Support of X.21, X.21 bis and X.20 bis Based Data Terminal Equipments (DTEs) by an Integrated Services Digital Network (ISDN) - Integrated Services Digital Network (ISDN) - Overall Network Aspects and Functions, ISDN User-Network Interfaces (Study Group
In this case, the two stage approach (rate adaption up to 8, 16 or 32 kbit/s followed by multiplexing to 64 kbit/s) as defined in $8 1.2 and 2.1 should be used.