џWPCL ћџ2BJ|xа АH аа АА X агга ХА6p&А6p&Х аеЄ† а Hр аааУ Уб cмˆ4 PŽТ б Fascicle VII.2 Р-Р Rec. U.20 PAGE1 Єее† а HH аааУ Уб cмˆ4 PŽТ бPAGE4 Fascicle VII.2 Р-Р Rec. U.20 еа H№ ааЬџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџHpи P Ј XА`ИhР!(#џџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџЬаа X  аб cмˆ4 PŽТ бTHE DRAWINGS CONTAINED IN THIS RECOMMENDATION HAVE BEEN DONE IN AUTOCAD. ‚У УRecommendation U.20 аЬџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџH јP Ј XА`ИhР!(#џџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџЬаСр(AСб cмˆ4 PŽТ бTELEX AND GENTEX SIGNALLING ON RADIO CHANNELS Ср(<С(SYNCHRONOUS 7Р-РUNIT SYSTEMS AFFORDING ERROR CORRECTION Ср(KСBY AUTOMATIC REPETITION)Ф Ф а H ааЬџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџHpи P Ј XА`ИhР!(#џџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџЬаУУб cмˆ4 PŽТ б(Geneva, 1956; amended at New Delhi, 1960, Geneva, 1964, Mar del Plata, 1968, and Geneva, 1972)ФФ аЬџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџHјP Ј XА`ИhР!(#џџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџЬаТа ТThe CCITT,ЦЦ аЬџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџHpи P Ј XА`ИhР!(#џџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџЬаУУconsidering аЬџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџH јP Ј XА`ИhР!(#џџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџЬаФФС  С(a)СјСthat numerous radiotelegraph circuits working in association with 5Р-Рunit startР-Рstop apparatus make use of errorР-Рcorrecting synchronous systemsй0€ errorР-Рcorrecting synchronous systems0йб cмˆ4 PŽТ б having a special errorР-Рdetecting 7Р-Рunit code enabling errors to be corrected by a request for a repetition (ARQ systemйYб cмˆ4 PŽТ бARQ systemб cмˆ4 PŽТ бYй); аЬџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџHpи P Ј XА`ИhР!(#џџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџЬаС С(b)СpСthat when they are usable for switched communications, on the radio section these synchronous systems use two combinations a and РРйCб cмˆ4 PŽТ б combinations a and РРCй, which characterize the permanent conditions of start polarity and stop polarity respectively in the startР-Рstop part of the connection (see Recommendation S.13 [1]); С С(c)СpСthat the special makeР-Рup of these systems is such that a change in the significant condition at the input to the system is not reproduced at the output with a constant delay; С С(d)СpСthat the experience acquired with telex and gentex switching through these radiotelegraph systems seems sufficient to justify the laying down of general rules specifying signalling arrangements for manual, semiР-Рautomatic and automatic working in such international radio channels, УУб cмˆ4 PŽТ бunanimously declares the view а H аФФС Сthat the signals, enumerated in Recommendation U.1, to be used in setting up international telex and gentex calls over radio channels comprising synchronous systems with error correction by automatic repetition should be characterized as follows: аЬџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџH јP Ј XА`ИhР!(#џџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџЬаТX  ТТX јТС€  СС€ HС‚У У1С  Сб cмˆ4 PŽТ бFree line conditionЦЦ аЬџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџHpи P Ј XА`ИhР!(#џџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџЬаб cмˆ4 PŽТ бФ Ф1.1С  СSuccessive a combinations on the forward and backward paths. аЬџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџH јP Ј XА`ИhР!(#џџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџЬаТX  ТТX јТС€  СС€ HС‚У У2С  Сб cмˆ4 PŽТ бCallЦЦ а H ааЬџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџHpи P Ј XА`ИhР!(#џџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџЬаб cмˆ4 PŽТ бФ Ф2.1С  СTransition from combination a to combination РР on the forward signalling path. Reception of two consecutive РР signals over the forward signalling path shall be interpreted as a calling signal. 2.2С  СOn circuits automatically operated in both directions, reception of a single РР signal at the end of the circuit remote from the calling subscriber must cause the outgoing equipment on this circuit at that end to be marked busy immediately. This busy condition must be applied until two a signals are received. 