1、INTERNATIONAL TELECOMMUNICATION UNION)45G134 TELECOMMUNICATIONSTANDARDIZATION SECTOROF ITU).4%2.!4)/.!,G0G0!.!,/5%G0G0#!22)%2G0G03934%-3%.%2!,G0G0#(!2!#4%2)34)#3G0G0#/-/.G0G04/G0G0!,!.!,/5%G0G0#!22)%2G1342!.3-)33)/.G0G03934%-32%#/-%.$!4)/.3G0G02%,!4).G0G04/G0G04(%!#52!#9G0G0/Recommendation G.361 for
2、 3-channel open-wire systems;Recommendations G.326 and G.327 3 for (12 + 12) cable systems.2 Measure of alignment of the master oscillatorsThe recommendation in 1 above cannot be met without some measure of alignment of the master oscillators atthe various stations in which modulation occurs._1)In f
3、act, the chain considered for these calculations comprised 16 (instead of 12) modulator/demodulator pairs to allow for thepossibility that submarine cables with equipments in conformity with Recommendation G.235 might form part of the chain. Noallowance was made, however, for the effects of Doppler
4、frequency-shift due to inclusion of a non-stationary satellite; values forthis shift are given in CCIR Report 214 2.2 Fascicle III.2 - Rec. G.225Carrier-transmission systems are formed into “partial networks“ extending over the whole or a part of a country.Synchronization of the master oscillators o
5、f a partial network is ordinarily based on national frequency comparisons;international comparisons may be made if necessary.2.1 National frequency comparisonsIt is necessary that, within the same partial network of coaxial carrier systems, the master oscillators in stationswhere frequencies are gen
6、erated should be “coordinated“. This “coordination“ can consist of a control of one oscillatorwith respect to another to give one of the following three conditions:1) synchronization, i.e. identical frequency and fixed phase relationship;2) isochronization, i.e. identical frequency only;3) different
7、ial control to correct differences between the frequencies at intervals.Also, automatic devices can be used to give an alarm if the difference in frequency between the checking pilotand a local oscillator exceeds a certain fixed value.The CCITT has not recommended any particular method of comparing
8、or controlling the master oscillators atdifferent stations, and “routine frequency comparison“ of the master oscillators may be thought sufficient; thiscomparison being followed if necessary by automatic or manual regulation, the master oscillators in each partial networkbeing compared periodically
9、with a national frequency standard, if possible.The routine comparison of the frequencies generated by the master oscillators is made by means of a “frequencycheck pilot“ transmitted to line for this purpose. It is not necessary to compare phases.2.2 International frequency comparisonsThe case may a
10、rise, either of a country that has a national frequency standard with no facilities for distributing itthroughout the country (particularly in an area in which a coaxial carrier system is to be set up), or of a country that hasno national frequency standard. Recommendation M.540 4, describes methods
11、 by which such countries may obtain astandard frequency by radio, or may have a controlled frequency sent over a telephone circuit.References1 CCITT Recommendation Hypothetical reference connections, Vol. III, Rec. G.103, Figure 1/G.103.2 CCITT Report The effects of doppler frequency-shifts and swit
12、ching discontinuities in the fixed satelliteservice, Vol. IV, Report 214, Dubrovnik, 1986.3 CCITT Recommendation Valve-type systems offering 12 telephone carrier circuits on a symmetric cable(12 + 12) systems, Orange Book, Vol. III-1, Rec. G.327, ITU, Geneva, 1977.4 CCITT Recommendation Routine maintenance of carrier and pilot generating equipment, Vol. IV,Rec. M.540.
