This document provides an overview of Integrated Services Digital Network (ISDN). It describes ISDN as a set of standards that define end-to-end digital connectivity for carrying voice, data, and videos concurrently. The key benefits of ISDN include higher speeds, faster call setup times, and the ability to run voice and data simultaneously. ISDN services include Basic Rate Interface (BRI) and Primary Rate Interface (PRI). BRI provides 2 B channels for user data and 1 D channel for control, while PRI provides more B channels and higher speeds for carrier use. The document also outlines ISDN devices, reference points, call processing, and its relationship to the OSI model layers.

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Dar es Salaam institute of Technology (DIT)
ETU 07420
Switching Systems
Ally, J
jumannea@gmail.com

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Integrated Service Digital Network
(ISDN)

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Integrated Service Digital Network (ISDN)
introduction
 ISDN refers to a set of communication protocols
proposed by telephone companies to permit telephone
networks to carry data, voice, and other source material
 ISDN provides a set of digital services that concurrently
deliver voice, data, text, graphics, music, videos, and
information to end users
 ISDN is comprised of digital telephony and data
transport services offered by regional telephone carriers.
 ISDN is generally viewed as an alternative to frame relay
and T1 wide area telephone services (WATS)

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What is ISDN
 ISDN is a set of standard that defines an end-to-end
digital network
 The ISDN standards define the hardware and call setup
schemes for end-to-end digital connectivity
 Digital connectivity via ISDN to the local site has many
benefits include:
 Carries many types of network traffic like data, voice, and videos,
etc
 Much faster call setup using out-of-band (D channel) signaling
than modem connections
 Much faster data transfer rate using bearer (B channel) services
at 64Kbps per channel

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The advantages of ISDN
 The advantages of ISDN over analog
connections include:
 Higher speed
 Faster call setup
 Cheaper than leased lines
 Voice and data can be run simultaneously

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ISDN Devices
 ISDN devices include the following
 Terminal Equipment 1 (TE1)
 Terminal Equipment 2 (TE2)
 Terminal Adapter (TA)
 Network Termination Type 1 (NT1)
 Network Termination Type 2 (NT2)
 ISDN reference points include the following:
 R—The reference point between non-ISDN equipment and a TA.
 S—The reference point between user terminals and the NT2.
 T—The reference point between NT1 and NT2 devices.
 U—The reference point between NT1 devices and line-
termination equipment in the carrier network. The U
reference
point is relevant only in North America, where the NT1
function is not provided by the carrier network.

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ISDN Configuration Between Devices and
Reference Points

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ISDN Components
Component Description
Terminal Equipment Type 1 (TE1) Designates a device that is compatible with an ISDN
network (A specialized device created for ISDN.). A TE1
connects to a network termination of either type 1 or type
2 (NT1 or NT2).
Terminal Equipment Type 2 (TE2) Designates a device that is not compatible with an ISDN
network and requires a terminal adapter (TA).
Terminal Adapter (TA) Converts standard electrical signals into a form used by
ISDN so that non-ISDN devices can connect to the ISDN
network.
Network Termination Type 1 (NT1) Connects four-wire ISDN subscriber wiring to the
conventional two-wire local loop facility.
Network Termination Type 2 (NT2) Connects four-wire ISDN subscriber wiring to the
conventional two-wire local loop facility. NT2 is a more
complicated device, typically found in digital Private
Branch eXchanges (PBXs), that performs Layer 2 and
Layer 3 protocol services.

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Physical Representation of BRI
Reference Points
Cisco ISDN
router
To ISDN
service
Wall
jack
4-wire
circuit
2-wire
circuit
S/T U
TA NT1NT1
S/T bus
S/T bus
S/T interface as a
multipoint connection
To non-ISDN
device (TE2)
R
ISDN
phon
e

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ISDN services
 There are two types of services associated with ISDN:
 Basic Rate Interface (BRI)
 Primary Rate Interface (PRI)
 ISDN BRI Service
 The ISDN Basic Rate Interface (BRI) service offers two B channels and one
D channel (2B+D).
 BRI B-channel service operates at 64 kbps and is meant to carry user data;
 BRI D-channel service operates at 16 kbps and is meant to carry control and
signaling information, although it can support user data transmission under
certain circumstances.
 ISDN PRI Service
 ISDN PRI service offers 23 B channels and 1 D channel in North America
and Japan, yielding a total bit rate of 1.544 Mbps (the PRI D channel runs at
64 kbps).
 ISDN PRI in Europe, Australia, and other parts of the world provides 30 B
channels plus one 64-kbps D channel and a total interface rate of 2.048
Mbps.
 The PRI physical layer specification is ITU-T I.431.

