This Recommendation specifies the interface between packet mode DTEs and public data networks for physical links which have data rates higher than those of the scope of Recommendation X.25.

Full interworking compatibility between X.25 DTEs and X.45 DTEs is provided. In particular credit based flow control is available for both directions of each virtual circuit at each DTE/DCE interface. For X.45 DTEs an unassured virtual circuit option is offered for virtual circuits where data losses are preferred to extra delays in case of transmission errors.

Protocol of this Recommendation can be used on underlying layers which are functionally equivalent to HDLC layer 2.a, in particular on ATM-VPs and VCs used with the CPCS of AAL 5.