2. Telex machine to telex exchange, Space () = 1850 Hz; Mark 1 ever for normal telex use the local record is generated by linking the = 1650 Hz. REQUEST CONNECTED CLEAR SELECTION Figure 59.3 Single current call progression Answerback 59/5 REQUEST SELECTION CLEAR TRANSMIT CLEAR CONFIRM CONFIRM RECEIVE Figure 59.4 Double current call progression Introduction of this signalling system also gave the opportunity Answerback for telex machines to operate at speeds up to 300 baud. CCITT recommendation F.60 specifies that every telex machine 59.3.4 Type A and Type B signalling 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 Two further subdivisions of the signalling systems have evolved, will trigger the distant telex to transmit the answerback code em- corresponding to the different types of national network employed. bedded in the distant telex to the sending telex. This technique It has been agreed that, where two countries with different types of enables telex users to confirm that they have reached the correct signalling system are trying to make connection, the country setting distant party, before message transmission can begin, SO protecting up the call will convert its outgoing signals to that of the country the confidentiality of the message. receiving the call. Both types of signals convey the same informa- tion, the difference is in the detail of implementation. In general Type A signalling has been used where the telex keyboard has been used to send the address signals to the network and Type B signall- Telex protocol ing where the address signals were sent to the network with dial pulses, much like a telephone. Figure 59.5 shows the sequence of As telex signalling is machine to machine operation and the distant signals in each system. end may be unattended, then a protocol has been established. Sev- TYPE A TYPE 8 TRANSMIT RECEIVE TRANSMIT RECEIVE FREE LINE FREE