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ISDN technology involves
many functional devices, also known as functional groups. The ISDN
reference points define the communication protocols of these
devices. Each functional group has identical elements, but one group
includes the Network Termination 2 customer premises equipment (NT2
CPE); the other does not. The following functional groups are
illustrated in the Figure :
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Terminal
equipment 1 (TE1) designates a device that is compatible
with the ISDN network. A TE1 connects to a network termination
of either Type 1 or Type 2, such as a digital telephone, a
router with ISDN interface, or digital facsimile equipment.
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Terminal
equipment 2 (TE2) designates a device that is not compatible
with ISDN and requires a terminal adapter, such as terminals
with X.21, EIA/TIA-232, or X.25 interfaces, or a router without
ISDN interface (AGS+, and so on).
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Terminal adapter
(TA) converts standard electrical signals into the form used
by ISDN, so that non-ISDN devices can connect to the ISDN
network. An example would be converting V.35 or EIA/TIA-232 to
ISDN or a router without ISDN interface (AGS+, and so on).
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Network
Termination type 1 (NT1) connects four-wire ISDN subscriber
wiring to the conventional two-wire local-loop facility. The NT1
is part of the CPE in the United States and it is part of the
local exchange in Europe.
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Network
Termination type 2 (NT2) directs traffic to and from
different subscriber devices and the NT1. The NT2 is an
intelligent device that performs switching and concentrating.
Often, a private branch exchange (PBX) is the NT2 device.
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Line termination
(LT) (located at the exchange side) functions are identical
to those of an NT1.
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Exchange
termination (ET) Subscriber line cards in the ISDN exchange
LT and ET are sometimes referred to as the LE (local exchange),
which is the ISDN switch for which we must configure.
Reference points are architectural definitions that may or may not have a physical realization as an interface. ISDN reference points are as follows:
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U reference point
(user reference point)-This reference point is located
between the NT1 and LT (it corresponds with a subscriber line).
There are no ITU-T standards for the U interface. This is the
American National Standards Institute (ANSI) standard for the
United States.
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T reference point
(terminal reference point)-This reference point is located
between NT1 and NT2 (or between the NT1 and TE1 or TA, if there
is no NT2 device). The T interface uses the same characteristics
as the S interface.
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S reference point
(system reference point)-This reference point is located
between NT2 and TE1 or TA. It connects the terminals to the ISDN
network. This is the most important interface for the users. The
S interface uses the same characteristics as the T interface.
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R reference point
(rate reference point)-This reference point is located
between TA and TE2 (non-ISDN interface). The TE2 connects to the
TA via a standard physical-layer interface. These standards
include EIA/TIA-232-C (formerly RS-232-C), V.24, X.21, and V.35.
The S/T interface is
governed by the ITU I.430 standard. The ANSI T1.601 or ITU I.431
standards govern the U interface, depending on the country. Some
manufacturers define a V reference point in LEs between LT and ET.
This reference point identifies the network node interface and is
transparent to users. Given all the ISDN abbreviations such as T, S,
U, S/T, and so on, what do all of these components and reference
points look like in the real world?
As shown in Figure , a connection is made from the wall jack with a standard two-wire cable to the NT1, and then out of the NT1 with a four-wire connection to your ISDN phone, terminal adapter, Cisco ISDN router, or maybe to an ISDN fax. The S/T interface is implemented using an eight-wire connector to allow for powering the NT and TE capabilities.
Because all these
connectors look similar (such as RJ-11, RJ-45s, and so on), you must
be careful about what you plug in and where. The S/T reference point
is a four-wire interface (TX and RX). It is point-to-point and
multipoint (passive bus). It uses the ITU I.430 specification. The
S/T interface defines the interface between a TE1 or TA and an NT.
The U interface defines
the two-wire interface between the NT and the ISDN cloud. The R
interface defines the interface between the TA and an attached
non-ISDN device (TE2). An NT1 and NT2 combination device is
sometimes referred to as an NTU.
ISDN Protocols
The ITU-T groups and organizes the ISDN protocols according to
general topics:
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E-series-Telephone network, for example
E.164-International ISDN addressing
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I-series-Concepts and interfaces, for
example I.430-BRI interface
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Q-series-Switching and signaling, for
example q.921- Link Access Procedures on the D channel (LAPD)
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