Telex

Telex ( teleprinter for EXchange ) is a system for the transmission of text messages over a telephone network with the similar, switching telecommunications network, the telex network, used in the Telegraph. It was an important means of communication for business and government in the 20th century. With the emergence of forms of communication fax and e- mail it rapidly lost importance and is now used only in a few niche areas.

Today used telex systems have features such as speed dial, DSS, newsletters, attributing fees, class of service, etc.

Technical standard

  • As a character set of the CCITT Code No. 2 ( " Baudot code " colloquially ) used, some characters may be used ( may ).
  • The transfer speed was in Europe for a long time 50 Baud ( = 50 steps per second). Since each transmitted character from start step 5 characters steps and 1.5 times the stopping step, ie from 7.5 steps is given, a maximum of 6.67 characters per second can be transmitted. Later Telegraph had repluggable gears and so could also be converted to 75 and 100 Bd Bd.
  • It is only half-duplex operation is possible.
  • Each individual connection of a participant in the telex network has its own Telex number and can be dialed on this.
  • A Telegraph on telex network must be reachable. Local operation is only permitted if it can be interrupted due to an incoming call. Through this provision, the Telex has retained an important role, despite technological backwardness far: Messages can be delivered safely.
  • Each teletype must have a remote inquiry identifier encoder whose content ( telex number and a name abbreviation and a country code or the like. ) Is fixed from the network operator and must not be modified by the operator. Thus, the sender can find a message by retrieving the remote ID encoder reliable to be connected to the correct remote station. The repeated call of the identification generator after the end of a transmission can also be assured that the message has been completely transmitted. Appear on the document above and below the sender of the actual text in red is the recipient data. Misdirection of important data, such as allegedly occur today when sending and were practically excluded the telex.

Teilnehmeranschaltung

A telegraph in the original sense only consists of a transmitter and a receiver, which are driven by a common motor. To bind the Telegraph to the Telex network, a remote control device is used, which takes over the call signaling, the machine on and off and if appropriate signals between the telegraph and telex network physically.

For connecting the subscriber to the central office, various approaches exist, which, in some cases, however, require usually a two-wire line and a four-wire cable to the party.

The classical connection between Telegraph and remote switching device is the 40 mA current loop. In later Telegraph the remote switching unit is installed directly often. In the area of ​​electronic machinery other connections are used at this point. The logical distinction between remote switching device and Telegraph was largely maintained.

Hand placement ( HV)

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

  • In the idle state of the line the switch feeds the terminal with negative polarity, one also speaks of " sweeping rest position ". It will only be a very small current of 5 mA through a resistor in the remote control unit.
  • If the participant wishes to establish a connection, it presses the call button on the remote control unit, which it joins the Telegraph to the line. Thus, the loop current increases to 40 mA.
  • The exchange now swap them the lead. This is detected by a built- in remote switching device polarized relay. The relay now switches to the motor Telegraph.
  • The connection is established. The participant can now with the other party, typically the agency staff to communicate directly by telex.
  • To termination of the call of the subscriber presses the end key on the remote control unit. Wherein the line is disconnected. The exchange notices this and terminates the connection. Here, the connection is switched back to the downside rest position, which is also the Telegraph of participants shuts down again and her current loop separates from the line. It flows to the low quiescent current return again.
  • For an incoming connection, the switch reverses the polarity of the line. Thus, the Telegraph is turned on and connected to the line.

TW39

The originally used in Germany automatic system TW39 (TW stands for " subscriber selection " ) is technically compatible with the largely hand- mediated system. TW39 -enabled remote switching devices are simultaneously full HV Remote Switching. They also have a dial, and some have a recognition of the election prompted the exchange.

The connection in TW39 system is as follows:

  • The participant presses the call button on the remote control unit, which it joins the Telegraph to the line. Thus, the loop current increases to 40 mA.
  • The switch reverses the polarity of the line not to, but signaled the Wählbereitschaft a line interruption of 20 ms duration.
  • Now, the target number is dialed on the dial connected in the loop (exactly as in the phone).
  • When the connection is established, the switch reverses the polarity of the line. The further procedure is identical to that in the manual switchboard.
  • Errors in the connection setup ( eg if the destination is busy ) polarizes the exchange the line only for a brief moment and then back to the downside in rest position. Thus, the Telegraph after a short run. Subsequently, the remote control device is again in its initial state.