2.3С  СIf the motor of the FRXD (fully automatic reperforator transmitter distributor) or equivalent motorР-Рdriven storage device is not already running, it must be started without delay, in order to accept the subsequent selection signals. Furthermore, if the motor of the storage device at the called end is not already working, it must be started. а H а2.4С  СIt is desirable that, in the busy hour at least, the starting of the motor of the storage device should not be dependent on the calling signal for а H аeach call. One simple method of meeting this requirement is to provide a device that delays the switching off of the motor until about 5 minutes after the call has been cleared. аЬџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџH јP Ј XА`ИhР!(#џџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџЬаТX  ТТX јТС€  СС€ HС‚У У3С  Сб cмˆ4 PŽТ бCallР-Рconfirmation signalФ ФЦЦ а H ааЬџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџHpи P Ј XА`ИhР!(#џџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџЬаб cмˆ4 PŽТ б3.1С  СTransition from combination a to combination РР on the backward signalling path. The reception of two consecutive РР signals over the backward signalling path shall be interpreted as a callР-Рconfirmation signal. 3.2С  СThe return of this signal can be initiated either by the switching equipment or by the radio equipment. Not more than one second shall elapse at the incoming end between the moment when two РР signals have been received and the return of the first РР signal of the callР-Рconfirmation signal. 3.3С  СWith manual switching the callР-Рconfirmation signal shall be returned independently of the operator's answer. 3.4С  СFor retest purposes radio circuits may be considered faulty when the callР-Рconfirmation signal is not received within three seconds. аЬџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџH јP Ј XА`ИhР!(#џџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџЬаТX  ТТX јТС€  СС€ HС‚У У4С  Сб cмˆ4 PŽТ бSignals preceding selectionЦЦ ТX  ТТX јТС€  СС€ HСб cмˆ4 PŽТ бФ Ф4.1С   СУУб cмˆ4 PŽТ бProceedР-РtoР-Рselect signalЦЦ аЬџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџHјP Ј XА`ИhР!(#џџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџЬаТа ТС€ HСФФб cмˆ4 PŽТ б4.1.1УУС јСSemiР-Рautomatic workingЦЦ а H ааЬџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџHpи P Ј XА`ИhР!(#џџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџЬаФФ4.1.1.1СpСIf the automatic switching equipment at the receiving end can receive the selection information immediately after the sending of the callР-Рconfirmation signal, the callР-Рconfirmation signal shall constitute the proceedР-РtoР-Рselect signal. 4.1.1.2СpСIf the automatic switching equipment at the receiving end cannot receive the selection information immediately after the sending of the callР-Рconfirmation signal, a distinctУУ proceedР-РtoР-РselectФФ signal, combination No. 22, shall be returned over the backward signalling path after the callР-Рconfirmation signal. For 99% of calls in the busy hour, this signal must be returned not more than 3 seconds after the transmission of the callР-Рconfirmation signal begins. (For some existing systems this delay will be 4 seconds.) аЬџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџHјP Ј XА`ИhР!(#џџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџЬаТа ТС€ HС4.1.2С јСУУFullyР-Рautomatic workingЦЦ а H ааЬџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџHpи P Ј XА`ИhР!(#џџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџЬаФФ4.1.2.1СpСThe proceedР-РtoР-Рselect signal, combination No. 22, returned over the backward signalling path shall always be distinct from the а H аcallР-Рconfirmation signal and should be returned within the limits specified under semiР-Рautomatic working. аЬџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџH јP Ј XА`ИhР!(#џџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџЬаТX  ТТX јТС€  СС€ HС4.2С   Сб cмˆ4 PŽТ бУУProceedР-РtoР-Рtransmit signalЦЦ а H ааЬџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџHpи P Ј XА`ИhР!(#џџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџЬаФФб cмˆ4 PŽТ б4.2.1С СOn the backward signalling path teleprinter signals indicating the called operator's position. а H а4.2.2С СThe sending of the proceedР-РtoР-Рselect or the proceedР-РtoР-Рtransmit signal should be delayed until two consecutive РР signals have been correctly received over the backward signalling path. Two consecutive РР signals can be presumed to have been or to be received when four РР signals have been accepted by the storage of the errorР-Рcorrecting equipment at the sending end. (This allows for the loss of one РР signal as an undetected error.) а H а4.2.3С СThe receiving equipment shall be arranged so that when two РР signals are received and followed immediately by teleprinter signals [representing the callР-Рconfirmation and proceedР-РtoР-Рselect (or proceedР-РtoР-Рtransmit) signals in rapid succession] the recognition of the two РР signals as the callР-Рconfirmation signal will allow the teleprinter signals to be preceded by 140 ms (minimum) of stop polarity. 