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Configuring Dial Services

2-6

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.

Figure 2-4. ISDN Interfaces with ISDN Devices

Note: Any device that can connect to an S reference point can connect to a T

reference point, because they are the same electrically.

Digital

Line

TE1 NT1

Router with

ISDN/BRI

module

ISDN

Network

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Avaya Configuring Dial Services Manual do Utilizador Página 48