The method described corresponds to the " B signaling " on two-wire lines with single stream.

However, there was also a connection with double power via four-wire lines. However, this method was in practical use at the subscriber's very rare. Here was the " signaling A" used. The remote control device had to implement it between single power Telegraph and dual stream connection.

With the dual power supply two current loops are used: one in the central office fed to transfer from there to the participants and a long-distance switching device powered for transmission in the central office. In this case, both loops are not interrupted. The encoding of the data and the call signaling is done by changing the line polarity. Here, by separating layer (logic 1 ) and character position is (logic 0) distinguished.

  • At rest, both loops are in character position.
  • When you press the call button the remote switching unit switches its loop into character position.
  • The switch signals Wählbereitschaft
  • It is chosen by pulses in character position
  • When trying to connect the switch reverses the polarity of its loop around to the rest position, the Telegraph is turned on. This also applies to incoming connections.
  • To disconnect the continuous loop is switched to character position permanently again when the final key.
  • Then, the switch also turns her bow to character position.

ED1000

The system developed by Siemens system was introduced in the early 1970s. To connect the subscriber to the exchange a two- wire cable is used, a telephone leased line to the exchange is possible. The information is transmitted FSK modulated. From a logical point of view this corresponds to a TW39 remote connection with one line for each direction. Therefore, the signaling A as in TW39 is used, but without number switch choice. The selection will be carried out as with the hand placement on the Telegraph, however, automatically. To this end, the choice must be terminated with a plus sign. As the Telegraph is already switched on during the election itself, the switch automatically queries the callee from the identification generator. So does the caller by receiving the identifier of the callee that it is connected. In case of errors, the ED1000 - mediation able to output the error cause in short messages before the connection is dropped.

Electronic Telegraph usually have a built - ED1000 remote switching unit as standard. But it is also possible, by means of said " AGT " adapter to operate in a terminal - TW39 ED1000 line.

History

Telex networks have been built worldwide since the 1930s, but lose since the 1980s as a communication medium and their significance were increasingly replaced by more modern media such as fax and e- mail later.

Main reasons for the general decline of the telex system were:

  • The low transmission rate (see below), adjusted to the mechanical speed printer at the time when
  • The need for a separate network infrastructure (eg, a separate phone line for a telex machine )
  • The comparatively high flexibility and capacity of electronic data transmission via modem, DSL, or LAN at a much lower cost per amount of data

Many of the remaining users are from the banking and financial industry and have for security reasons ( both fax and e- mail the other hand, best-effort techniques are ) not been able to adapt. However, this application will disappear with advances in encryption and certification of digital communication.

In large companies, embassies, airports, railway operators and the like Telegraph still be used during message exchange with many small and technologically less developed countries, since telex connections work the most reliably. In air traffic control, the telex is still standard when it comes to the transfer flight operational information between air traffic control centers.

The military in many countries continues to operate telex networks. Although these (due to their hierarchical star topology ) more susceptible to the destruction of nodes as eg the Internet, however, has the comparatively simple technique a higher security against EM pulses or hacker attacks.

Telex in Germany

In the German Empire, the first telegraph service was introduced in 1933, after a trial operation since 1926 had occurred. In the Federal Republic of Germany Telex network was expanded by the German Federal Post Office further and had a speed of 50 baud - around 6.67 characters per second. In 1978 there were in West Germany and West Berlin in total 123 298 Telex terminals. In the late 1990s there were only a few thousand connections to the operated by Deutsche Telekom telex network. Especially in newsrooms telex machines were still widespread. On 31 December 2007 finally ended the German Telekom the service. However, telex terminals in Germany continue to move in by the company Swiss telex.

Telex in Switzerland

In Switzerland are still about 300 business customers of Swisscom, which spun off this area of the company Swiss telex, on the telex. This figure stood in the heyday (1987 ) compared with about 40,000 users.

Telex in Austria

In Austria, the Telex service was discontinued at the end of the third quarter 2006. 2002, "Legacy Communications GmbH Luze " the service acquired by Telekom Austria and found a closed user group.

ITU-T Baudot-Code Deutsche Bundespost Teletex
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