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Chapter 10

Wide Area Networking Protocols

ISDN Terminals

Devices connecting to the ISDN network are known as terminal equipment
(TE) and network termination (NT) equipment. There are two types of each:

TE1

A terminal equipment type 1 (TE1) device refers to those termi-

nals that understand ISDN standards and can plug right into an ISDN
network.

TE2 A terminal equipment type 2 (TE2) device refers to those that pre-
date ISDN standards. To use a TE2, you have to use a terminal adapter
(TA) to be able to plug into an ISDN network.

NT1 The network termination 1 (NT1) device implements the ISDN
Physical layer specifications and connects the user devices to the ISDN net-
work by converting the network from a four-wire to the two-wire network
used by ISDN.

NT2 The network termination 2 (NT2) device is typically a provider's
equipment, such as a switch or PBX. It also provides Data Link and Net-
work layer implementation. It's very rare at customer premises.

TA A terminal adapter (TA) converts TE2 non-ISDN signaling to sig-
naling used by the ISDN switch and then connects into an NT1 device for
conversion into a two-wire ISDN network.

ISDN Reference Points

Reference points are a series of specifications that define the connection
between the various equipment used in an ISDN network. ISDN has four
reference points that define logical interfaces:

R The R reference point defines the point between non-ISDN equipment
(TE2) and a TA.

S The S reference point defines the point between the customer router
and an NT2. Enables calls between the different customer equipment.

T The T reference point defines the point between NT1 and NT2
devices. S and T reference points are electrically the same and can perform
the same function. Therefore, they are sometimes referred to as an S/T
reference point.

The U reference point defines the point between NT1 devices

and line-termination equipment in a carrier network. (This is only in

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