Integrated Services Digital Network

1. Introduction

Integrated Services Digital Network

The Integrated Services Digital Network (ISDN) is a development of the plain

ordinary telephone system (POTS) that enables it to carry data and other traffic as

well as voice calls. Instead of using a continuously changing analogue voltage on the

line between the network and your house, it uses pulses having one of a few discrete

voltage levels to encode a series of digits. This is known as "pulse code modulation"

(PCM). It is the original "digital subscriber loop" (DSL) technology. It is more

complicated than the POTS way, but has some big advantages:

Two simultaneous phone calls can be made (or more on primary rate), using

the same pair of wires that your POTS telephone used to connect to. This is

achieved by interleaving the data for each call, a technique called "time

division multiplexing" (TDM). The phone company doesn't have to dig up the

road to change to ISDN and effectively give you a second line.

Calls can be connected much more quickly - typically within one second over

ISDN, compared with 20 seconds or more over POTS. This is especially

important when connecting a home computer to an office network ("wide area

networking") or validating credit card transactions, for example.

Data can be sent faster (64,000 bits per second in each direction) and more

reliably, so data calls can be shorter and therefore cheaper. You don't need a

modem to exchange data between computers, although you will probably need

a cheaper "terminal adapter" (TA) or ISDN card instead. There is no modem

"training" time to wait for (and perhaps pay for) after the call connects.

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Integrated Services Digital Network

Noise, distortion, echoes and crosstalk become inaudible, because the

telephone no longer has to measure an exact analogue value, it only has to

decide which of a few discrete voltages is present at any particular instant.

In most countries, the "trunk" network between telephone exchanges has

already been converted to digital technology, for this reason. ISDN just

extends it the "last mile" to your home.

The digits can represent any data, including faxes, files, web pages, sound,

pictures and ordinary voice calls. This is the meaning of "integrated

services".

(Figure Of ISDN)

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Integrated Services Digital Network

So why isn't everyone using ISDN:-

ISDN may not be available in your area, because it is expensive to upgrade a

telephone exchange to support it. A "network termination unit" (NTU) may

also have to be installed at the user end (not necessary in North America),

and any "loading coils" removed from the line. Also, it won't work if you

live more than about 5 km from your local telephone exchange, which

affects around 10% of users, depending on location.

In order to make fast data calls, both ends must have digital connections.

Most internet service providers (ISPs) already support this. Voice calls can

be made from ISDN terminals to ordinary POTS phones without problems.

Long-distance ISDN data calls may be considerably more expensive than

voice calls because they can't be compressed. ISDN data calls may also be

charged by time in places where POTS calls are not normally timed. Voice

calls over ISDN typically cost the same as over POTS, however.

Supplementary services such as "caller display", "ring back when free" and

"charge advice" work differently (usually better) than on POTS lines, but are

not always available and may cost extra. 

ISDN terminals often need a local power supply, which can be a problem in

emergencies. POTS phones normally take their power from the line.

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Integrated Services Digital Network

Telephone companies developed ISDN (Integrated Services Digital

Network) as part of an effort to standardize subscriber services.

This included the User-Network Interface (UNI), better known as the local

loop.

The ISDN standards define the hardware and call setup schemes for end-to-

end digital connectivity.

These standards help achieve the goal of worldwide connectivity by ensuring

that ISDN networks easily communicate with one another.

In an ISDN network, the digitizing function is done at the user site rather than

the telephone company.

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Integrated Services Digital Network

(Figure of ISDN Working)

Unlike POTS, ISDN is digital from end to end.

With asynchronous connections (POTS) the local loop is analog and requires

PCM (Pulse Code Modulation) - explained later.

Benefits of ISDN include:

Carries a variety of user traffic signals, including data, voice, and video

Offers much faster call setup than modem connections

B channels provide a faster data transfer rate than modems

B channels are suitable for negotiated Point-to-Point Protocol (PPP)

links

ISDN also provides more bandwidth than a traditional 56 kbps dialup

connection.

ISDN uses bearer channels, also called B channels, as clear data paths.

Each B channel provides 64 kbps of bandwidth.

An ISDN connection with two B channels would provide a total usable

bandwidth of 128 kbps.

Each ISDN B channel can make a separate serial connection to any other site

in the ISDN network.

ISDN lines can be used in conjunction with PPP encapsulation.

2.ISDN DEVICES

Terminal Equipment 1 (TE1)

This is any device that understands isdn signaling standards.

