ISDN reference point
In the local loop of ISDN, which extends from the digitallocal exchange (DIVO) to the terminal, important reference or benchmark points for the local loop have been defined by the International Telecommunication Union( ITU-T). A number of parameters and functions have been defined for these reference points, which are reflected in the various ISDN interfaces. This ensures that devices can communicate with each other via the respective assigned interface.
A reference point can be an interface, but it does not have to be. Starting from the subscriber side, the reference points have been designated consecutively with letters, beginning with the letter "R", which marks the reference point for non-ISDN-capable terminals. Other reference points are "S", on which the S interfaces are based, "T", which lies between network terminations 1 and 2, and "U". The latter is the reference point for the U interfaces and forms the interface to the DIVO.
The reference points can be combined into one reference point in certain configurations, for example in the case of the multi-device connection, where network termination 2 is omitted and the reference points "S" and "T" are thus combined to form S/T. The reference points are dealt with in detail in the ETSI standards. For example, these standards describe exactly which service features are offered with which connection configurations.
In addition to the reference points mentioned above, the European Computer Manufacturers Association( ECMA) has defined a Q reference point for the networking of PBXs. For this reference point, there is a separate signaling protocol, the Q-Interface Signalling Protocol( QSIG), which supports the interconnection oftelecommunications equipment from different manufacturers.