1、INTERNATIONAL TELECOMMUNICATION UNION)45G134 TELECOMMUNICATIONSTANDARDIZATION SECTOROF ITU).4%2.!4)/.!,G0G0!.!,/5%G0G0#!22)%2G0G03934%-3).$)6)$5!,G0G0#(!2!#4%2)34)#3G0G0/ G0 G0 G9G03934%-3=)45G134G0G0RecommendationG0G0 (Extract from the “LUEG0“OOK)NOTES1 ITU-T Recommendation G.325 was published in F
2、ascicle III.2 of the Blue Book. This file is an extract fromthe Blue Book. While the presentation and layout of the text might be slightly different from the Blue Book version, thecontents of the file are identical to the Blue Book version and copyright conditions remain unchanged (see below).2 In t
3、his Recommendation, the expression “Administration” is used for conciseness to indicate both atelecommunication administration and a recognized operating agency. ITU 1988, 1993All rights reserved. No part of this publication may be reproduced or utilized in any form or by any means, electronic ormec
4、hanical, including photocopying and microfilm, without permission in writing from the ITU.Fascicle III.2 - Rec. G.325 1Recommendation G.325Fascicle III.2 - Rec. G.325GENERAL CHARACTERISTICS RECOMMENDED FOR SYSTEMSPROVIDING 12 TELEPHONE CARRIER CIRCUITS ON ASYMMETRIC CABLE PAIR (12 + 12) SYSTEMSSyste
5、ms of the (12 + 12) type on symmetric pair in cable are used for carrier working either on old deloadedcables or on cables specially constructed for the purpose (without a second cable being required). These systems may beused in regional or local relations, or in long-distance relations, trunk or i
6、nternational.This Recommendation applies to systems for long-distance relations making use of the kinds of cable at presentrecommended by the CCITT (see Recommendation G.611) and to multiple-twin quad cables with conductors of 0.9 mmdiameter, with an effective capacitance of 35 to 40 nF/km or other
7、kinds of deloaded cables of equivalent quality. Forsystems used for local or regional relations, some clauses of the present Recommendation may be made less stringent.1 Frequency spectrum transmitted to lineThe CCITT recommends that the line-frequency spectrum should be in accordance with Scheme 1 o
8、r 2 ofFigure 1/G.325.FIGURE 1/G.325Arrangement of line-transmitted frequenciesfor international (12 + 12) cable systemsAdministrations concerned in setting up such an international system should agree to use either one or the otherof the two schemes.2 Line-regulating pilotsThe following frequencies
9、are recommended: with Scheme 1: 60 kHz and 72 kHz; with Scheme 2: 54 kHz and 60 kHz.2 Fascicle III.2 - Rec. G.325The recommended accuracy is 1 Hz for the 60-kHz pilot. The frequency tolerance for other pilots will bedecided by agreement between the Administrations concerned.All these pilots should b
10、e transmitted at power level of -15 dBm0.3 Hypothetical reference circuit for (12 + 12) symmetric-pair systemThis is 2500 kilometres long, and for each direction of transmission comprises a total of: three channel translation pairs; nine special translation pairs translating a basic group into the b
11、and transmitted to line, and vice versa.This circuit is carried on a (12 + 12) symmetric-pair system in cable, with pairs assumed to be of conductors of0.9-mm diameter, with an effective capacitance of 35 to 40 nF/km.Figure 2/G.325 shows one of the three identical parts of which this hypothetical re
12、ference circuit is made up.All in all, it has 18 homogeneous sections, each 140 kilometres long.FIGURE 2/G.325Basic diagram of one-third of the hypothetical reference circuitfor symmetric pair (12 + 12) systemsNote 1 - There are only half as many translation pairs as there are homogeneous sections,
13、because one of thetwo bands transmitted to line corresponds to a basic group (see Figure 2/G.325).Note 2 - With systems using frequency-frogging in the repeaters, the appropriate modulators form part of thehigh-frequency line.4 Design objectives for circuit noiseThe objectives set forth in Recommend
14、ation G.222 apply to the hypothetical reference circuit for symmetric-pair (12 + 12) systems, in the circumstances described in Recommendation G.223.Fascicle III.2 - Rec. G.325 3In practice, it will suffice to check by calculation that the mean psophometric power at the end of everytelephone channel
15、 as defined by the hypothetical reference circuit, at zero relative level, does not exceed 10 000 pW0pduring any hour.Provisionally, it is recommended that this overall limit be apportioned between the total noise components asfollows: line noise (including noise due to special translation equipment
