Single channel voice frequency signalling



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Single channel voice frequency signalling



Telex communications

Introduction

Telex has its roots in telegraphy and the first practical telegraph was installed by Wheatstone and Owke in 1857The development of telegraphy has been marked by continuous mechanical improve­ments. Hand sending of Morse code was laborious and so an electric typewriter was developed to a point where it could be connected to a telephone line and operated by a typist (the teleprinter).

The ability to send/receive typed messages from one part of the world to the other became ol enormous importance to business and as the machine was able to acknowledge receipt of the transmission of the information in some areas the telex message was accepted as a legally binding. The first telex machines were connected to the telephone network alongside a telephone, by setting up a telephone call and then transferring the line to a teleprinter. As the need for the service grew in business the need for a separate telex network became apparent. Initially the telex networks were switched via manual switchboards, but as they were exclusive to business use the networks were quickly converted to automatic working in the late 1950s.

Characteristics of telex

Telex differs from telephony in that it carries the written rather than spoken communication and is almost exclusively used in the busi­ness world. A key element of telex communication is the production of a local record, exactly duplicating the message sent to and received at the distant end. Further, telex can be operated with the distant end completely unattended, which makes it ideal for inter­national working where businesses operate in differing time zones. As telex transmits the written word, then it also makes it ideal for communication between people of different languages, as time can be spent on translating the message and then formulating the reply. Since most countries that adopted telex also had a telephony net­work, they naturally used the telephony network as the basis for the design of the telex network, which in turn led to a wide variety of signalling systems to communicate between telex machines and the network

Telex signalling

To send the written word, a character set had to be produced and standardised on by all users of the telex system. As the teleprinter grew out of telegraphy and Morse code, then so did the telex character set. The Murray Gxie , unlike the Morse code, is com­posed in such a manner that all characters contain the same number of units and each unit takes exactly the same time to transmit. The unequal dot and dash of Morse code is replaced by equal duration 'mark' and 'space'. Each character requires 5 units giving 32 char­acters in total. The character set is extended by defining two shift characters, figures shift and letters shift.

This code makes up the basis of CCITT International Alphabet Number 2 which is fundamental to the operation of all telex ma­chines (see Figure 59.1). As the telex service grew, the need for a more comprehensive character set was required, particularly when operating over high frequency radio circuits. The CCITT Interna­tional Alphabet Number 5. with its 7 unit error detecting code and automatic retransmission to correct errors, was adopted for telex working and a translation defined between both character sets. In essence this meant that a TELEX machine sending in CCITT ITA 2 would be translated via a code conversion to CCITT ITA 5.

When a character is sent to line it is preceded with one start element followed by 5 information elements and then one stop element (see Figure 59.2). With the pulse durations shown in Figure 59.1, the telex operates at 50 bits per second.

A variety of line signalling systems exist for telex operation, using voltage and current signalling and mainly designed around the needs of electromagnetic systems, where the transmit and receive elements were contacts and electromagnets respectively, loiter single channel voice frequency signalling was added to the telex system. There are therefore three major signalling systems in use throughout the world all with minor country differences depending on the type of telex exchange operating in the local country. These are double current signalling, single current signalling, single chan­nel voice frequency signalling, as described in the following sec­tions. In addition some telex machines are connected to private networks or point to point private wires using similar signalling systems. The mode of working is also important i.e. full duplex discrete send and receive paths and half duplex common send and receive path. Public telex networks are invariably half duplex in operation.

 

Start Stop Signal Code

Figure 59.1CCITT International alphabet number 2

 

Figure 59.2Start stop telex signal

 

Single current signalling

This describes a system using the presence or absence of current to indicate a start or stop polarity on the line. Usually the transmitter and receiver are connected in series to obtain the local record. The current source is led from the telex exchange to the machine, down Ihe telex line. The polarity of the current can change during signall­ing as well as being at an intermediate level. Single current oper­ation normally works from a 120 volt exchange feed. (Figure 59.3)

Double current signalling

This describes a system using voltage/currents of equal magnitude hut opposite polarity, to indicate start or stop information on the line. The system generally has discrete send and receive paths with a common return and is inherently full duplex in operation. Mow-ever for normal telex use the local record is generated by linking the transmitter to the receive path in the machine and hence making this system effectively half duplex. Telex machines operate at the fol­lowing voltages and currents at 20mA, 40mA, and 60mA Line currents for negative mark or positive space: (Figure 59.4)

1. 80.0-80 Volts.

2. 60-0-60 Volts.

3. 48-0-48 Volts.


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

 

 

Telex communications

Introduction

Telex has its roots in telegraphy and the first practical telegraph was installed by Wheatstone and Owke in 1857The development of telegraphy has been marked by continuous mechanical improve­ments. Hand sending of Morse code was laborious and so an electric typewriter was developed to a point where it could be connected to a telephone line and operated by a typist (the teleprinter).

