11 - ericssonhistory.com
11 - ericssonhistory.com 11 - ericssonhistory.com
In a new Recommendation U 11, CCITT has introduced a new signalling system called Type C. The recommendation is so recent, however, that it is still being revised (CCITT, JWP Study Groups 1 and X. Melbourne 1966), and it is difficult to predict the use to which Type C signalling may be put. Signals in L M Ericsson Telex System The signalling used in L M Ericsson's telex system may be characterised as CCITT Type B with keyboard selection and printed service signals. As a large number of telex exchanges in the world have Type B signalling, it was natural in the development of L M Ericsson's telex system to adopt Type B. As the use of registers, moreover, was obviously desirable, it was possible to introduce keyboard selection, which is preferable to dial selection owing to its greater rapidity. In conjunction with the introduction of keyboard selection it is also desirable to use printed service signals. Signalling of Numerical Information by Register The A subscriber sends the numerical information to the register at the transit centre. This register transmits the numerical information either to a register at another transit centre or to a code receiver at a terminal exchange, after which the first mentioned register leaves the circuit and the subsequent setting up of the connection is taken over by the next unit. This method of linkwise setting up of the connection is suitable on account of the small number of signals available for telex signalling. Backward signals of different kinds would unnecessarily complicate the equipment. Digital transmission between registers and between register and code receiver is done with teleprinter signals in accordance with CCITT Alphabet No. 2. On interexchange circuits it is of course of great importance that the digital transmission is effected as quickly as possible, i.e. with teleprinter signals. This applies especially on expensive international and intercontinental circuits. As a large part of the telex traffic is international, there will be an increasing demand for the use of teleprinter signals for digital transmission on national circuits. Fig. 5 I: Part of telegraph signalling generator «hich generates all 32 combinations of the 5-unit alphabet and forms service signals such as OCC, NA, NC etc. II: Electronic telegraph signalling receiver for reception, for example, of subscriber's keyboard selection signals. 6
Keyboard Selection In order to avoid receiving equipment both for teleprinter and dial signals in registers, among other reasons, it is advisable that the subscriber's selection information should be sent from the teleprinter keyboard. As mentioned above, the transmission of the selection information should be quicker than with dial selection, owing both to the telex subscriber's familiarity with the use of keyboards and to the fact that this procedure enables the subscriber to use his automatic tape transmitter also for sending the selection information. (This is convenient for the subscriber especially on international calls, which may involve a large number of digits.) Fig. 6 Form and meaning of printed service signals. Signal A-subscriber reads 1 Proceed-to-select GA 2 Time announcement e.g. 09.11 3 Busy 09.11 occ 4 No-circuit 09.11 NC 5 Connection not admitted 09.11 NA 6 Not a working line 09.11 NP 7 Office closed, e.g. for holidays 09.11 ABS 8 Out-of-order 09.11 DER With keyboard selection it may happen that, between two digits, the telex operator keys another character than a digit. Instead of, in this case, sending NP (see below) to the A subscriber, which may cause confusion as the latter is presumably not aware of the error he has committed, other characters than digits are accepted but not registered by the register and the connection is established in the normal way when the necessary digits have been received. Before the subscriber starts numerical transmission he sends a "prepare for digits" signal which consists of Combination No. 30 (figure-shift) and which "opens" the register. The possibility both of keyboard and dial selection was achieved through the fact that the register can be equipped with a special decadic receiver and through the use of classification (see below) for informing the register whether the subscriber uses keyboard or dial selection. Printed Proeeed-to-Select Signal With the use of keyboard selection the teleprinter motor must start at an early stage. The sound of the starting of the motor might be used as a proceed-to-select signal. On the other hand silent electronic teleprinters may come into use within the not too distant future. It is also necessary that the teleprinter motor runs up to speed before the subscriber starts to send selection information. It is therefore advisable to give the subscriber a clear indication that he can start, which is done in the form of a teleprinter signal GA (go ahead), as the subscriber's attention is directed to the teleprinter after the motor has started, not to the control unit. Printed Service Signals Primed service signals in accordance with CCITT recommendations could be introduced as the teleprinter motor starts at an early stage in the setting up of the connection. Such signals give the A subscriber more detailed information concerning the reason for failure of a connection than a simple busy-andcongestion signal. Unnecessary renewed attempts at connection can thereby be avoided. The service signals used are as follows: OCC NC NA ABS DER NP Subscriber engaged No circuits, e.g. all trunks busy Connection not admitted Subscriber absent or office closed Out of order Not a working line Each service signal is preceeded by a time announcement, e.g. 09.11, as shown in fig. 6. 7
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In a new Re<strong>com</strong>mendation U <strong>11</strong>, CCITT has introduced a new signalling<br />
system called Type C. The re<strong>com</strong>mendation is so recent, however, that it is<br />
still being revised (CCITT, JWP Study Groups 1 and X. Melbourne 1966),<br />
and it is difficult to predict the use to which Type C signalling may be put.<br />
Signals in L M Ericsson Telex System<br />
The signalling used in L M Ericsson's telex system may be characterised<br />
as CCITT Type B with keyboard selection and printed service signals. As a<br />
large number of telex exchanges in the world have Type B signalling, it was<br />
natural in the development of L M Ericsson's telex system to adopt Type B.<br />
As the use of registers, moreover, was obviously desirable, it was possible to<br />
introduce keyboard selection, which is preferable to dial selection owing to its<br />
greater rapidity. In conjunction with the introduction of keyboard selection it<br />
is also desirable to use printed service signals.<br />
Signalling of Numerical Information by Register<br />
The A subscriber sends the numerical information to the register at the<br />
transit centre. This register transmits the numerical information either to a<br />
register at another transit centre or to a code receiver at a terminal exchange,<br />
after which the first mentioned register leaves the circuit and the subsequent<br />
setting up of the connection is taken over by the next unit. This method of<br />
linkwise setting up of the connection is suitable on account of the small number<br />
of signals available for telex signalling. Backward signals of different kinds<br />
would unnecessarily <strong>com</strong>plicate the equipment.<br />
Digital transmission between registers and between register and code receiver<br />
is done with teleprinter signals in accordance with CCITT Alphabet<br />
No. 2. On interexchange circuits it is of course of great importance that the<br />
digital transmission is effected as quickly as possible, i.e. with teleprinter signals.<br />
This applies especially on expensive international and intercontinental<br />
circuits. As a large part of the telex traffic is international, there will be an<br />
increasing demand for the use of teleprinter signals for digital transmission on<br />
national circuits.<br />
Fig. 5<br />
I: Part of telegraph signalling generator<br />
«hich generates all 32 <strong>com</strong>binations of<br />
the 5-unit alphabet and forms service<br />
signals such as OCC, NA, NC etc.<br />
II: Electronic telegraph signalling receiver<br />
for reception, for example, of subscriber's<br />
keyboard selection signals.<br />
6
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