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On physical subscriber lines other signals may also occur, e.g. 1000 Hz signals for stepping of private meters at the subscribers' premises. The signalling principles and signals are likewise affected by the exchange equipment in use. Register-controlled systems, for example, permit digital transmission between exchanges in the form of teleprinter signals and at a speed of 50 bauds, which implies a transmission speed of 6.7 digits per second. Signals and Signalling Conditions Recommended by CCITT CCITT has defined a number of signals and signalling conditions, of which the most important are listed below. Free line condition. The circuit is free for traffic. Calling signal. Transmitted on the forward signalling path. Call-confirmation signal. Returned on the backward signalling path to indicate reception of calling signal. Proceed-to-select signal. Returned on the backward signalling path to indicate that the distant terminal equipment is prepared to receive selection information. Selection signals. Transmitted on the forward signalling path to indicate the number required. Call-connected signal. Returned on the backward signalling path to indicate that the call has reached the B subscriber and that the latter is free. Idle-circuit condition. An established connection ready for exchange of information. Clearing signal. Transmitted on either signalling path. Service signals. Returned on the backward signalling path, e.g. if the B subscriber is engaged. Type A and Type B Signalling According to the CCITT Blue Book, Volume VII. Recommendation U 1, the following signalling conditions are equivalent in Type A and Type B signalling. Start polarity. The polarity of the start pulse in the International Telegraph Alphabet No. 2. Stop polarity. Polarity of the stop pulse in the same alphabet. Free line condition. Permanent start polarity on the forward and backward signalling paths. Call. On the forward signalling path a reversal of polarity takes place to permanent stop polarity. Idle-circuit condition. On an established connection a permanent stop polarity is sent on the forward and backward signalling paths. Clearing signal. Consists of a reversal of polarity to permanent start polarity on either signalling path. The start polarity is interpreted by the exchange equipment as a clearing signal within 300-1000 ms. Clear-confirmation signal. Consists of a reversion to start polarity on the other signalling path in response to a clearing signal. When a clearing signal has reached the receiving line relay set, a confirmation signal must be sent back in the other direction within 350-1500 ms after reception of the reversal of polarity. The minimum period will be increased to 400 ms in future systems. 4
The table below shows the signals recommended lor use in subscribercontrolled fully automatic telex traffic which differ in the two types of signalling. Signal Type A Type B Call-confirmation Proceed-to-select Selection Call-connected Busy Permanent stop polarity 40 ras (± X ms) pulse of start polarity Teleprinter signals 150 ms (± 11 ms) pulse of start polarity followed by stop polarity for at least 2 sec. and possibly teleprinter signals Teleprinter signals followed by clearing signal 25 ms pulse of stop polarity (between 17.5 and 35 ms) 25 ms pulse of stop polarity (between 17.5 and 35 ms) Dial pulses or teleprinter signals Stop polarity for at least 2 sec. a. 200 ms pulse of stop polarity followed by start polarity for 1500 ms (tolerance ± 30 %)* b. 200 ms pulse of stop polarity (tolerance ± 30 %) followed by teleprinter signals and start polarity for 1500 ms (tolerance ± 20 %)* Out-of-order, number changed and number unobtainable Clearing signal normally preceded by teleprinter signals a. Permanent start polarity** b. 200 ms pulse of stop polarity followed by start polarity for 1500 ms (tolerance ± 30%)* c. 200 ms pulse of stop polarity (tolerance ± 30%) followed by teleprinter signals and start polarity for 1500 ms (tolerance ± 20 %)* * This sequence of signals may be repeated until a clearing signal is sent over the forward signalling path. ** The use of this signal should be avoided if possible. U20 and Type C Signalling In telex networks radio channels are often used over large distances. In order that these channels may operate reliably, a 7-unit alphabet is employed which provides 35 combinations, all of which have the ratio 3 : 4 between stop and start polarity pulses. Thirty-two combinations are used to represent the characters of the 5-unit alphabet and the remaining three have special significations. At the receiving end of a connection the signals are analysed and those not having the ratio 3 : 4 give rise to a request for signal repetition, whereupon an RQ signal is sent. The RQ signal is one of the three aforementioned special signals. The two others are used to characterize the two polarity conditions, start and stop, and are called a and /» signals. Fig. 4 Time circuit used, among other purposes, for determining the operate or release times of relays within the range 1-1000 sec. Is used, for example, on repeater racks for checking the efficiency on radio channels with ARQ equipment and employing U 20 signalling. During free line condition, for example, signals are sent continuously in both directions on an ARQ circuit. A call is signalled by substituting / signals for a signals in the forward direction. The entire signalling procedure is defined in Recommendation U 20 in the CCITT Blue Book, Volume VII, from which the name U 20 signalling derives. All intercontinental telex signalling on cable, satellite and radio circuits is now Type A, Type B or U 20. 5
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