1、INTERNATIONAL TELECOMMUNICATION UNION)45G134 TELECOMMUNICATIONSTANDARDIZATION SECTOROF ITU%.%2!,G0G0!30%#43G0G0/b) have a limited life and application;c) already be available in a number of realizations,recommendsthat FDM codecs should conform with the following requirements:1 GeneralThis Recommenda
2、tion gives details of the analogue interfaces, the overall analogue-to-analogue performance ofa coder/decoder pair and certain details of the digital interfaces to which FDM codecs should conform. The CCITT doesnot recommend any particular relationship between FDM assemblies and the digital hierarch
3、ies to be used in any codecrealization nor does it recommend any particular frame structure or encoding law. Administrations intending to usecodecs in their networks should ensure that compatible designs of codec are used at each end of a link. For internationallinks, the codecs to be used should be
4、 by the agreement of the Administrations concerned. An Annex to thisRecommendation gives details supplied by some Administrations of a number of FDM codec realizations.The application of FDM codecs in a network is explained in Supplement No. 28.2 Analogue interfaces2.1 FDM assembliesThe constitution
5、 of the FDM assemblies at the analogue input and output should conform toRecommendation G.211, Figure 1 a)/G.211, for the basic group and Recommendation G.233, Figures 1/G.233 through5/G.233, as appropriate for the basic supergroup, mastergroup, supermastergroup and 15 supergroup assemblies.2.2 Impe
6、dances and relative levelsThe impedances and relative levels at the analogue transmission ports should be as indicated inRecommendation G.233, 3 through 6.2.3 Return lossThe return loss against the nominal impedance of all analogue transmission ports should be at least 20 dB in thewanted frequency b
7、and. This limit relates to the intrinsic return loss, i.e. that is obtained when the cords connecting themeasuring apparatus to the equipment are as short as possible. In view of the station cabling encountered in practice, thereturn loss recorded at the distribution frame of groups, supergroups, et
8、c., may differ from the intrinsic return loss. Thisfactor should be taken into account in designing and making links.2.4 Accuracy of carrier frequenciesDesigners of FDM codecs may find it expedient to translate the analogue signal frequency before coding and2 Fascicle III.4 - Rec. G.795after decodin
9、g. The accuracy of any carrier frequencies used should conform to Recommendation G.225. It is possible tolock the carriers to the digital signal so that no overall frequency error is caused by the FDM codecs.3 Digital interfacesDigital interfaces should conform to the appropriate sections of Recomme
10、ndation G.703.4 Encoding law and frame structureAt present the CCITT does not recommend any particular encoding law or frame structure. In some instances itmay not be technically or economically feasible to encode one standardized FDM assembly into one standardizedhierarchical bit rate. In these cas
11、es it is possible that more than one encoded FDM assembly or an encoded FDMassembly and lower order hierarchical bit streams may be combined to form one standardized hierarchical bit rateconforming to Recommendation G.703. Where one or several encoded FDM assemblies are combined with some lowerorder
12、 hierarchical bit streams, then the multiplexing techniques used must be plesiochronous.5 Analogue performanceThe analogue performance is recommended in terms of the overall performance of a coder/decoder pair.5.1 NoiseA maximum value of 800 pW0p is recommended. In practice, this magnitude of noise
13、is expected to occur onlyon codecs for the higher order FDM assemblies and that significantly lower values will be achieved with codecs for thesmaller FDM assemblies (see the Annex to this Recommendation). The use of FDM codecs on comparatively shorttransmission paths becomes possible when lower lev
14、els of codec noise are achieved. The recommended allowance ofnoise is intended to take account of all sources of noise, i.e. noise due to:a) analogue processing before the coder and following the decoder,b) quantization,c) errors and jitter on the received digital signal as indicated in the G.900 se
15、ries of Recommendations.Noise should be measured in accordance with Recommendation G.230 under the loading conditions given inRecommendation G.222, 4, for the particular FDM assembly used (see Note).Note - The contribution to this noise made by errors on the digital transmission path is likely to be
16、 small. Theeffect of errors is to give rise to impulsive type interference and its expression in pW0p depends upon the statistics of theerror distribution. However, for design purposes, it should be assumed that errors occurring on the digital line systemhave a Poisson distribution with a long-term
17、mean error ratio of 10-7.5.2 Performance under conditions of light loadingUnder conditions of light loading, the quantizing distortion caused by a discrete tone (e.g. a test tone orsignalling frequency) may give rise to a structured noise spectrum containing components considerably in excess of thea
18、verage quantizing distortion level per channel. However, in practice, the presence of a small number of systemreference pilots and carrier leaks is sufficient to maintain an adequately uniform noise spectral distribution.5.3 Overload pointShould be as given in Recommendation G.233, 6 (see Note).Note
19、 - A higher loading is appropriate if digital speech interpolation techniques or 3 kHz spaced channels(Recommendation G.235) are used.5.4 Frequency responseThe amplitude/frequency response, the ratio between wanted and unwanted components and the group delayFascicle III.4 - Rec. G.795 3distortion re
20、commended is that given in Recommendation G.242 for through connections of the relevant FDMassemblies. This performance will be adequate to allow direct connection of the FDM codec analogue ports to the lowfrequency side of following translating equipment. However, if the analogue ports of the FDM c
21、odec are to be directlyconnected to the high frequency side of translating equipment, then the performance required of the FDM codec mayappropriately be that performance normally required by the Administration of its translating equipment.5.5 Go-return crosstalkThe go-return crosstalk ratio should n
22、ot be worse than 80 dB.This level of crosstalk may be difficult to measure because of the digital processing in the transmission path. Itmay be necessary to add to the disturbed path a low level activating signal (a sine wave or band limited white noise) toavoid gain enhancement effects.5.6 Unwanted
23、 modulation by harmonics of the power supply and other low frequenciesThe combined effect of a coder/decoder pair should correspond to a minimum side component attenuation of 57dB (Recommendation G.229).5.7 Phase jitterThe phase jitter on a signal caused by a coder/decoder pair should not exceed 1 p
24、eak-to-peak when measuredin the frequency band given in Recommendation G.229, 2.Note - The value quoted above is indicated as guidance for design purposes. In practical applications, the codecshould tolerate the jitter of the digital interfaces as specified in Recommendations G.823 and G.824.6 Fault
25、 conditions and consequent actionsThe decoder should detect:a) loss of frame alignment;b) loss of digital input signal;c) the presence of Alarm Indication Signal (AIS) on the digital input port.For all these conditions, the analogue output signal should be suppressed.Note - Other conditions and cons
26、equent actions are under study.4 Fascicle III.4 - Rec. G.795ANNEX A(to Recommendation G.795)FDM codecsAdministration Analogue interface Digital interface Noise performanceBritish Telecom Supergroup(312-552 kHz)8 448 kbit/s 140 pW0pBritish Telecom 15 SG assembly(312-4025 kHz)68 736 kbit/s 700 pW0pChina Mastergroup(812-2044 kHzor 60-1300 kHz)34 368 kbit/s 783 pW0pNTT Group(60-108 kHz)1 544 kbit/s 340 pW0p