The ability to send/receive typed messages from one part of the world to the other became ol enormous importance to business and as the machine was able to acknowledge receipt of the transmission of the information in some areas the telex message was accepted as a legally binding. The first telex machines were connected to the telephone network alongside a telephone, by setting up a telephone call and then transferring the line to a teleprinter. As the need for the service grew in business the need for a separate telex network became apparent. Initially the telex networks were switched via manual switchboards, but as they were exclusive to business use the networks were quickly converted to automatic working in the late 1950s.

Characteristics of telex

Telex differs from telephony in that it carries the written rather than spoken communication and is almost exclusively used in the busi­ness world. A key element of telex communication is the production of a local record, exactly duplicating the message sent to and received at the distant end. Further, telex can be operated with the distant end completely unattended, which makes it ideal for inter­national working where businesses operate in differing time zones. As telex transmits the written word, then it also makes it ideal for communication between people of different languages, as time can be spent on translating the message and then formulating the reply. Since most countries that adopted telex also had a telephony net­work, they naturally used the telephony network as the basis for the design of the telex network, which in turn led to a wide variety of signalling systems to communicate between telex machines and the network

Telex signalling

To send the written word, a character set had to be produced and standardised on by all users of the telex system. As the teleprinter grew out of telegraphy and Morse code, then so did the telex character set. The Murray Gxie , unlike the Morse code, is com­posed in such a manner that all characters contain the same number of units and each unit takes exactly the same time to transmit. The unequal dot and dash of Morse code is replaced by equal duration 'mark' and 'space'. Each character requires 5 units giving 32 char­acters in total. The character set is extended by defining two shift characters, figures shift and letters shift.

This code makes up the basis of CCITT International Alphabet Number 2 which is fundamental to the operation of all telex ma­chines (see Figure 59.1). As the telex service grew, the need for a more comprehensive character set was required, particularly when operating over high frequency radio circuits. The CCITT Interna­tional Alphabet Number 5. with its 7 unit error detecting code and automatic retransmission to correct errors, was adopted for telex working and a translation defined between both character sets. In essence this meant that a TELEX machine sending in CCITT ITA 2 would be translated via a code conversion to CCITT ITA 5.

When a character is sent to line it is preceded with one start element followed by 5 information elements and then one stop element (see Figure 59.2). With the pulse durations shown in Figure 59.1, the telex operates at 50 bits per second.

A variety of line signalling systems exist for telex operation, using voltage and current signalling and mainly designed around the needs of electromagnetic systems, where the transmit and receive elements were contacts and electromagnets respectively, loiter single channel voice frequency signalling was added to the telex system. There are therefore three major signalling systems in use throughout the world all with minor country differences depending on the type of telex exchange operating in the local country. These are double current signalling, single current signalling, single chan­nel voice frequency signalling, as described in the following sec­tions. In addition some telex machines are connected to private networks or point to point private wires using similar signalling systems. The mode of working is also important i.e. full duplex discrete send and receive paths and half duplex common send and receive path. Public telex networks are invariably half duplex in operation.

 

Start Stop Signal Code

Figure 59.1CCITT International alphabet number 2

 

Figure 59.2Start stop telex signal

 

Single current signalling

This describes a system using the presence or absence of current to indicate a start or stop polarity on the line. Usually the transmitter and receiver are connected in series to obtain the local record. The current source is led from the telex exchange to the machine, down Ihe telex line. The polarity of the current can change during signall­ing as well as being at an intermediate level. Single current oper­ation normally works from a 120 volt exchange feed. (Figure 59.3)

Double current signalling

This describes a system using voltage/currents of equal magnitude hut opposite polarity, to indicate start or stop information on the line. The system generally has discrete send and receive paths with a common return and is inherently full duplex in operation. Mow-ever for normal telex use the local record is generated by linking the transmitter to the receive path in the machine and hence making this system effectively half duplex. Telex machines operate at the fol­lowing voltages and currents at 20mA, 40mA, and 60mA Line currents for negative mark or positive space: (Figure 59.4)

1. 80.0-80 Volts.

2. 60-0-60 Volts.

3. 48-0-48 Volts.


Single channel voice frequency signalling

Single channel voice frequency signalling (SCVF) was adopted by CCITT R 20 to improve the telex network for the following reasons:

1. To reduce the incidence of single or double current high level signals, inducing noise in adjacent cable pairs.

2. To reduce the power consumption of the telex exchange.

3. To enable connections to be made between telex machines over non metallic circuits.

4. To achieve full duplex transmission using relatively inexpens­ive CCITT V2I modems over 2 wire circuits.

The method of signalling is based on CCITT V21 with the follow­ing frequency allocations:

1. Telex exchange to telex machine, Space 0 = 1180 Hz; Mark 1= 980 Hz.

2. Telex machine to telex exchange, Space 0 = 1850 Hz; Mark I= 1650 Hz.

 

Figure 59.4 Double current call progression



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