4.2.4С СMeasures should be taken so that, if proceedР-РtoР-Рselect or proceedР-РtoР-Рtransmit signals are relayed by the FRXD (or equivalent storage device), the switching equipment does not return these signals until the motor has reached its full speed. аЬџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџH јP Ј XА`ИhР!(#џџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџЬаТX  ТТX јТС€  СС€ HС‚У У5С  Сб cмˆ4 PŽТ бSelection signalsЦЦ а H ааЬџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџHpи P Ј XА`ИhР!(#џџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџЬаб cмˆ4 PŽТ бФ Ф5.1С  СFor manual working, teleprinter signals over the forward signalling path. 5.2С  СFor semiР-Рautomatic working, teleprinter signals over the forward signalling path, as follows: Та ТР-РТ№ Тthe prepareР-РforР-Рdigits signal shall be combination No. 30 (figureР-Рshift);ЦЦ а H аТа ТР-РТ№ Тdigits of the called subscriber's number (preceded by transit access codes, if required) in International Telegraph Alphabet No. 2;ЦЦ а H аТа ТР-РТ№ ТendР-РofР-Рselection signal, combination No. 26. This may be followed by another combination characterizing the class of traffic in the incoming country.ЦЦа HH а а H№ а5.3С  СFor fullyР-Рautomatic working: teleprinter signals over the forward signalling path, as follows: Та ТР-РТ№ Тthe prepareР-РforР-Рdigits signal shall be combination No. 30 (figureР-Рshift);ЦЦ а H аТа ТР-РТ№ Тdigits of the called subscriber's number (preceded by transit access codes, if required) in International Telegraph Alphabet No. 2;ЦЦ а H аТа ТР-РТ№ Тif an endР-РofР-Рselection signal is required, this should be combination No. 26. This may be followed by another combination characterizing the class of traffic in the incoming country.ЦЦ а H а5.4С  СThe transmission of the selection signals should be delayed if the motor of the FRXD has not yet gained speed. а H а5.5С  СWhere the incoming system uses a uniform numbering plan so that the number of digits in the number can be determined from the initial digit, the outgoing Administration must transmit an endР-РofР-Рselection signal if this is required by the incoming country. Where the incoming system has a nonР-Рuniform numbering scheme the endР-РofР-Рselection signal cannot be made obligatory. However, for such a system it may be advantageous to use this signal subject to the agreement of the outgoing Administration, in those cases where the outgoing system can readily insert the signal. To avoid undue occupation of trunks and equipment, Administrations should take all reasonable steps to ensure that selection signals are transmitted over radio circuits without undue delay. аЬџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџH јP Ј XА`ИhР!(#џџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџЬаТX  ТТX јТС€  СС€ HС‚У У6С  Сб cмˆ4 PŽТ бCallР-Рconnected signalФ ФЦЦ аЬџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџHpи P Ј XА`ИhР!(#џџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџЬаб cмˆ4 PŽТ б6.1С  СManual working: the codeУ У DFФ Ф over the backward signalling path. а H а6.2С  СSemiР-Рautomatic working: either answerback signals or the signals defined for fullyР-Рautomatic working below. 6.3С  СFullyР-Рautomatic working: combination No. 32, followed by 11 to 13 combinations No. 29 (letterР-Рshift) followed by the obtained subscriber's answerback code. The insertion of the combinations No. 29 must not cause mutilation of the subsequent signals in the sequence. а H а6.4С  СIn the case of transit operation where the first circuit in the connection is an ARQ radio circuit and the second circuit in the connection uses Type A or B signalling to a country that returns the answerback automatically, the number of combinations No. 29 of the radio callР-Рconnected signal may be reduced to eight to avoid mutilating the answerback. аЬџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџH јP Ј XА`ИhР!