1、INTERNATIONAL TELECOMMUNICATION UNION)45G134 1 TELECOMMUNICATIONSTANDARDIZATION SECTOROF ITU%.%2!,G0G02%#/-%.$!4)/.3G0G0/.G0G04%,%0(/.%37)4#().G0G0!.$G0G03).!,).3).!,).G0G0).G0G04(%G0G0).4%2.!4)/.!,G0G0-!.5!,3%26)#%3).!,G0G02%#%)6%23G0G0 itmust not exceed 2.5 microwatthours or 9000 microwattseconds
2、at a zero relative level point. A reasonable value for thenumber of calls, or attempted calls, on a circuit during the busy hour was assumed and 2 seconds was assumed to be thesending duration of the signalling current to line by operation of the operators ringing key.On outgoing circuits from an in
3、ternational exchange, where the 500/20 Hz signals are liable to be sent overwideband carrier systems (coaxial carrier systems) it is desirable, to avoid overloading the repeaters, that the durationof the 500/20 Hz signals sent to line should not exceed 2 seconds and they should be limited to this va
4、lue by automaticmeans.Since, in general, the Instructions for the International Telephone Service (Article 32) 1 require thesignalling current sent over an international circuit to have a duration of at least 2 seconds to avoid the risk of signalsbeing undetected at the incoming end, the means for l
5、imiting the sending duration of the signalling current willgenerally consist of an arrangement which controls the sending duration independently of the time the ringing key isoperated and which automatically fixes that duration at 2 seconds.Note - In the case of short 2-wire circuits, it may be econ
6、omical to use, by agreement between theAdministrations concerned, a low-frequency signalling current (either between 16 and 25 Hz or 50 Hz).ANNEX A(to Recommendation Q.1)Basic technical clauses of a model specification for the provisionof 500/20-Hz voice-frequency signalling sets(signal transmitters
7、 and receivers) intendedfor manually-operated circuitsA.1 Sending of signalsPower - The signal transmitted shall supply a sinusoidal current at a frequency of 500 Hz 2% interrupted at afrequency of 20 Hz 2%.The effective mean power of the 500/20-Hz current is fixed at 0.5 milliwatt or an absolute po
8、wer level of-3 dBm (with a tolerance of 1 dB) at a zero relative level point.Every precaution should be taken to avoid unbalance effects in the circuit during the transmission of a500/20-Hz signalling current.2 Fascicle VI.1 - Rec. Q.1Reception of signalsSensitivity - The signal receiver shall opera
9、te correctly when the 500/20-Hz current at the input to the signalreceiver is within the following limits:-8.5 + n N + 2.5 + n dBwhere n is the relative power level at the point of the circuit at which the signal receiver is connected.The limits take account of the tolerances indicated above for the
10、 transmitted power level and include a marginof 4.5 decibels on the nominal absolute power level of the 500/20 Hz current received at the input to the signalreceiver. This margin allows for variations in transmission conditions on international circuits.Tuning - Tuning should be such that the signal
11、 receiver operates only at a frequency of 500 Hz guaranteed towithin 2% and at an interrupting frequency of 20 Hz guaranteed to within 2%.Delay - The delay, i.e. the time which elapses between the application of the signalling voltage and theoperation of the signal receiver, must be long enough for
12、the signal receiver to remain insensitive to all speech currentswhich normally flow in the circuit to which it is connected. The duration of this delay must, however, be less than1200 milliseconds. (In other words, 1200 milliseconds is the maximum signal recognition time within which a signalhas to
13、be recognized.)Selectivity (resulting from the tuning of the resonant circuit and the delay mentioned above) - The receipt of aspeech (or noise) current circulating in the circuit must not give rise to a current liable to cause the operation of thesignalling equipment and, in consequence, to cause a
14、 wrong indication to be given on the international positions eventhough the speech (or noise) voltage reaches the maximum value likely to be met in practice. In particular, the signalreceiver must not operate when a speech power not exceeding 6 milliwatts is applied at a zero relative level point.In
15、sertion loss - The insertion loss introduced by the signal receiver in the circuit with which the signalling setis associated must be less than 0.3 dB for any frequency effectively transmitted by the circuit.Reference1 CCITT Instructions for the international telephone service (1 October 1985), ITU, Geneva, 1985.
