Type A and Type B signalling

Two further subdivisions of the signalling systems have evolved, corresponding to the different types of national network employed. It has been agreed that, where two countries with different types of signalling system are trying to make connection, the country setting up the call will convert its outgoing signals to that of the country receiving the call. Both types of signals convey the same informa­tion, the difference is in the detail of implementation. In general Type A signalling has been used where the telex keyboard has been used lo send the address signals to the network and Type B signall­ing where the address signals were sent lo the network with dial pulses, much like a telephone. Figure 59.5 shows the sequence of signals in each system.

Answerback

CCITT recommendation F.60 specifies that every telex machine will have a unique answerback code made up of a one or two letter country code, telex line number and abbreviated subscriber's name. Sending a 'who are you' when connected through to the distant end will trigger the distant telex lo transmit the answerback aide em­bedded in the distant telex to the sending telex. This technique enables telex users to confirm that they have reached the correct distant party, before message transmission can begin, so protecting the confidentiality of the message.

Telex protocol

As telex signalling is machine to machine operation and the distant end may be unattended, then a protocol has been established. Several exist hut the ones given in Table 59.1 are generally internation­ally understood. All telex messages should commence with the sender's answerback so that the distant end can easily find the caller. The sequence of setting up a call on the telex network is seizing the line, receiving call confirmation and proceed to select indication from the telex network. The telex terminal then transmits the ad­dress information and if the call set up is successful the distant machine will automatically transmit to the caller the answerback code indicating it is ready to accept the message. If the call is unsuccessful for any reason the failure will be indicated by a service code of up to 3 letters transmitted from the telex network to the originating machine and printed out on the local record of the telex message.

The telex network has facilities for testing the telex lines and can detect if a machine has its power disconnected or the line is out of order, and then send the corresponding service message from the network to the originating machine.

Figure 59.5 Type A and B signalling

 

Telex terminals

Modern telex terminals are highly sophisticated pieces of equip­ment built around the principles of the personal computer. They comprise a microprocessor, disk drive, keyboard, VDU, and heavy duly printer with paper roll holder. These machines are still required to interwork with the older electromechanical machines, or paper tape transmitters or receivers, that exist on the telex networks. The modern telex terminal has in addition the ability to communicate with the telex network word processing software, to enable the telex operator to prepare messages, edit, store and finally transmit the messages. This makes the telex terminal a very powerful communi­cation tool for business use.

 

Table59.l Telex protocol (indicates some of the typical service codes)

1. Learn the words & word combinations:

Telex message	Телексное сообщение
Manual (operator) switchboard	Ручной коммутатор
Handsending	Отправка вручную
To acknowledge	Подтверждать прием, квитировать
Local record	Локальная запись
Duplicate	Снимать копию, копировать
Unattended	Несопровождаемый, без внимания
A character set	Набор знаков, набор символов
Teleprinter	Телетайп, стартстопный телеграфный аппарат
Shift character	Знак (символ) смены регистра
Figures shift	Установка регистра цифр
Letters shift	Установка регистра букв
Code conversion	Преобразование кода, перекодировка
Voce frequency	Тональная частота
Telex exchange	Станция телексной связи
Double current signaling	Телеграфирование на 2-х фазном токе
Single current signaling	Телеграфирование на 1-фазном токе
Full duplex	Дуплексный (двусторонний) режим Дуплексная (одновременная двусторонняя) связь
Half duplex	Полудуплексная (поочередная двусторонняя) связь
Invariably	Неизменный
Feed	Питание, подпитка
Magnitude	Величина, значение (величины)
Inherent(ly)	Присущий
Frequency allocation	Распределение частоты
Frequency-shift signaling	Частотно-манипулированный сигнал
Baud	Бод (единица скорости передачи информации)
To evolve	Развиваться
Address signal	Сигнал адресации
Dial pulse	Импульс набора номера
Answerback	Ответ (в протоколе передачи данных); автоответ; реакция на сигнал дистанционного управления.
Answerback code	Код автоответчика
Trigger	Триггер; запускающий сигнал
Trigger	Запускать, инициировать, включать
VDU – video display unit	Дисплей, видеомонитор
Heavy duty	С тяжелым режимом работы; с высокой нагрузкой; мощный
Paper roll holder	Держатель бумажного рулона
Word processing software	ПО пословной обработки; ПО обработки текстов