16、) 9000 pW0p noise due to channel translating equipment . 1000 pW0pApportionment of total noise inherent in the system among: basic noise, intermodulation noise, noise due to crosstalk,is left entirely to the discretion of the carrier system designer, up to 1000 pW0p for channel translating equipment
17、 and9000 pW0p for the line.Note - In accordance with all recommendations on cable systems in the Series G Recommendations, the designobjective as regards noise power does not take into consideration noise from external sources; it is assumed that this isnegligible compared with the figure of 10 000
18、pW0p.With regard to real circuits, Administrations must take whatever steps are required in each individual case toensure that clicks arising on audio-frequency pairs in the same cable as a (12 + 12) system and transmitted by crosstalkdo not create excessive noise on the circuits of that system whic
19、h may be used for international communications.5 Error on the reconstituted frequencyThe difference between a frequency sent at the origin of a homogeneous section 140 km long (see 3 aboveand Figure 2/G.325) and the frequency received at the end of that section, should not exceed a figure provisiona
20、lly fixedat 0.3 Hz; this figure is the same whether there is frequency-frogging in the intermediate repeaters or not.6 Direct line interconnectionWhen Administrations desire the direct line interconnection of two systems (with, of course, the same allocationof line-transmitted frequencies) it is rec
21、ommended that each of these systems should meet the following requirements onthe interconnection section (except where agreed otherwise between the Administrations concerned):1) Relative level per channel, at all frequencies, at the output of the frontier repeaters: -15 dBr1).2) Attenuation of the f
22、rontier elementary cable section at the highest frequency transmitted to line: 25 dB1).Note - For composite cables, agreement should be reached between the two Administrations concernedto fix the attenuation of the frontier section in such a way that the repeaters of the symmetric pairs andthose of
23、the coaxial cables can be housed in the same frontier stations.3) Matching of the impedances of the frontier repeaters and the line. The modulus of the return-currentcoefficient between the input (or output) impedance of a repeater and the characteristic impedance of theline should not exceed the lo
24、wer of the two values:_1)These values apply to low-gain systems. They are not valid for high-gain systems, i.e. for systems whose gain issubstantially above 30 dB.4 Fascicle III.2 - Rec. G.3257 Interconnection in a main stationIf such interconnection is necessary, either for operating reasons or bec
25、ause the two systems to beinterconnected use different allocations of frequencies transmitted to line, one of the following procedures may befollowed:1) interconnection at a group distribution frame, with use of the basic group, levels and impedance appliednormally by the Administration to which the
26、 frame belongs;2) direct interconnection between the two systems. If they use different allocations of frequencies transmittedto line, the two Administrations concerned shall reach agreement on which of them shall install thenecessary demodulators (the line of separation between the two types of equ
27、ipment will then be CC orDD on Figure 3/G.325).FIGURE 3/G.325Direct interconnection of two (12 + 12) systems using different allocationsof frequencies transmitted to lineIn the absence of such an agreement, each incoming system must comprise equipment required for the outgoingsystem, in each directi
28、on of transmission (the separating line in Figure 3/G.325 would then be the oblique DC).Unless there is a specific agreement, the relative power level will be -36 dBr at sending (input of each system -points C and D in the case of Figure 3/G.325). The points considered do not correspond to points T
29、and T defined inRecommendation G.213. In particular, a translating equipment of any type cannot be connected to it withoutprecautionary measures (see the levels indicated in the Table 1/G.233).Fascicle III.2 - Rec. G.325 5By agreement between Administrations, interconnection can be effected as indic
30、ated in Figure 4/G.325, amethod whereby it is possible to replace three modulators by one.FIGURE 4/G.325Method of interconnection that can be used by agreement between Administrations(the numbers on this diagram show frequencies in kHz; A and B indicate the basic group concerned)8 Essential clauses for a model specificationSee Recommendation G.326.
