1、INTERNATIONAL TELECOMMUNICATION UNION)45G134 2 TELECOMMUNICATIONSTANDARDIZATION SECTOROF ITU4%,%2!0(94%,%2!0(G0G042!.3-)33)/.!00%!2!.#%G0G0/amended at Arnhem, 1953 and Geneva, 1964)The CCITT,in view ofRecommendation R.80, on the causes of disturbances affecting signals in telegraph channels, and the
2、ir effect onthe distortion of telegraph signals,considering(a) that precautions should be taken with circuits used in switched teleprinter services to prevent theappearance of parasitic signals that would give rise to false calling and clearing signals;(b) that special monitoring or indicating devic
3、es should be provided on voice-frequency telegraph (VFT)systems, the channels of which are used for international switched circuits;(c) that special steps might well be taken to discover the causes of false signals due to transient changes intransmission level or momentary increases in noise level,
4、on VFT circuits;(d) that it would be desirable to draw up operating standards in this connection,unanimously declares the view(1) that the following precautions should be taken to avoid false calling and clearing signals: the security and stability of power supplies and of sources of carrier frequen
5、cies, both telegraph andtelephone, should be ensured; a characteristic marking should be used to denote telegraph and telephone-type circuits used for theoperation of switched teleprinter circuits, both in terminal and intermediate stations; precise instructions should be given to staff in order tha
6、t false entry into the above-mentioned circuits maybe avoided; the number of non-soldered connections should be reduced as much as possible, together with the numberof break points; unsoldered connections, e.g. U-links and screw terminals, etc., should be checked withparticular care. In this connect
7、ion, attention is drawn to the methods of inspection by vibration tests; the amplitude of level variations in VFT bearers should be limited, and abrupt variations in the level shouldbe particularly avoided; limit the crosstalk mentioned in Recommendation R.80; limit induced voltage caused by electri
8、c power and traction systems; limit the microphonics of valves in repeaters and of valves in VFT; reduce the sensitivity of voice-frequency modulators to disturbing signals; avoid, in switched teleprinter services, the use of supervision signals having a short duration in relation tothe transitory p
9、henomena due to filters and time-constants in the level-regulators of VFT systems.(2) These precautions, inasmuch as they concern telephone-type circuits used for voice-frequency telegraphy,must be taken simultaneously on normal and reserve circuits.(3) For the permanent monitoring of VFT systems th
10、e channels of which are used for international switchedcircuits, it is advisable to use a pilot channel. An alarm should be given to indicate when either the system or the pilotchannel is out of order (see Recommendation R.78).(4) It would be advisable to record the transmission level, in order to discover and localize the causes of thefalse signals on circuits behaving particularly badly.(5) It is not yet possible to lay down operating standards in this connection.
