
Configuring Dial Services
2-6 114062 Rev. A
Reference Points
Reference points, also called interfaces, bring two functions together. A reference
point may be physical, along the ISDN line, or merely a conceptual point where
functions merge. There are four main reference points in an ISDN network:
• R reference point -- The point between the non-ISDN device and a terminal
adapter. It is the boundary between your equipment and the ISDN network.
• S reference point -- The reference point that sits between the terminal
equipment and the switching device at your premises.
• T reference point -- The reference point that is the boundary between your
switching device and the subscriber side of the local loop, that is, the wire
between your phone and the phone company’s central office.
• U reference point -- The point between the NT1 (where the phone company’s
line ends at your premises) and the phone company’s central office. It is where
the subscriber side and network side of the local loop meet.
This point is different in North America than in Europe. In North America,
the subscriber side of the local loop includes the NT1, so the U point is farther
out on the network. In Europe, the NT1 is part of the provider or network side
of the local loop -- so the S/T point becomes the place where the subscriber
and the network meet.
Figures
2-4 and 2-5 show two ISDN networks with functional groups and
reference points.
Note: Any device that can connect to an S reference point can connect to a
T reference point, because they are the same electrically.
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