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2、0RecommendationG0G0- (Extract from the “LUEG0“OOK)NOTES1 ITU-T Recommendation M.910 was published in Fascicle IV.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 fil
3、e are identical to the Blue Book version and copyright conditions remain unchanged (see below).2 In this Recommendation, the expression “Administration” is used for conciseness to indicate both atelecommunication administration and a recognized operating agency. ITU 1988, 1993All rights reserved. No
4、 part of this publication may be reproduced or utilized in any form or by any means, electronic ormechanical, including photocopying and microfilm, without permission in writing from the ITU.Fascicle IV.2 - Rec. M.910 1Recommendation M.910Fascicle IV.2 - Rec. M.910SETTING UP AND LINING UP AN INTERNA
5、TIONAL LEASED GROUP LINKFOR WIDE-SPECTRUM SIGNAL TRANSMISSION1 General1.1 The international leased group links in this Recommendation relate to corrected group links in theRecommendation cited in 1.1.2 The composition of a leased group link and the terminology used for maintenance purposes is given
6、inRecommendation M.900.1.3 The procedure for setting up an international leased group link should as far as possible follow the principlesgiven in Recommendation M.460 2.1.4 For the purpose of this Recommendation, the constitution and subsequent line-up and maintenance practiceassumes that the group
7、 link between renters premises is operated throughout in the frequency range 60-108 kHz.1.5 Where, as in some cases, modems are fitted at the terminal national centres the group link is defined as existingbetween defined access points at these centres.In such cases the terminal national section is t
8、reated for lining-up and maintenance purposes as a separatesection and not part of the group link as defined in Recommendation M.900.1.6 In some cases, where the wide-spectrum transmission equipment located at the renters premises is notfrequency band restricted, it may be found necessary to include
9、 a through-group filter at the terminal national centre inthe transmitting direction of transmission, in order to prevent interference by the wide-spectrum signals into adjacentgroups in the carrier systems, over which the group is routed.Also, when measuring at the terminal national centre in the r
10、eceiving direction of transmission, a through-groupfilter may be necessary in the measuring circuit, in order to prevent signals from adjacent groups affecting themeasurement results.2 Setting-up of an international leased group link2.1 National and international main sectionWith the exception of th
11、e terminal national section, the provisions of Recommendation M.460 2 shall apply tothe setting-up and the interconnection of the group sections constituting the national and international main section.2.2 Terminal national sectionsBecause of the particular arrangements adopted for providing these s
12、ections within the country concerned, thesetting-up of such sections will follow the practice determined within the country concerned.2.3 Application of the group reference pilotThe application of a group reference pilot (preferably 104.08 kHz), whether injected into the group path at therenters pre
13、mises or whether it is injected at the terminal national centre, should conform to the requirements ofRecommendation M.460 2.3 Lining up an international leased group link3.1 Lining up the national and international main sections3.1.1 The reference test frequency to be used should be 84 kHz.3.1.2 Th
14、e lining-up of these sections should follow the procedure and method given in the Recommendation cited in3.3.1.3 The national main sections may be lined up separately from the international main sections since nointernational cooperation is needed.2 Fascicle IV.2 - Rec. M.9103.1.4 The limits given i
15、n Table 2/M.460 4 should apply to these main sections. In addition, the group-delaydistortion of the national and international sections should be measured and the results recorded.3.2 Terminal national sectionsThe lining-up of these sections will follow the national practice of the country concerne
16、d.3.3 Interconnection of terminal national sections and national main sectionThe levels and impedances in the frequency band concerned of the terminal national sections and the nationalmain section at the terminal national centre should be made compatible with the levels and impedances specified for
17、 theaccess point at this centre.3.4 Overall line-up of the linkWhen the national and international main sections have been lined up and interconnected using the necessarythrough-group equipment, measurements should be made between the terminal access points, either at the renterspremises or in excep
