The 1TR6 protocol was the national ISDN protocol for the D channel from Deutsche Telekom. This technical directive (TR) was changed to the European DSS1 protocol, Digital Subscriber System
no. 1 (DSS1), in the course of European harmonization. The differences between the two protocols lie, among other things, in the transmission of call number information. With the 1TR6 protocol, this was only done in one direction from the caller to the called party. In the European protocol DSS1, backward transmission is also possible. Furthermore, with the 1TR6 protocol, all ISDN terminals on an ISDN basic access could have the same call number as well as be addressed individually by means of the terminal selection digit. With global call, the last digit of the call number is a 0. The selection of a compatible device was made with the service identifier.