1、INTERNATIONAL TELECOMMUNICATION UNION)45G134 2 TELECOMMUNICATIONSTANDARDIZATION SECTOROF ITU4%,%2!0(94%,%2!0(G0G042!.3-)33)/.53%G0G0/amended at New Delhi, 1960 and Mar del Plata, 1968)1 Composition and nomenclatureFigure 1/R.77 illustrates the composition of an international voice-frequency telegrap
2、h (VFT) system and thenomenclature used.2 The international voice-frequency telegraph system2.1 This is the whole of the assembly of apparatus and lines, including the terminal VFT equipment. InFigure 1/R.77 the system illustrated provides 24 duplex international telegraph circuits but other numbers
3、 of telegraphcircuits can be provided.2.2 The international VFT bearer circuit2.2.1 Four-wire telephone-type circuits are used as VFT bearer circuits. The circuit comprises two unidirectionaltransmission paths, one for each direction of transmission, between the terminal VFT equipments.2.2.2 The VFT
4、 bearer circuit consists of an international line together with any terminal national sections connectingthe international line to the VFT terminal equipment and may be constituted entirely on carrier channels (on symmetricpair, coaxial pair or radio-relay systems) or an audio-frequency lines or com
5、binations of such lines._1)See also Recommendation M.800 1.2 Fascicle VII.1 - Rec. R.772.2.3 VFT bearer circuits have no terminating units, signalling equipment or echo suppressors.2.3 The international line of a VFT bearer circuit2.3.1 The international line of a VFT bearer circuit may be constitut
6、ed by using a channel in a carrier group orchannels in tandem on a number of groups. National and international sections can be interconnected to set up aninternational line. See Figure 1/R.77 but note that 2.3.2 below details the preferred method. The international linecould equally well be set up
7、between, for example, only A and C or between C and D, in which case A and C, or C andD would be the terminal international centres.2.3.2 Wherever possible an international line for a VFT bearer circuit should be provided on channels of a singlecarrier group, thereby avoiding intermediate audio-freq
8、uency points. In some cases, such a group may not exist or, forspecial routing reasons, it may not be possible to set up the international line in the preferred way. In such cases, theinternational line will consist of channels in tandem on two or more groups with or without audio sections, dependin
9、gon the line available and the routing requirements.2.4 Terminal national sections connected to the international line of a VFT bearer circuitIn many cases the VFT terminal equipment is remote from the terminal international centre of the internationalline (Figure 1/R.77), and such cases necessitate
10、 the provision of terminal national sections in order to establishinternational VFT bearer circuits. These sections may be in short-distance local audio cables, amplified or unamplified,or may be routed in long-distance carrier groups or amplified audio plant as available.3 Reserve arrangements for
11、international VFT bearer circuits3.1 General3.1.1 All necessary action should be taken to enable the duration of interruptions on international VFT bearer circuitsto be reduced to a minimum and, for this purpose, it is expedient to standardize some of the methods to be adopted forreplacing defective
12、 portions of the circuit.3.1.2 Although it does not appear necessary for these methods to be the same in detail in every country, it would beadvisable to reach agreement regarding the general directives to be followed.3.1.3 The make-up of the reserve VFT bearer circuits will in general be similar to
13、 that of the normal VFT bearercircuits. However, if the VFT terminal equipment is not located at the terminal international centres, the line portion ofan international telephone circuit can be used to replace only the international line of the VFT bearer circuit.3.2 Reserve international lines3.2.1
14、 Wherever possible a reserve international line should be provided between the two terminal internationalcentres by means of the international line of an international telephone circuit (between A and B in Figure 1/R.77).3.2.2 The telephone circuit used as a reserve should be chosen wherever possibl
15、e so as to follow a different routefrom that of the normal international line. Where this cannot be done, as much as possible of the circuit or its sectionsshould be alternatively routed.3.2.3 If there is a choice, the use of manually-operated circuits as reserve lines for VFT is technically andoper
16、ationally preferable to the use of automatic circuits. It should be possible after prior agreement between thecontrolling officers at the international terminal exchanges concerned for an operator to break into a call in progress toadvise the correspondents that the circuit is required and that the
17、call should be transferred to another circuit if it lastslonger than six minutes.3.2.4 If the reserve telephone circuit is automatic or semi-automatic a direct indication should be given at thechangeover point. If it is not available when needed the reserve circuit should be blocked against any furt
18、her call.3.3 Reserve sections for the sections of the international VFT bearer circuit3.3.1 Where it is not possible to provide reserve international circuits either because there are no suitable telephonecircuits or because the number of telephone circuits does not permit the release of a circuit f
19、or reserve purposes, reservesections should be provided wherever possible for each of the component sections. For these sections, national orinternational telephone lines or, where they exist, spare channels, circuits, etc., should be used.Fascicle VII.1 - Rec. R.77 33.4 Reserve arrangements for the
20、 terminal national sections connecting the VFT terminal equipment to theinternational line3.4.1 Reserve sections should be provided by means of national telephone circuits or by the use of spare channels,particularly in the case of long sections and of sections forming part of a category B VFT beare
21、r circuit (see 2).3.5 Changeover arrangements from normal to reserve lines3.5.1 When an international telephone line (i.e. part of an international telephone circuit) is used to provide a reservefor the international line (or for one of its sections as mentioned in 3.3 above), there should be change
22、overarrangements to enable the changeover from the normal line to the reserve line to be made as rapidly as possible. Thechangeover arrangements (Figure 2/R.77) should be such that on changeover, all signalling equipment, echosuppressors, etc., associated with the telephone circuit that is used as a
23、 reserve for the international line, aredisconnected on the line side. When the fault is cleared on the normal line, it should be possible to join it to thesignalling equipment, echo suppressors, etc., and put it into service as part of the telephone circuit until the agreed timefor the restoration
24、of the line to the normal routing. It is desirable to introduce as little disturbance as possible whenchanging back from reserve to normal. Arrangements of cords and parallel jacks can be devised to achieve this.3.5.2 The changeover arrangements shown in Figure 2/R.77 could be applied to sections of
25、 the international linementioned under 3.3 above when it is not possible to obtain an overall reserve for the international line. Normalsections and the corresponding reserve sections should be routed via suitable changeover arrangements at the stationsconcerned.3.5.3 Should the alarm indicating tha
26、t the VFT bearer circuit is faulty be received by a station other than the groupcontrol station, this other station shall interrupt the return direction of the alarm channel towards the group controlstation in order to advise the latter to take the necessary action.3.5.4 Making manual, automatic or
27、semi-automatic international telephone circuits available for reserve circuits forvoice-frequency telegraphy should be in accordance with the instructions issued and the arrangements made by therespective Administrations. Should the normal and reserve lines both be faulty, the technical services of
28、theAdministration concerned should take immediate joint action to find a temporary remedy.4 Fascicle VII.1 - Rec. R.773.6 Designation and marking3.6.1 Normal and reserve circuits, etc., should be clearly distinguishable from other circuits both from the point ofview of designation (see Recommendatio
29、n M.140 3) and marking (see Recommendation M.810 4).References1 CCITT Recommendation Use of circuits for voice-frequency telegraphy, Rec. M.800.2 CCITT. White Book, Preface to Vol. IV, ITU, Geneva, 1969.3 CCITT Recommendation Designation of international circuits, groups, etc., Rec. M.140.4 CCITT Recommendation Setting-up and lining-up an international voice-frequency telegraph link for publictelegraph circuits (for 50, 100 and 200 baud modulation rates), Rec. M.810.
