For the use of ISDN Tk systems in the terminal connection area, the ZVEI has specified a variant of the D-channel protocol. This protocol is called DKZ-E and offers some functions that are not supported by the D-channel protocols 1TR6 and DSS1
. The DKZ-E protocol is based on the 1TR6 protocol, but has additional features. Since compatibility with the D-channel protocols is ensured, communication between telecommunication systems with DKZ-E protocol and terminal devices with the D-channel protocols is possible.