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ISDN standards define functional groups as
devices or pieces of hardware that enable the user to access the
services of the BRI or PRI. Vendors can create hardware that supports
one or more functions. ISDN specifications define four reference
points that connect one ISDN device to another.
 Each device
in an ISDN network performs a specific task to facilitate end-to-end
connectivity.
To connect devices that perform specific
functions, the interface between the two devices needs to be well
defined. These interfaces are called reference points.
 The
reference points that affect the customer side of the ISDN connection
are as follows:
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R – References the connection
between a non-ISDN compatible device Terminal Equipment type 2 (TE2) and a
Terminal Adapter (TA),
for example an RS-232 serial interface.
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S – References the points
that connect into the customer switching device Network Termination
type 2 (NT2) and enables
calls between the various types of customer premises equipment.
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T – Electrically identical to the S
interface, it references the outbound connection from the NT2 to the
ISDN network or Network Termination type 1 (NT1).
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U – References the connection
between the NT1 and the ISDN network owned by the telephone company.
Because the S and T references are
electrically similar, some interfaces are labeled S/T interfaces.
Although they perform different functions, the port is electrically
the same and can be used for either function.
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Interactive Media Activity
Drag and Drop: ISDN Reference Points
When the student has completed this activity, the student
will be able to correctly identify the ISDN reference points.
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