ITU-T - International Telecommunication Union/ITU Telcommunication Sector
适用范围
Introduction For the purposes of this Recommendation@ an international exchange is a collection of equipment regarded as an entity by the Administration concerned. In the case of an international transit centre@ it extends from the end of the incoming international line to the beginning of the outgoing international line (e.g. between such points as A and D in Figure 1/Q.45 or any other suitable pair of points). In the absence of an international agreement on the choice of the points delimiting an international exchange@ it has proved impossible to draw up model specifications showing the limits to be observed for quantities measured between these points. The CCITT recommendations given hereafter have been issued regardless of the actual arrangement. Automatic international exchanges should be provided with circuit test access points (see Recommendation M.700 [1]) complying with the Recommendation cited in [2]. This Recommendation will ensure that circuit line-up and maintenance testing procedures are referred to points at or near the switchblock The essential transmission requirements for an international exchange are: a) The transmission loss through the centre should be substantially constant with time and independent of the routing through the centre. b) Crosstalk and noise should be negligible. c) The distortions introduced should be small. These include attenuation distortion@ non-linear distortion and intermodulation products. d) Impedance and balance with respect to earth at the points in the international exchange to which the lines are connected should be closely controlled. The following recommendations apply to new automatic 4-wire international exchanges of the electromechanical type. It is desirable that they should apply to new national 4-wire exchanges. They may also be applicable to electronic exchanges having metallic contact crosspoints. These recommendations are intended to be used only as type tests@ acceptance tests@ or for special investigations. They do not constitute a complete specification. Generally the recommended tests should be conducted on a sampling basis.