Recommendation U.3
                                   ARRANGEMENTS IN SWITCHING EQUIPMENT
                            TO MINIMIZE THE EFFECTS OF FALSE CALLING SIGNALS
                         (former CCIT Recommendation E.3, Geneva, 1956)
             The CCITT,
       considering
             (a) that transmission systems at present in  use  for  international  telex
       trunks are liable to generate false calling signals;
             (b) that  such  false  calling  signals  can  seize  and  engage  switching
       equipment,  thereby  reducing  the  grade  of  service.  This  is  of  particular
       importance with systems in which common equipment normally used only  to  set  up
       calls is seized by false calling signals;
             (c) that the ill effects of false  calling  signals  can  be  minimized  by
       delaying  the  operation  of  the  calling  relay  at  the  termination  of   the
       international telex trunk circuit;
             (d)  that,  however,  when  direct  dial  selection  is  employed  over  an
       international trunk line, unless it is a manually selected circuit  not  preceded
       by a stage of automatic selection, there is normally insufficient time  available
       between successive digits to permit the use of slow operating relays;
             (e) that, nevertheless, Administrations may agree among one another to  use
       digit storage at the outgoing end of the circuit so that  the  inter-train  pause
       can be increased to permit the calling relays to be made slow to operate,
       unanimously declares the view
             (1) that the design and maintenance of transmission systems should be  such
       as to reduce to a minimum the number and duration of false  calling  signals.  In
       this  connection  attention  is  drawn  to  the  merits  of   frequency-modulated
       voice-frequency telegraph systems, particularly with long overhead lines;
             (2) that              possible,     calling     relays     on
       international telex trunk circuits should have an operation lag  of
       at least 100 milliseconds. Administrations using circuits on  lines
       prone to long-duration false  calling  signals  may  agree  to  use
       calling relays with longer operation lags.





































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