If the end-device at the remote extreme is digital such as, for example, a PC with an ISDN card or a digital fax device (fax group 4), or there is ISDN access to a network or to a service provider (Datex-J-Service, CompuServe, Internet Provider), then data exchange through the ISDN B-channel usually conforms to either the X.75 or V.110 standards.
The X.75 protocol uses the full bandwidth of ISDN, so the data transmission rate is 64,000 bit/s per ISDN B-channel. If the remote station can only operate at a data rate less than 64,000 bit/s, for instance CompuServe on ISDN, then the transmission rate has to be modified. This is called bit rate adaptation. For that purpose, fill bits are inserted to make up the difference between 64,000 bit/s and the required bit rate.

This process is defined in the V.110 standard for 2,400 to 38,400 bit/s and it can only be applied with ISDN cards that support this standard. If application flexibility is considered to be important, an adapter capable of supporting both standards should be selected.