1、INTERNATIONAL TELECOMMUNICATION UNION)45G134 TELECOMMUNICATIONSTANDARDIZATION SECTOROF ITU).4%2.!4)/.!,G0G0!.!,/5%G0G0#!22)%2G0G03934%-3%.%2!,G0#(!2!#4%2)34)#3G0G0#/-/.G04/G0G0!,!.!,/5%G0#!22)%2G1342!.3-)33)/.G0G03934%-3!335-04)/.3G0G0 further amended)1 Nominal mean power during the busy hourTo simp
2、lify calculations when designing carrier systems on cables or radio links, the CCITT has adopted aconventional value to represent the mean absolute power level (at a zero relative level point) of the speech plussignalling currents, etc., transmitted over a telephone channel in one direction of trans
3、mission during the busy hour.The value adopted for this mean absolute power level corrected to a zero relative level point is -15 dBm0 (meanpower = 31.6 microwatts); this is the mean with time and the mean for a large batch of circuits.Note 1 - This conventional value was adopted by the CCIF in 1956
4、 after a series of measurements andcalculations had been carried out by various Administrations between 1953 and 1955. The documentation assembled atthe time is indicated. in 1. The adopted value of about 32 microwatts was based on the following assumptions:i) mean power of 10 microwatts for all sig
5、nalling and tones (Recommendation Q.15 2, givesinformation concerning the apportionment on an energy basis of signals and tones);ii) mean power of 22 microwatts for other currents, namely:- speech currents, including echoes, assuming a mean activity factor of 0.25 for one telephonechannel in one dir
6、ection of transmission;- carrier leaks (see Recommendations G.232, 5; G.233, 11; G.235, 5); and theRecommendations cited in 3 and 4;- telegraph signals, assuming that few telephone channels are used for VF telegraphy systems(output signal power 135 microwatts (the Recommendation cited in 5) or photo
7、telegraphy(amplitude modulated signal with a maximum signal power of about 1 milliwatt (theRecommendation cited in 6).On the other hand, the power of pilots in the load of modern carrier systems has been treated as negligible.The reference to “the busy hour“ in 1 is to indicate that the limit (of -1
8、5 dBm0) applies when transmissionsystems and telephone exchanges are at their busiest so that the various factors concerning occupancy and activity of thevarious services and signals are to be those appropriate to such busy conditions.It is not intended to suggest that an integrating period of one h
9、our may be used in the specification of the signalsemitted by individual devices connected to transmission systems. This could lead to insupportably high short-term powerlevels being permitted which give rise to interference for durations of significance to telephony and other services.Note 2 - The
10、question of reconsidering the assumptions leading to this conventional value arose in 1968 for thefollowing reasons:- changes in the r.m.s. power of speech signals, due to the use of more modern telephone sets, to adifferent transmission plan, and perhaps also to some change in subscriber habits;- c
11、hange in the mean activity factor of a telephone channel due, inter alia, to different operatingmethods;- increase in the number of VF telegraphy bearer circuits and sound-programme circuits;- introduction of circuits used for data transmission, and rapid increase in their number.2 Fascicle III.2 -
12、Rec. G.223During several Study Periods these points have been under study and various Administrations carried outmeasurements of speech signal power and loading of carrier systems. The results are shown in Supplement No. 5. Theseresults indicate that there is no sufficiently firm information to just
13、ify an alteration to the conventional mean value of-15 dBm0 (32 W0) for the long-term mean power level per channel.Indeed, the steps envisaged by Administrations to control and reduce the levels of non-speech signals indicate atendency to limit the effect of the increase in the non-speech services.A
14、s regards the subdivision of the 32 W into 10 W signalling and tones and 22 W speech and echo, carrierleaks, and telegraphy, again there is no evidence which would justify proposals to alter this subdivision.As a general principle, it should always be the objective of Administrations to ensure that
15、the actual loadcarried by transmission systems does not significantly differ from the conventional value assumed in the design of suchsystems.Note 3 - The CCITT has agreed to the following rules concerning the maximum permissible number ofVF telegraph bearer circuits:1) For a 12-channel system, both
16、 the load capacity and the intermodulation requirements are determined bythe statistics of speech; hence there is no reason to limit the number of channels in a 12-channel systemwhich may be used as VF telegraphy bearer channels.2) For a 60-channel system, the load capacity is determined by the stat
17、istics of speech but the intermodulationrequirements for a mixed VF telegraph and speech loading become controlling when the VF telegraphbearers exceed about 30% of the total. Hence it is possible, without change of specifications, to allow upto 20 channels in this system to be used for VF telegraph
