This Recommendation sets out the fundamental parameters of a transmission scheme that should be used for 48-kbit/s data signalling rate for interworking of networks that make use of the following structures: a) 8-bit envelope (see Explanatory Notes 1 and 2 of Recommendation X.50); b) 10-bit envelope (see Explanatory Note 3 of Recommendation X.50); in the case where at least one of the networks is structured according to a).