(#џџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџЬаТX  ТТX јТС€  СС€ HС‚У У7С  Сб cмˆ4 PŽТ бIdle circuit conditionЦЦ аЬџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџHpи P Ј XА`ИhР!(#џџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџЬаб cмˆ4 PŽТ бФ Ф7.1С  СCombinations РР on the forward and backward signalling paths. аЬџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџH јP Ј XА`ИhР!(#џџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџЬаТX  ТТX јТС€  СС€ HС‚У У8С  Сб cмˆ4 PŽТ бClearingЦЦ ТX  ТТX јТС€  СС€ HСб cмˆ4 PŽТ бФ Ф8.1С   СУУб cмˆ4 PŽТ бClearing signalЦЦ а H ааЬџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџHpи P Ј XА`ИhР!(#џџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџЬаб cмˆ4 PŽТ бФФ8.1.1С СThe appearance of a combinations in the direction in which the clearing signal is sent. Reception of two consecutive a signals will have to be interpreted as a clearing signal. а H а8.1.2С СOn recognition of the clearing signal received over the radio circuit any text remaining in the store, at the point where the clearing signal is recognized, must be destroyed. а H а8.1.3С СOn recognition of the clearing signal received over the land line, any text remaining in store, at the point where the clearing signal is recognized, must be transmitted before the a signals are sent over the radio path. аЬџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџH јP Ј XА`ИhР!(#џџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџЬаТX  ТТX јТС€  СС€ HС8.2С   Сб cмˆ4 PŽТ бУУClearР-Рconfirmation signalЦЦ а H ааЬџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџHpи P Ј XА`ИhР!(#џџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџЬаФФб cмˆ4 PŽТ б8.2.1С СThe appearance of a combinations in the direction opposite to that from which the clearing signal was sent. Reception of two consecutive a signals will be interpreted as a clearР-Рconfirmation signal when a clearing signal of 7 a signals has been accepted by the storage of the radio equipment without a request for repetition. The transmission of 7 a signals in this way ensures that, allowing for the loss of one a signal as an undetected error, the clearing signal can be presumed to have been received and recognized at the distant end. а H а8.2.2С СFor radio circuits using an eightР-Рcharacter cycle with four characters stored, a sequence of 8 a signals shall be used in place of the above sequence of 7 a signals. For radio circuits using an eightР-Рcharacter cycle with seven characters stored, a sequence of 11 a signals shall be used in place of the above sequence of 7 a signals. а H а8.2.3С СIt is desirable that the equipment shall be arranged so that the clearing and clearР-Рconfirmation signals do not cause spurious characters (including combinations No. 32) to be transmitted over the radio path. Where electronic storage devices are used it is possible to arrange for these spurious characters to be suppressed in the storage device. Where electroР-Рmechanical storage devices are used, the generation of spurious characters by the clearР-Рconfirmation signal can be minimized by arranging that when the clearing signal is received over the radio circuit, the input to the storage device is blocked. 8.2.4С СIn order to ensure that, on transit calls, switching equipment and possibly the subscriber's teleprinter set are not unnecessarily held because of delay in transmitting the clearing and clearР-Рconfirmation signals over the radio path, the radiotelegraph equipment should return the clearР-Рconfirmation signal to the switching equipment without waiting for the exchange of clearing and clear confirmation signals over the radio path. аЬџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџHјP Ј XА`ИhР!(#џџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџЬа8.3Тh  ТУУGuard delayЦЦ а H ааЬџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџHpи P Ј XА`ИhР!(#џџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџЬаФФ8.3.1С СThe circuit shall be guarded on release as specified in Recommendation U.1 except that the delay shall be measured from the moment when the equipment has both: а H аТа ТТ№ ТС€ Сa)СpСtransmitted 7 a signals over the radio path without request for repetition;ЦЦ а H аТа ТТ№ ТС€ Сb)СpСhas received two consecutive a signals over the other signalling path.