5 mA flows through a resistor in the remote switching device.

When the subscriber wants to establish a connection, he presses the call button on the remote switching device, which then connects the teletype to the line. This increases the loop current to 40 mA.

The exchange now reverses the polarity of the line. This is detected by a polarized relay built into the remote switching device. The relay now switches on the motor of the teleprinter.

The connection is established. The participant can now communicate directly with the remote station.

To terminate the connection, the subscriber presses the end button on the remote switching device. This disconnects the line. The exchange notices this and clears the connection. The connection is switched back to the sweeping position, which also switches off the telex machines of the participants and disconnects their current loops from the line. The small sweeping calm current flows again.

In the case of an incoming connection, the exchange reverses the polarity of the line. This will turn on the teletype and connect it to the line.

TW39

The automatic system TW39 originally used in Germany (TW stands for "subscriber selection") is technically largely compatible with the manual system. TW39-capable remote switching devices are also fully-fledged HV remote switching devices. They also have a rotary dial and, in some cases, a recognition of the dial request from the exchange.

The connection establishment in the TW39 system proceeds as follows:

The subscriber presses the call button on the remote switching device, which then connects the telex to the line. This increases the loop current to 40 mA.
The exchange does not yet reverse the polarity of the line, but instead signals that it is ready to dial by interrupting the line for 25 ms.
Now the target number is dialed on the dial in the loop (with the pulse dialing method as with the telephone).
When the connection is established, the exchange reverses the polarity of the line. The rest of the process is identical to that for manual switching.
In the event of errors in connection establishment (e.g. if the destination is busy), the switching center only reverses the polarity of the line for a brief moment and then returns to the calm position. This causes the telex to start up briefly. The remote switching device is then back to its original state.

The procedure described corresponds to "Signaling B" on two-wire lines with single current.

There was also a connection with double current via four-wire lines. However, this procedure was very rare in practical use by the participant. "Signaling A" was used here. The remote switching device had to switch between the single current of the teleprinter and the double current of the connection. Two current loops are used in the double-current connection: one fed in the exchange for transmission from there to the subscriber and one fed in the remote switching device for transmission to the exchange. Both loops are not interrupted. The coding of the data and the call signaling is done by changing the line polarity. A distinction is made between separating layer (logical 1) and character layer (logical 0).

In the idle state, both loops are in character position.
When the call button is pressed, the remote switching device switches its loop to the character position.
The exchange signals that it is ready to dial
It is selected by impulses in the character position.
When the connection is established, the exchange reverses its loop to the idle position, and the telex is switched on. This also applies to incoming connections.
To terminate the connection, the outgoing loop is switched back to the character position permanently when the close button is pressed.
The exchange then switches its loop back to character position.

ED1000

Connection device AGT10

The system developed by the manufacturer Siemens was introduced in the early 1970s. A two-wire line is used to connect the subscriber to the exchange; a dedicated line to the exchange is also possible. The information is transmitted frequency shift- modulated. From a logical point of view, this corresponds to a TW39 remote connection with one line for each direction. Therefore signaling A is used as with TW39, but without dialing a number switch. The dialing is carried out again as with the manual exchange via the teleprinter, but automatically. To do this, the choice must be concluded with a plus sign. Since the teletype is already switched on during the dialing, the exchange automatically asks the called party for the identifier. By receiving the identifier of the called party, the caller knows that he is connected. In the event of errors, the ED1000 switch is able to output the cause of the error in short messages before the connection is disconnected.

Electronic teleprinters usually have an ED1000 remote control unit installed as standard equipment. However, it is also possible to operate TW39 end devices on an ED1000 line using an adapter called "AGT".

Connection via modem

In some countries teleprinters were connected to normal telephone lines using audio frequency modems . In some cases, dedicated lines independent of the telephone network were also used, for example when the connection was made via a transformer and therefore the use of a power interface was not possible. Most modem connections were only used for the last mile . In the USA, however, the TWX system was based entirely on the telephone network, with its own network, as in other countries, only logically available.

i-Telex

Now that almost all public telex networks have been switched off, collectors of historical devices brought the TelexPhone network to life. A direct connection between two conventional telex devices can be established here via the public telephone network.

The "i-Telex" expansion card was developed in order to create a telex option even without an analog telephone connection, since modem connections with sufficient quality are not possible via VoIP. The project is now being continued exclusively under the name i-Telex, the telexphone network is no longer being expanded, and almost all telexphone connections have migrated to i-Telex or have been switched off.

