1、INTERNATIONAL TELECOMMUNICATION UNION)45G134 - TELECOMMUNICATIONSTANDARDIZATION SECTOROF ITU-!).4%.!.#% ).4%2.!4)/.!,G0G04%,%2!0(G0G03934%-3G0G0!.$0(/4/4%,%2!0(G0G042!.3-)33)/.53%G0G0/&G0G0#)2#5)43G0G0&/26/)#%G13&2%15%.#9G0G04%,%2!0(9)45G134G0G0RecommendationG0G0- (Extract from the “LUEG0“OOK)NOTES1
2、 ITU-T Recommendation M.800 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 file are identical to the Blue Book version and copyright c
3、onditions 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 part of this publication may be reproduced or utilized
4、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.800 1Recommendation M.8001)Fascicle IV.2 - Rec. M.800USE OF CIRCUITS FOR VOICE-FREQUENCY TELEGRAPHY1 Composition and nomenclatureFigure 1/M.80
5、0 illustrates the composition of an international voice-frequency telegraph system and thenomenclature used.1.1 The international voice-frequency telegraph systemThis is the whole of the assembly of apparatus and lines including the terminal voice-frequency telegraphequipment. In Figure 1/M.800 the
6、system illustrated provides 24 duplex telegraph circuits, but other numbers oftelegraph circuits can be provided.1.2 The international voice-frequency telegraph link (sometimes referred to as the bearer circuit)1.2.1 Four-wire telephone-type circuits are used for international voice-frequency telegr
7、aph links. The linkcomprises two unidirectional transmission paths, one for each direction of transmission, between the terminalvoice-frequency telegraph equipments.1.2.2 The international voice-frequency telegraph link consists of an international voice-frequency telegraph linetogether with any ter
8、minal national sections connecting the international telegraph line to the voice-frequency telegraphterminal equipment and may be constituted entirely on carrier channels (on symmetric pairs, coaxial pairs, radio-relaysystems, etc.) or on audio-frequency lines or combinations of such lines.1.2.3 The
9、 nominal links for voice-frequency telegraphy have no terminating units, signalling equipment or echosuppressors.1.3 The international voice-frequency telegraph line1.3.1 The international voice-frequency telegraph line may be constituted by using a channel in a carrier group orchannels in tandem on
10、 a number of groups. National and international sections can be interconnected to set up aninternational voice-frequency telegraph line. See Figure 1/M.800, but note that 1.3.2 below details a preferred method.The international voice-frequency telegraph line could equally well be set up between, for
11、 example, onlyA and C or between C and D, in which case A and C or C and D would be the terminal international centres._1)See also Recommendations R.77 1 and H.21 2.2 Fascicle IV.2 - Rec. M.800d01-scFIGURE 1/M.800.D011.3.2 Wherever possible, an international voice-frequency telegraph line should be
12、provided on a channel of a singlecarrier group, thereby avoiding intermediate audio-frequency points. In some cases, such a direct group may not exist or,for special routing reasons, it may not be possible to set up the international telegraph line in the preferred way. In suchcases, the internation
13、al telegraph line will consist of channels in tandem on two or more groups with or without audiosections, depending on the line available and the routing requirements.1.4 Terminal national sections connected to the international voice-frequency telegraph lineIn many cases the voice-frequency telegra
14、ph terminal equipment is remote from the terminal internationalcentre of the international voice-frequency telegraph line (Figure 1/M.800), and such cases necessitate the provision ofterminal national sections in order to establish international voice-frequency telegraph links. These sections may be
15、 inshort-distance local audio cables, amplified or unamplified, or may be routed in long-distance carrier groups or onamplifier audio plant.2 Reserve arrangements for international voice-frequency telegraph linksAll necessary action should be taken to enable the duration of interruption of internati
16、onal voice-frequencytelegraph links to be reduced to a minimum and, for this purpose, it is expedient to standardize some of the methods tobe adopted for replacing defective portions in the link.Although it does not appear necessary for these methods to be the same in detail in every country, it wou
17、ld beadvisable to reach agreement regarding the general directives to be followed.The make-up of a reserve voice-frequency telegraph link will in general be similar to that of the normalvoice-frequency telegraph link. However, if the voice-frequency telegraph terminal equipment is not located at the
18、terminal international centres, the line portion of an international telephone circuit can be used to replace only theinternational voice-frequency telegraph line of the voice-frequency telegraph link.Fascicle IV.2 - Rec. M.800 32.1 Reserve international lines2.1.1 Wherever possible, a reserve inter
