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Signalling in L M Ericsson Telex System E. STRINDLUND & C. PAULSSON, TE LEFON AKTIE BOL AG ET L M ERICSSON, STOCKHOLM UDC 621.394.63 621.394.34 LME 815 831 Signalling occurs in a telex network in conjunction with the setting up and clearing of connections between exchanges and takes place, respectively, before and after the transmission of information between the connected subscribers. An account is given below of the signalling principles used in L M Ericsson's telex system within the framework of the CCITT recommendations for establishing, supervising, clearing and charging national, international and intercontinental telex connections. The signalling used in national networks may be characterized as CCITT Type B with keyboard selection and printed service signals. International and intercontinental exchanges have equ.pment for Type A, Type B, Type C Table II and U 20 signalling. L M Ericsson Telex System The system comprises the following types of exchanges: • National terminal exchanges type ARB 111 • National transit exchanges type ARM 201 and ARM 503 • International transit exchanges type ARM 201 • Intercontinental transit exchanges type ARM 201 Fig. 1 shows how a national network can be built up with these types of exchanges. ARB Fig. 1 Telex network International transit exchange ARM — ARB I 11 National transit exchange ARM 201 or ARM 503 : ARB 111 Terminal exchange ARB III Remote or local subscriber 2
The skeleton of the network consists of transit centres ARM, with which one or more terminal exchanges A lift 111 are always integrated. The remaining terminal exchanges, in the number required, can be connected to the transit centres on VF telegraph channels. Up to 15 classes of subscribers can be connected to ARB 111. Examples of such classes are ordinary telex subscribers, police, banks, news agencies, embassies, subscribers with Roman and subscribers with Arabic alphabet. Every subscriber is given a corresponding classification marking which is signalled in the form of 1 out of 15 selected combinations from CCITT's Alphabet No. 2, which is shown in its entirety in fig. 2. The class signal is called K in subsequent signalling diagrams. A country can be divided into a large number of tariff zones. To any one terminal exchange ARB 111 it is possible to connect subscribers situated in up to 20 zones and base the tariffs on the geographical distance between the communicating A and B subscribers. In accordance herewith every subscriber is given a tariff zone marking consisting of 1 of 20 telegraph characters also selected from Alphabet No. 2 and called T in the signal diagrams. Fig. 2 International 5-unit Telegraph Alphabet No. 2 For single current For double current O No current Start polarity A • Current • Stop polarity Z For meaning of symbols see fig. 3. To achieve the greatest possible rapidity and reliability in switching, all information is signalled in telegraph code both between exchanges and from the calling subscriber to the exchange equipment. The signalling principles that have been chosen are accordingly aimed at achieving the most efficient signalling possible within the framework of a telegraph speed of 50 bauds. One example of this is that, at the same time as the calling subscriber receives the printed proceed-to-select signal GA from the terminal exchange as a request to deliver selection information (either by automatic tape transmitter or manually), the exchange equipment sends the T och K information concerning the subscriber to the superior transit centre. For further information reference should be made to Ericsson Review No. 2/1967. An account will be given first of the signalling principles employed and thereafter, in the form of signalling diagrams, examples of signalling for different switching procedures. E] Hyphen ® Answer-back tripping signal El Full-stop EPIus [
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