1、INTERNATIONAL TELECOMMUNICATION UNION)45G134 1 TELECOMMUNICATIONSTANDARDIZATION SECTOROF ITU%.%2!,G0G02%#/-%.$!4)/.3G0G0/.G0G04%,%0(/.%37)4#().G0G0!.$G0G03).!,).).4%2.!4)/.!,G0G0!54/-!4)#G0G0!.$3%-)G13!54/-!4)#G0G07/2+).4%#(.)#!,G0G0b) when pressing the buttons after an international call has been s
2、et up, the signalling frequencies for push-button sets may cause interference to foreign signalling systems on the connection. However, thesubscriber can be warned of the possible disadvantages of touching the buttons in conditions differentfrom those prescribed.2 There can be no doubt that, owing t
3、o the high dialling speed which can be obtained with push-button sets,their use is bound to spread widely and rapidly and it is desirable for the signalling methods for such sets to beinternationally standardized.One factor in favour of such standardization is the advantage it offers for countries w
4、hich have to import theirequipments from various other countries. This argument, admittedly, applies to any type of telephone equipment.Other advantages of standardization are:- the possibility of using the push-button of such sets for signalling directly from one subscriber to anothersubscriber via
5、 a national and/or international connection;- the standardized allocation of signalling frequencies for push-button sets facilitates the choice ofsignalling frequencies in the frequency band of a telephone circuit for any other use (data transmission,telephone signalling system, etc.) for which prov
6、ision might have to be made. The risk of mutualinterference among the signalling systems (see Recommendation Q.25) makes it necessary to have anorderly arrangement of the spectrum of frequencies used for signalling.3 The general use of push-button sets for purposes other than telephone dialling is e
7、nvisaged by someAdministrations. However, some Administrations observe that it would seem advisable to reserve such uses for anetwork of relatively limited extent; in their view the reliability of standards for data transmission should not make anydemands on the push-button set system other than tho
8、se required for the transmission of telephone numericalinformation to the local exchange, if the design of push-button sets is to remain within economical limits compatiblewith their widespread use.However, the CCITT considered, at Mar del Plata in 1968 that, even if the transmission of data from a
9、push-button telephone set is at present to be envisaged in international traffic on a limited scale only, it would nonetheless bewise not to rule out the possibility of such transmission of data on a general scale.4 In choosing a signalling system for push-button sets, Administrations may be guided
10、by conditions whichvary considerably from one country to another. Economic considerations may, for instance, lead them to prefer a directcurrent system which might be less expensive than a voice-frequency system. The numerical dialling informationwould then be transmitted only as far as the telephon
11、e exchange to which the subscriber is connected. There are notones that could affect the connection after its establishment. Data would not be transmitted from the push-button setsunless a suitable converter were used in the exchange.Standardization of a direct current system for signalling from pus
12、h-button sets does not seem justified at theinternational level; it may depend on the conditions peculiar to the local networks of the country concerned.5 The signalling system for push-button sets recommended by the CCITT applies solely to voice-frequencysignals.2 Fascicle VI.1 - Rec. Q.23A multifr
13、equency code for such signalling is recommended in which the dialling signal is composed of twofrequencies emitted simultaneously when a button is pressed. It is planned to have 10 decimal digits and 6 reservesignals, making 16 signals in all. The two frequencies composing each signal are taken from
14、 two mutually exclusivefrequency groups of four frequencies each, a code known as the “2 (1/4) code“.6 The low group frequencies of this 2 (1/4) code are:697, 770, 852, 941 Hz.The high group frequencies are:1209, 1336, 1477 and 1633 Hz.The allocation of frequencies to the various digits and symbols
15、of a push-button set appears in Figure 1/Q.23.7 The frequency variation tolerances and the permissible intermodulation products are defined as follows:7.1 each transmitted frequency must be within 1.8% of the nominal frequency;7.2 the total distortion products (resulting from harmonics or intermodul
16、ation) must be at least 20 dB below thefundamental frequencies.8 The CCITT determined, at Mar del Plata in 1968, that it was not practicable to specify a standardization ofthe levels for the frequencies transmitted when a push-button is pressed, as these level conditions depend essentially onnational transmission plans which are not the same in all countries.However, the sending level conditions must be such that on an international connection they do not exceed thevalues specified in Recommendation Q.16 (maximum permissible value for the absolute power level of a signallingpulse).
