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, wherever 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.