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ISDN Services and Channelized
E1 and T1
64 kbps
64 kbps
16 kbps
144
kbps
2B
D }{
BRI
T1 1.544
Mbps
or
E1 2.048
Mbps
(includes
sync)
23B (T1) or
30B (E1)
D
64 kbps
each
64 kbps}PR
I
2.048 Mbps
(includes sync)
31 64 kbps
channels }E1
1.544 Mbps
(includes sync)
24 64
kbps
channel
s
}T1

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31
BRI Call Processing
ISDN
service provider
4
B
channelD channel/SS7 signaling
2 SS7
Q.931 signaling Q.931 signaling

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ISDN and OSI Layer
 Physical layer-The ISDN Basic Rate
Interface (BRI) physical layer
specification is defined in ITU-T
standardization sector 1.430. The
ISDN Primary Rate Interface (PRI)
physical layer is defined in ITU-T
1.431
 Data link layer-The ISDN data link
layer specification is based on Link
Access Procedure on the D-Channel
(LAPD) and is formally specified in
ITU-T Q.920 and ITU-T Q.921
 Network layer-The ISDN network
layer is defined in ITU-T 1.450 (also
known as ITU-T Q.930) and ITU-T
1.451 (also known as ITU-T Q.930.
Together these two standards
specify user-to-user, circuit-switched,
and packet-switched connection

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ISDN Physical Layer (Layer 1)
 ISDN physical layer (Layer 1) frame formats differ
depending on whether the frame is outbound (from
terminal to network) or inbound (from network to
terminal).
 The frames are 48 bits long, of which 36 bits represent
data. The bits of an ISDN physical layer frame are used
as follows:
 F—Provides synchronization
 L—Adjusts the average bit value
 E—Ensures contention resolution when several terminals
on a passive bus contend for a channel
 A—Activates devices
 S—Is unassigned
 B1, B2, and D—Handle user data

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ISDN Physical Layer Frame Formats
Differ Depending on Their Direction

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ISDN Physical Layer operation
 Multiple ISDN user devices can be physically attached to one circuit. In
this configuration, collisions can result if two terminals transmit
simultaneously.
 Therefore, ISDN provides features to determine link contention. When
an NT receives a D bit from the TE, it echoes back the bit in the next E-
bit position. The TE expects the next E bit to be the same as its last
transmitted D bit.
 Terminals cannot transmit into the D channel unless they first detect a
specific number of ones (indicating “no signal”) corresponding to a pre-
established priority. If the TE detects a bit in the echo (E) channel that
is different from its D bits, it must stop transmitting immediately.
 After successful D-message transmission, the terminal has its priority
reduced by requiring it to detect more continuous ones before
transmitting.
 Terminals cannot raise their priority until all other devices on the same
line have had an opportunity to send a D message.

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IDSN Data Link Layer (Layer 2)
 Layer 2 of the ISDN signaling protocol is Link Access Procedure, D
channel (LAPD). LAPD is similar to High-Level Data Link Control
(HDLC) and Link Access Procedure, Balanced (LAPB).
 As the expansion of the LAPD acronym indicates, this layer is used
across the D channel to ensure that control and signaling information
flows and is received properly.

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ISDN Network Layer (Layer 3)
 Two Layer 3 specifications are used for ISDN
signaling: ITU-T I.450 (also known as ITU-T
Q.930) and ITU-T I.451 (also known as ITU-T
Q.931).
 Together, these protocols support user-to-user,
circuit-switched, and packet-switched
connections.
 A variety of call-establishment, call-termination,
information, and miscellaneous messages are
specified, including SETUP, CONNECT,
RELEASE, USER INFORMATION, CANCEL,
STATUS, and DISCONNECT.

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Typical stages of an ISDN circuit-switched
call

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Thanks!
Technology changes but communication lasts.