18、y.3) For a 120-channel system, about 12% of the total could be allowed for VF telegraph bearers. The numberof reserve circuits for VF telegraphy is excluded from these limits for both 60- and 120-channel systems.The number of channels for these systems should be distributed more or less uniformly th
19、roughout theline-frequency band.4) For systems with 300 or more channels, the CCITT is not yet able to define any specific limit, owing to themany complicated factors such as mean power, peak power, overload capacity, intermodulation, noise-performance and pre-emphasis, which have to be taken into c
20、onsideration.5) For groups and supergroups no conclusion could be obtained. From information available, it would beunwise, without special consideration, to exceed two VF telegraph systems per supergroup in a widebandsystem.6) For transmission systems not exceeding 1000 km the permissible number of
21、telegraph systems may beincreased if the power per telegraph channel is reduced according to Table 1/G.223.A similar table in respect of transmission systems longer than 1000 km cannot be drawn up at this time.There is evidence to suggest that for systems considerably longer than 1000 km a reduction
22、 in telegraphsignal power gives rise to unacceptable levels of telegraph distortion and character error rates.TABLE 1/G.223Total number ofcircuits provided bythe transmissionsystem (N)Approximate number of circuits that may be used for24-channel FM voice frequency telegraph systems withthe indicated
23、 power level/TG channel (dBm0)-22.5 -22.5 -27.0 -28.51260120300 or more122014N/30126042N/10126084N/51260120NFascicle III.2 Rec. G.223 32 Loading for calculation of intermodulation noise2.1 It will be assumed for the calculation of intermodulation noise below the overload point that the multiplexsign
24、al during the busy hour can be represented by a uniform spectrum random noise signal, the mean absolute powerlevel of which, at a zero relative flat level point, is given by the following formulae:andn being the total number of telephone channels in the system and the power of the random noise signa
25、l inmilliwatts.Examples are shown in Table 2/G.223 of the results given by these formulae for some typical values of n.TABLE 2/G.223n 10 log10(dBm0)n 10 log10(dBm0)12243648601203.34.55.25.76.17.32403006009601 8002 70010 8008.89.812.814.817.619.325.3These results apply only to systems without pre-emp
26、hasis and using independent amplifiers for the twodirections of transmission.2.2 For 2-wire systems having common amplifiers for the two directions of transmission (n + n systems), it isnecessary to assume a different conventional loading. When the relative levels are the same for both directions of
27、transmission the conventional load is given by the following formulae:andwhereis defined in 2.1 above and n is the number of channels in each direction of transmission.4 Fascicle III.2 - Rec. G.2232.3 When use is made of a call concentrator having the effect of multiplying the number of circuits est
28、ablished ona system by a coefficient a, for the determination of the conventional load, the number of channels should be multipliedby a and the activity coefficient should remain unchanged (see also Note 5 below). The following formulae then replacethose given in 2.2 above:andn being the total numbe
29、r of telephone channels in the system and the power of the random noise signal inmilliwatts.Note 1 - The mean absolute power level of a uniform-spectrum random noise test signal deduced from theseformulae may be used in calculating the intermodulation noise on a hypothetical reference circuit, when
30、there is nooverloading. It is considered that these formulae give a good approximation in calculating intermodulation noise whenn 60. For small numbers of channels, however, tests with uniform-spectrum random noise are less realistic owing tothe wide difference in the nature of actual and test signa
31、ls.Note 2 - In view of the conventional character of these calculations, it was not considered useful to take intoaccount the power transmitted for programme transmissions over carrier systems. Moreover, the mean value of 0.25 wasassumed for the activity factor of a telephone channel and it was not
32、deemed useful to study any deviations from thismean.Note 3 - Care must be taken in interpreting the results of tests with uniform-spectrum random noise loading,especially in systems in which the dominant noise contribution in certain channels arises from a particular kind ofintermodulation product (
33、e.g. A-B). In such cases, the weighting factor used in relating the performance of the channelto that under real traffic conditions must be carefully determined. The curve given by the transfer function of thenetwork used to define the conventional telephone signal (see Recommendation G.227) may be
