With ISDN, the subscriber has three connection types at his disposal - the ISDN dial-up connection, the ISDN leased line and the preset permanent dial-up connection - which he can use optimally according to his applications.
The ISDN dial-up connections are set up in the same way as telephone connections and can be used from the ISDN line via the S0 interface with the standard transmission rate of 64 kbit/s duplex. The connection setup to another ISDN line is initiated by dialing the phone number. The transmission of the call number to the digital local exchange (DIVO) takes place in the D channel. The connection setup time depends on the interconnected ISDN switching computers and can be between 0.8 s and 2 s in the national range. A dial-up connection can only be set up if the other end is unoccupied
The ISDN dial-up connections can be established by conventional terminals with appropriate terminal adapters.
ISDN leased lines are permanently provided connections that are similar to direct data connections. There are four different ISDN leased lines, which differ in the number of user and control channels: Group 0 Type 4 leased lines are monopole lines with one 64 kbit/s channel and the S0FV interface( CCITT I.430), as an analogue to the basic connection with S0 interface; Group 0 Type 6 leased lines are monopole lines with 30 user channels and one control channel; Group 2 Type 1.1 leased lines correspond to the basic Group 0 Type 4 leased line with S0FV interface, but also have one control channel; and Group 2 Type 1.2 leased lines are basic leased lines with two basic channels and one control channel.
The S0FV interface is not identical to the S0 interface, since the subscriber's terminal equipment is not directly connected to the ISDN DIVO's switching network; both the basic leased lines and the primary multiplex leased lines. The user is provided with the various groups and types via the S0FV interface.
The pre-ordered permanent dial-up connection is a temporary leased line (formerly SPV, semi-permanent connection), which is provided on a case-by-case basis for a specified duration and operates at the ISDN standard transmission rate of 64 kbps. The connection is duplex between two fixed ISDN lines. Pre-ordered permanent dial-up connections, in contrast to ISDN leased lines, are routed via the digital local exchange (DIVO) switching network. This means that a prearranged permanent dial-up connection between two ISDN lines can only be set up using the 1TR6 protocol. The prearranged permanent dial-up connection is provided in ISDN as a technical feature of the ISDN exchange for ISDN lines and is billed by Deutsche Telekom at monthly flat rates. The service is not provided on European convention( DSS1 protocol) lines.