ЦЦ а H а8.3.2С СDuring the guard period the free line condition shall be maintained on both signalling paths of the international circuit. а H а8.3.3С СBecause it is possible for the circuit to be opened for traffic at one end before the equipment at the other end has completed the transmission of the 7 a signals, it is possible that an incoming call may be received before the 7 a signals have been transmitted. Where this occurs, the call should be accepted but the callР-Рconfirmation signal should not be returned until the transmission of the 7 a signals has been completed. (See РSР 8.2.2 above.) аЬџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџH јP Ј XА`ИhР!(#џџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџЬаТX  ТТX јТС€  СС€ HС‚У У9С  Сб cмˆ4 PŽТ бRegister congestionЦЦ а H ааЬџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџHpи P Ј XА`ИhР!(#џџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџЬаб cмˆ4 PŽТ бФ Ф9.1С  СSemiР-Рautomatic working: the return of a signal indicating congestion may be allowed; theУ У NCФ Ф sequence with the standard form of service signal should be used to indicate the situation. а H а9.2С  СFullyР-Рautomatic working: the return of a signal indicating congestion is prohibited. аЬџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџH јP Ј XА`ИhР!(#џџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџЬаТX  ТТX јТС€  СС€ HС‚У У10С   Сб cмˆ4 PŽТ бService signalsЦЦ аЬџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџHpи P Ј XА`ИhР!(#џџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџЬаб cмˆ4 PŽТ бФ Ф10.1С  СTeleprinter signals (У УOCCФ Ф,У У NCФ Ф,У У NCHФ Ф,У У NAФ Ф,У У NPФ Ф,У У DERФ Ф,У У ABSФ Ф) preceded by the carriageР-Рreturn, lineР-Рfeed and letterР-Рshift signals and followed by а H аlineР-Рfeed (preferably together with carriageР-Рreturn) and then immediately by the clearing signal in all cases. аЬџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџHјP Ј XА`ИhР!(#џџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџЬа‚У У11Тр  ТBothР-Рway workingФ ФЦЦ аЬџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџHpи P Ј XА`ИhР!(#џџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџЬа11.1С  СFor bothР-Рway ARQ radio circuits used in the fullyР-Рautomatic telex and gentex services, the following action to minimize the incidence of headР-Рon collision is recommended: а H аТа ТТ№ ТС€ Сa)СpСthat inverse order testing, or a close approximation to it by testing the route in small groups of circuits in fixed order, always starting the search from the same initial position, should be adopted at opposite ends of a bothР-Рway group of trunk circuits.ЦЦ а H аТа ТТ№ ТС€ Сb)СpСthat calls should be offered in such a way that each circuit is tested once only for the minimum period of time necessary to ascertain whether it is free or busy, and the outgoing selectors should not have facilities for delayed hunting.ЦЦ а H а11.2С  СThe absence of the proceedР-РtoР-Рselect signal will serve to detect a headР-Рon collision when the group of circuits is totally occupied or very nearly totally occupied. The two calls will then be cleared down unless there are still free circuits in the route. а H аС СУУNoteФФ Р-Р The recognition of the calling, callР-Рconfirmation, clearing and clearР-Рconfirmation signals requires the detection of two consecutive signals РР or a as specified. The detection device should, in new equipment, beд Д-д arranged to recognize two consecutive signals even though these may be separated by a period of automatic correction, i.e. the discrimination involves counting. In some existing equipments the detection device requires that the two signals to be recognized shall occur in consecutive character periods, i.e. the discrimination involves timing. The transmission of the callР-Рconfirmation, clearing and clearР-Рconfirmation signals requires that the appropriate number of РР or a signals shall be offered to the storage of the radio equipment without a request for repetition, i.e. the control should be by a timing device that is reset when automatic correction occurs. ‚Ср LСб cмˆ4 PŽТ бFIGURE 1/U.20 Љ CCITT 48381 б cмˆ4 PŽТ б аЬџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџH јP Ј XА`ИhР!(#џџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџЬаТX  ТТX јТУ УReferenceФ ФЦЦ а H ааЬџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџHpи P Ј XА`ИhР!(#џџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџЬа[1]Тh  ТCCITT Recommendation УУUse on radio circuits of 7Р-Рunit synchronous systems giving error correction by automatic repetitionФФ, Rec. S.13ЦЦ