34、used in this case todetermine the weighting factor for the wideband signal.Note 4 - The formulae in 2.2 above for (n + n) type 12-channel systems are the same as those given in 2.1above (4-wire systems), assuming that the number of channels is doubled but that there is no correlation between thechan
35、nel activities in each direction of transmission. For the purposes of this assumption, the fact that in an (n + n)system the two directions of transmission of a telephone circuit are not active at the same moment is ignored.Calculations have shown that the resultant error is negligible and in any ca
36、se is on the safe side.Note 5 - The formulae in 2.3 above are only valid in the case when all channels are equipped with callconcentrators. They are not applicable when only some of the channels are equipped with call concentrators, because thedistribution of these channels generally will not be uni
37、form over the band of the multiplex signal.3 Component characteristics and levelsThe values of the characteristics of circuit components and the levels to be used in calculations will be thenominal values.Note - When specifying equipments, a reasonable margin should be allowed for the ageing of comp
38、onents andfor tolerances on levels, supply voltages temperature, etc.4 Psophometric weights and weighting factorFor calculating psophometric power, use should be made of the Table of psophometer weighting for commercialtelephone circuits which is given in Table 4/G.223.Fascicle III.2 Rec. G.223 5If
39、uniform-spectrum random noise is measured in a 3.1-kHz band with a flat attenuation/frequencycharacteristic, the noise level must be reduced by 2.5 dB to obtain the psophometric power level. For another bandwidth,B, the weighting factor will be equal to:When B = 4 kHz, for example, this formula give
40、s a weighting factor of 3.6 dB.5 Calculating noise in modulating (translating) equipments(See also Recommendation G.230.)5.1 For group, supergroup, etc., modulating equipments, in calculating intermodulation noise (below the overloadpoint), the following conventional values, already accepted, will b
41、e assumed for the load at a zero relative level point:- for 12-channel group modulators: 3.3 dBm0;- for 60-channel supergroup modulators: 6.1 dBm0;- for 300-channel mastergroup modulators: 9.8 dBm0.5.2 The mean noise power in channel translating equipments due to interference from channels adjacent
42、to thedisturbed channel will be calculated as follows. In all the terminal equipment of the hypothetical reference circuit thereare six exposures to adjacent-channel disturbance. Five of these disturbing channels will be assumed to carry speech-likeloading signals each having a mean power of 32 W, i
43、.e. an absolute power level of - 15 dBm0 per channel at a zerorelative level point, while the sixth disturbing channel will be assumed to carry telegraphy, phototelegraphy or datatransmission with a conventional loading of 135 W applied at the zero relative level point, i.e. an absolute power of-8.7
44、 dBm0 uniformly distributed over the frequency range 380 to 3220 Hz.The conventional telephony signal defined in Recommendation G.227 may be used to simulate the speechsignals transmitted on the disturbing channels.Note - Limitation of crosstalk caused by channels adjacent to the disturbed channel i
45、s governed by an additionalclause in the channel equipment specification (see Recommendation G.232, 9.2). In addition, the power of signallingpulses is restricted by Recommendation G.224.5.3 In all cases allowance should, of course, be made for thermal noise.6 Overload point of amplifiers, the equiv
46、alent r.m.s. power of the peak of the multiplex signal and themargin against saturation6.1 overload pointThe overload point or overload level of an amplifier is at that value of absolute power level at the output atwhich the absolute power level of the third harmonic increases by 20 dB when the inpu
47、t signal to the amplifier isincreased by 1 dB.This first definition does not apply when the test frequency is so high that the third harmonic frequency fallsoutside the useful bandwidth of the amplifier. The following definition may then be used:Second definition - The overload point or overload lev
48、el of an amplifier is 6 dB higher than the absolute powerlevel in dBm, at the output of the amplifier, of each of two sinusoidal signals of equal amplitude and offrequencies A and B respectively, when these absolute power levels. are so adjusted that an increase of 1 dB inboth of their separate leve
49、ls at the input of the amplifier causes an increase, at the output of the amplifier, of20 dB in the intermodulation product of frequency 2A - B.6 Fascicle III.2 - Rec. G.2236.2 equivalent r.m.s. sine wave power of the peak of a multiplex telephone signalThis is the power of a sinusoidal signal whose amplitude is that of the peak voltage of the multiplex signal.Figure 1/G.223 shows the equivalent peak power level in terms of the number of channels. Up to 1000 channels, it isderived from Curve B, Figure 7 of Reference 7 taking into account th
copyright@ 2008-2019 麦多课文库（www.mydoc123.com）网站版权所有
备案/许可证编号：苏ICP备17064731号-1