18、tional cases at the terminal national centres.In addition to level measurements the group-delay distortion within the frequency band 68-100 kHz should bemeasured and the values relative to the minimum group-delay distortion within the band should be recorded forsubsequent maintenance use. If necessa
19、ry, group-delay equalizers have to be inserted into the link where appropriate.The procedure and method to be used for the line-up should follow that given in Recommendation M.460 2,but the limits to be achieved are those given below.3.4.1 Overall loss at the reference frequencyThe overall loss at t
20、he reference frequency between the renters premises cannot normally be specified becauseof the freedom accorded to Administrations to adopt nominal relative levels which is their national or agency practice.If, however, it is necessary to specify a particular value of overall loss as a result of a r
21、equest by a renter thismay be done only after prior consultation and agreement between the Administrations concerned.3.4.2 Loss/frequency distortionThe loss/frequency distortion of the overall link is shown in Figure 1/M.910. It should be measured over thefrequency range 60-108 kHz and equalized wit
22、h a group link equalizer as necessary to meet the limits with respect tothe loss at 84 kHz.Note 1 If the service channel is provided, additional equalization may be needed and there will be nopossibility of employing simplified through-group filters.Note 2 84 kHz is the reference frequency for the p
23、urposes of specifying and measuring attenuation distortion.The group reference pilot at 104.08 kHz may still be used as the regulating pilot, however, as required.Fascicle IV.2 - Rec. M.910 33.4.3 Group-delay distortion1)3.4.3.1 The group-delay distortion of the link should not exceed 45 s relative
24、to the minimum value within the bandof frequencies 68-100 kHz.3.4.3.2 If the group-delay distortion exceeds the value given in 3.4.3.1 above, equalization should be provided asagreed by the two terminal Administrations concerned to bring the group-delay distortion of the link within this valueand th
25、e results recorded.3.4.3.3 Where the group link terminates at the two terminal national centres, the value of the group-delay distortiongiven in 3.4.3.1 above should apply between these two centres.3.4.4 Level variationsIrrespective of whether the group link terminates at the two renters premises co
26、ncerned, or at the two terminalnational centres the link should be checked in accordance with the Recommendation cited in 5 in order to ensure thatno faults exist. The following limits should not be exceeded: short-term variations: 3 dB, long-term variations: 4 dB, relative to the nominal value._1)T
27、his limit can normally be met without overall link equalization for group links consisting of three group sections in tandemusing corrected through-groupe connection equipment.4 Fascicle IV.2 - Rec. M.9103.4.5 Carrier leakThe group link should be subjected to measurement of each carrier leak individ
28、ually at the receiving terminal inboth directions of transmission.The objective for the level of any carrier leak, appearing in the frequency band 60-108 kHz is 40 dBm0.In some cases, however, because of the composition of the link, which will generally involve the use of bothold and new types of eq
29、uipment it may not be possible to achieve this value.At all events, no carrier leak in the band 60-108 kHz should exceed 35 dBm0.Note The attention of users is drawn to the fact that failure to reach the value 40 dBm0 might causedifficulties in cases where links are used for data transmission.3.4.6
30、Impulsive noiseFor the specification of an impulsive-noise measuring instrument for wideband data transmissions, seeRecommendation H.16 (O.72) 6. No limit value can be given at the present time.3.4.7 Frequency errorThe frequency error over the group link should not exceed 5 Hz. When this measurement
31、 is necessary, it shouldbe made according to bilateral agreement between Administrations.3.4.8 Background noiseAt the present time it is not possible to specify a limit value for background noise for this class of group link.However, a check of the background noise should be made and recorded at eve
32、ry line-up.References1 CCITT Recommendation Characteristics of group links for the transmission of wide-spectrum signals, Vol. III,Rec. H.14, 2.2 CCITT Recommendation Bringing international group, supergroup, etc., links into service, Vol IV,Rec. M.460.3 Ibid., 7.2.4 Ibid., Table 2/M.460.5 Ibid., 8.6 CCITT Recommendation Characteristics of an impulsive-noise measuring instrument for wideband datatransmission, Vol. III, Rec. H.16.