19、national line should be provided between the two terminal internationalcentres by means of the line portion of an international telephone circuit (between A and B in Figure 1/M.800).2.1.2 The telephone line used as a reserve should be chosen wherever possible so as to follow a different route fromth
20、at of the normal international telegraph line. Where this cannot be done, as much as possible of the line or its sectionsshould be alternatively routed.2.1.3 If there is a choice, the use of manually-operated circuits as reserve lines for voice-frequency telegraphy istechnically and operationally pr
21、eferable to the use of automatic circuits.It should be possible, after prior agreement between the controlling officers at the international terminalexchanges concerned, for an operator to break into a call in progress to advise the correspondents that the circuit isrequired elsewhere and that the c
22、all will have to be transferred to another circuit if it lasts longer than six minutes.2.1.4 If the telephone circuit used as a reserve is automatic or semiautomatic a direct indication should be given atthe changeover point. If it is not available when needed the reserve circuit should be blocked a
23、gainst any further call.2.2 Reserve sections for the sections of the international voice-frequency telegraph linkWhere it is not possible to provide a reserve international line or a reserve international voice-frequencytelegraph link either because there are no suitable telephone circuits or becaus
24、e the number of telephone circuits does notpermit the release of a circuit for reserve purposes, a reserve section should be provided wherever possible for each ofthe component sections. For these sections, national or international telephone lines or, where they exist, spare channels,circuits, etc.
25、, should be used.2.3 Reserve arrangements for the terminal national sections connecting the voice-frequency telegraph terminalequipment to the international voice-frequency telegraph lineFor the terminal national sections of an international voice-frequency telegraph link, reserve sections should be
26、constituted using national telephone-type circuits or spare channels, lines, etc.2.4 Changeover arrangements from normal to reserve lines2.4.1 When an international telephone line (i.e. part of an international telephone circuit) is used to provide a reservefor the international voice-frequency tele
27、graph line (or for one of its sections as mentioned in 2.2 above), there shouldbe changeover arrangements to enable the changeover from the normal line to the reserve line to be made as rapidly aspossible. The changeover arrangements (Figure 2/M.800) should be such that on changeover, all signalling
28、 equipment,echo suppressors, etc., associated with the telephone circuit that is used as a reserve for the international voice-frequencytelegraph line are disconnected on the line side. When the fault is cleared on the normal line, it should be possible to joinit to the signalling equipment, echo su
29、ppressors, etc., of the telephone circuit used, until the agreed time for restoration tothe normal routing.It is desirable to introduce as little disturbance as possible when changing back from reserve to normal.Arrangements of cords and parallel jacks can be devised to achieve this.2.4.2 The change
30、over arrangements shown in Figure 2/M.800 could be applied to sections of the internationalvoice-frequency telegraph line mentioned under 2.2 above when it is not possible to obtain an overall reserve for theinternational voice-frequency telegraph line. Normal sections and the corresponding reserve
31、sections should be routedvia suitable changeover arrangements at the stations concerned.2.4.3 Making manual, automatic or semiautomatic international telephone circuits available for reserve purposes forvoice-frequency telegraphy should be in accordance with the instructions issued and the arrangeme
32、nts made by therespective Administrations. Should the normal and reserve lines both be faulty, the technical services of theAdministration concerned should take immediate joint action to find a temporary remedy.4 Fascicle IV.2 - Rec. M.800d02-scFIGURE 2/M.800.D022.5 Designation and identifying marks
33、Normal and reserve links, etc., should be clearly distinguishable from other circuits both from the point ofview of designation (see Recommendation M.140 3) and identifying marks (see Recommendation M.810).References1 CCITT Recommendation Use of bearer circuits for voice-frequency telegraphy, Vol. V
34、II, Rec. R.77.2 CCITT Recommendation Composition and terminology of international voice-frequency telegraph systems,Vol. III, Rec. H.21.3 CCITT Recommendation Designation of international circuits, groups, group and line links, digital blocks,digital paths, data transmission systems and related information, Vol. IV, Rec. M.140.
