ITU-T X.52 PDF

In the case where two synchronous data networks offer service for DTEs in user classes of service 1 and 2, the transfer of the anisochronous signals between the networks shall be performed using a synchronous user channel of 600 bit/s in the standardized multiplexing schemes given in Recommendations X.50 and X.51 if one or both of the networks nationally use the synchronous user channel of 600 bit/s.

In the case where two synchronous data networks offer service for DTEs in user classes of service 1 and 2 but do not provide the 600-bit/s rate, the transfer of the anisochronous signals between those two networks shall be performed using a synchronous user channel of 2400 bit/s in the standardized multiplexing schemes given in Recommendations X.50 and X.51.

The method of encoding signals from DTEs in user classes of service 1 and 2 into the synchronous bearer shall be independent of the multiplexing scheme used.

The method of encoding shall be as defined in this Recommendation.