The Digital Subscriber System no. 1 (DSS1) is a European ISDN protocol for the D channel based on ITU-T I.411 (ETS 300 102). The DSS1 protocol replaced the 1TR6 protocol
in the 1980s. The two protocols DSS1 and 1TR6 are completely identical in layers 1 and 2, and there are no significant differences in the protocol structure. The main differences are in performance characteristics and signaling. DSS1 offers bearer capability (BC), lower layer compatibility and higher layer compatibility. The European D-channel protocol supports caller
ID inboth directions, to the called party and from the called party to the caller.
With the DSS1 Euro protocol, there are no longer terminal selection digits; however, multiple numbers can be assigned for a basic access. ISDN line features can be defined independently for each individual number. With the DSS1 protocol, data transfer with packet-oriented transmission in the D channel is possible. The transmission speed can be up to 9.6 kbit/s This data transfer is made possible by connecting the packet-oriented X.25 data terminal to a frame handler via a fixed layer 2 address using a terminal adapter (TA X.25). This measure eliminates the typical ISDN signalling procedures in the network layer
The different code sets of DSS1The DSS1 protocol distinguishes between four code sets for information elements
Code set 0 corresponds to the Q.931 rule code set, code set 5 corresponds to the ETSI code set, code set 6 is for national applications and code set 7 is for private applications via the private branch exchange
. The network currently only uses code set 0. Since Telekom supports both protocols, communication relationships are also established between the two protocols. This interworking takes place in the destination exchange and essentially relates to the conversion of compatibility and address information elements.