To establish a connection between two ISDN terminal devices, control information is initially sent over the D channel. This information includes the caller's number, which then appears on the display of an ISDN telephone, an indication of the type of the service desired (e.g. voice or fax) as well as other connection-related technical data. Following the introduction of ISDN, several standards were developed for the protocols responsible for sending control

information over the D channel. This is the reason why, in Germany for example, two D channel protocols for ISDN exist. One is the national standard 1TR6 and the other is the European standardised protocol DSS1 or E-DSS1. This latter standard is known as the Euro-ISDN protocol, even though nowadays it is also used in many non-European countries. The number of operating companies offering Euro-ISDN has grown to 30 world-wide, 20 of which being in Europe. When using only the telephone, no problems should be encountered due to a difference in D channel protocols used at the respective locations. However, problems may arise when data are to be transmitted or if particular service features or line types are needed in ISDN that are offered only on a national basis.

In the meantime, given that Euro-ISDN has become a standard in many countries, wherever possible this type of interface should be used for new installations.