1、INTERNATIONAL TELECOMMUNICATION UNION)45G134 - TELECOMMUNICATIONSTANDARDIZATION SECTOROF ITU-!).4%.!.#% G0G0).4%2.!4)/.!,G0G0,%!3%$G0G02/50!.$G0G0350%22/50G0G0,).+353%G0G0/ to simplify the effort necessary for the satisfactory maintenance of the link.2.3.2 Where possible, it is desirable that each n
2、ational and international main section be provided on a single groupor supergroup section.In practice, however, it may not always be possible to satisfy this requirement. Two group or supergroupsections per main section should be considered as the normal limit, to be exceeded in exceptional circumst
3、ances only.2.4 Terminal national sectionsTerminal national sections will usually be provided on plant which differs from that normally used for nationalor international groups or supergroups.In most cases, such terminal sections may be provided on: symmetric pair cable specially provided between the
4、 terminal national centre (repeater station) and therenters premises; existing local line plant possibly involving intermediate installations (for example, telephone exchanges) inthe local network; combinations of the above.The particular routing arrangements and the constitution of such sections ar
5、e determined by the nationalpractice of the country concerned.2.5 Choice of the group position within a supergroupIt is very desirable, when choosing the routing of a group, to avoid the use of groups 1 and 5 as far as possiblebecause of the difficulties which may be experienced in equalizing for gr
6、oup-delay distortion due to the edgecharacteristics of such group sections.3 Provision of group or supergroup reference pilots and automatic regulators3.1 Pilots3.1.1 A group or supergroup reference pilot frequency as recommended by the CCITT (Recommendation M.460 4)should be transmitted on all inte
7、rnational leased links for maintenance and regulation purposes.Following national practice, the pilot may be injected either at the sending modem (as provided for in theRecommendation cited in 5, for example), or at the first repeater station (terminal national centre). When a pilot isinjected at a
8、renters premises, it is recommended that the frequency of the pilot shall be one of those mentioned inRecommendation M.460 4 (preferably 104.080 kHz and 547.920 kHz respectively), and that the pilot signal shallconform in all respects to the requirements in that Recommendation.3.2 Group and supergro
9、up link regulationAn automatic regulator should be provided on an international group or supergroup leased link in order toensure the necessary overall stability of the link.The point of insertion of such a regulator may be at the renters premises or at the terminal national centredepending upon the
10、 particular arrangement of the Administration concerned.4 Fascicle IV.2 - Rec. M.900References1 CCITT Recommendation Characteristics of group links for the transmission of wide-spectrum signals, Vol. III,Rec. H.14.2 CCITT Recommendation Characteristics of supergroup links for the transmission of wid
11、e-spectrum signals,Vol. III, Rec. H.15.3 CCITT Recommendation Definitions concerning international transmission systems, Vol. IV, Rec. M.300.4 CCITT Recommendation Bringing international group, supergroup, etc., links into service, Vol. IV,Rec. M.460.5 CCITT Recommendation Data transmission at 48 kilobits per second using 60-108 kHz group band circuits,Vol. VIII, Rec. V.35, 7.
