The CCITT considering (a) that the Recommendation X.1 defines the synchronous user classes of services to be provided by a public data network; (b) that Recommendation X.50 defines some envelope data formats which can be used in public data networks; (c) that the use of a full 64 kbit/s data stream for the transmission of a single lower speed synchronous data channel may be economically attractive in some applications, when a low cost 64 kbit/s channel is available; (d) that for customer access to circuit switched data services in the ISDN, a method for adaption of synchronous user data rates to a 64 kbit/s bearer rate is defined in Recommendation X.30, but that in certain applications as mentioned under (c) a simpler method is preferred.