While TelexPhone used an analog telephone connection, i-Telex connections are established via TCP / IP with the aid of a central number server (the actual connection between the participants then takes place directly). The dialing process and the subsequent use of the connection correspond to the TW39 standard. However, there are also ED1000 interfaces, a keyboard selection can also be set for TW39 cards, which then corresponds to the technical status of an ED1000 interface with an AGT10 connection device for older teleprinters. The i-Telex network is also used by some museums . There is also a gateway from i-Telex to TelexPhone connections (and vice versa) without a TCP / IP connection. B. was also possible from Teletex to Telex. Other special services and features, such as the so-called Minitelex (an option to receive telex via i-Telex with the help of a fax machine) and circular connections are being developed based on the original services of the Deutsche Bundespost / Telekom. The i-Telex network is now in use in over 20 countries worldwide and currently has a total of 147 subscribers, 110 of them in the Federal Republic of Germany (as of January 5, 2020), and the trend is rising.

history

Telex networks have been established around the world since the 1930s, but have lost their importance as a communication medium since the 1980s and have increasingly been replaced by more modern media such as fax and later e-mail . The main reasons for the decline of the telex system were:

the low transfer rate, adapted to the speed of mechanical printers at the time of its creation
the need for a separate network infrastructure (telex network and telephone network were always set up completely separately due to the different technical requirements)
the lower flexibility and capacity compared to electronic remote data transmission via modem , DSL or LAN at significantly higher costs per data volume
the problem-free transmission of drawings and texts in non-Latin alphabets by fax machines. For the latter reason, the fax was immediately well received, for example in Japan.

Still applies telex to be very safe and is known as certificate recognized. Many of the remaining users come from the banking and financial sector and have not yet been able to convert due to security concerns (both fax and e-mail, on the other hand, are best-effort techniques).

In large companies, embassies, at airports, with railway operators, shipping companies and the like, teleprinters are still used to exchange messages with many small countries and those where the corresponding technical infrastructure does not exist. In air traffic control , telex is the standard when it comes to the transmission of flight operations information between air traffic control units. The military in many countries also continues to operate telex networks. These are (due to their hierarchical star topology ) more susceptible to the destruction of nodes than z. B. the Internet, but the comparatively simple technology offers a higher level of security against EM pulses or hacker attacks .

Telex in Germany

Telex machine from Siemens

The first telex service was introduced in the German Reich in 1933 after a trial operation had taken place since 1926. In the Federal Republic of Germany, the telex network was further expanded by the Deutsche Bundespost . In 1978 there were a total of 123,298 telex connections in West Germany and West Berlin . At the end of the 1990s, there were only a few thousand connections on the telex network operated by Deutsche Telekom . Telex devices were still widespread, especially in newsrooms. On December 31, 2007, Deutsche Telekom finally ended its service. Telex connections in Germany can still be obtained from the provider Swisstelex . There are currently 35 telex connections in the Swisstelex public network (as of July 2016) and 147 (110 in Germany) subscribers (as of January 2019) in the I-Telex network.

Telex in Switzerland

In its heyday (1987), today's Swisscom telex network had around 40,000 users. In March 2006 this area was spun off into SwissTelex AG. This had 35 customers in Germany in 2016 and 22 customers in Switzerland, while operations in Austria had already ceased in March 2006 and had not resumed until 2016.

The Telephone Museum in Islikon has been connected to the I-Telex network since 2016. In total, there are two operators of a classic teleprinter in Switzerland.

Telex in Austria

In Austria, the telex service was discontinued at the end of the third quarter of 2006. In 2002 "Legacy Communications Luze GmbH" took over the service from Telekom Austria and made it available to a closed user group. However, there has been a telex subscriber again in the I-Telex network since 2017.

literature

Klaus R. Fellbaum: Electronic text communication. Technology, commitment, experience . Teletext, cable text, fax, telex, BTX. VDE, Berlin / Offenbach 1983, ISBN 3-8007-1260-1 .
Hans Mauruschat, Rolf Süring: Telex service . Published by the Ministry for Post and Telecommunications of the German Democratic Republic, Deutsche Post, Central Office for Vocational Training, Brandenburg 1985 (= teaching material for training and further education at Deutsche Post ).
The hour strikes the ticker. The last of the teletypes serve in the financial industry . In: NZZ , January 27, 2006