ISDN compatible device (Router with ISDN Interface)

TE1s connect to the ISDN network through a four-wire, twisted-pair

digital link

Example: Telephones, personal computers, fax machine or

Video conferencing machine.

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Integrated Services Digital Network

Terminal Equipment 2 (TE2)

This is any device that does not understand the isdn signaling standard.

ISDN Non-compatible devices.

Will require a terminal adapter.

Example: Analog phone or modem, requires a TA (TE2 connects to TA).

Network Terminator Type 1 (NT1)

When you connect a te1 device to the isdn network, you use an nt1. An NT1

will connect any 2-wire te1 device to the isdn network.

Network Terminator Type 2 (NT2)

Devices using 4 wire cables must use an NT2 to convert the 4 wire connector

to a 2 wire connector that can be physically connected to the isdn network

through an NT1.

Terminal Adaptor (TA)

A terminal adaptor allows a non isdn device (a device that is a te2) to

communicate with the isdn network via an nt1. This is typically needed where

the device uses a non-polar electrical signaling system. Isdn’s electrical

signaling is bipolar, thus a converter is needed.

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Integrated Services Digital Network

Local Termination (LT)

This is an nt1 at the ISP’s side of the connection.

Exchange Termination (ET)

This is the connection between the customer's last mile (local loop) connection

and the service provider's isdn network. Usually, this is the line card in the

isdn switch at the provider's local exchange office.

3.ISDN REFERENCE POINTS

R :-

This is the connection reference point designating the connection interface

between an isdn terminal adaptor and a non-isdn device. There really aren't

any standards for this reference point as this reference point was designated

for devices that allow non-isdn devices to communicate with the isdn devices.

Clearly there are many proprietary ways to do this, none of which are part of

any standard.

S/T :-

This is the connection reference point designating the connection interface

between an isdn capable device and a network terminator 1. Reference point’s’

is for user terminals that connect to the isdn network. Reference point’s’

defines connections between nt1 and nt2 devices. An s/t reference point

combines the functions of the s and the t reference points. S/t is governed by

the itu i.430 specification.

U:-

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Integrated Services Digital Network

This is the connection reference point designating the connection interface

between an isdn nt1 and the isdn services presented by the isdn switch.

V:-

This is the connection reference point between the line termination equipment

and the exchange termination equipment.

Following Figure illustrates a sample ISDN configuration and shows three devices

attached to an ISDN switch at the central office. Two of these devices are ISDN-

compatible, so they can be attached through an S reference point to NT2 devices. The

third device (a standard, non-ISDN telephone) attaches through the reference point to

a TA. Any of these devices also could attach to an NT1/2 device, which would

replace both the NT1 and the NT2. In addition, although they are not shown, similar

user stations are attached to the far-right ISDN switch.

Figure Sample ISDN Configuration Illustrates Relationships Between

Devices and Reference Points:-

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Integrated Services Digital Network

(FIGURE OF ISDN REFERANCE POINTS)

(FIGURE OF ISDN REFERANCE POINTS)

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4. ISDN Services

There are two types of services associated with ISDN: • BRI • PRI

(FIGURE OF ISDN SERVICES)

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Integrated Services Digital Network

4.1 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.

The D channel signaling protocol comprises Layers 1 through 3 of

the OSI reference model. BRI also provides for framing control

and other overhead, bringing its total bit rate to 192 kbps.

The BRI physical layer specification is International

Telecommunication Union-Telecommunications Standards Section

(ITU-T)

The B-channels and the D-channel provide the user with access to

the circuit switched network

Basic Rate Interface (BRI)

Two 64 Kbps B channels, one 16 Kbps D channel, and 48 Kbps

worth of framing and synchronization.

Available data bandwidth: 128 Kbps (2 x 64 Kbps)

User bandwidth: 144 Kbps (128 Kbps + a 16 Kbps D channel)

Total line capacity: 192 Kbps (144 Kbps + 48 Kbps framing)

Each B channel can be used for separate applications

Such as Internet and Voice

Allows individual B channels to be aggregated together into a Multilink

channel

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Integrated Services Digital Network

(FIGURE OF BRI SERVICE)

4.2 ISDN PRI Service

ISDN Primary Rate Interface (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.

A PRI connection can assign various 64 Kbps channels to both

ISDN and analog modem connections.

North America and Japan – PRI service has 23 64 Kbps B

channels, one 64 Kbps D channel, and 8 Kbps of

synchronization and framing for a total bit rate of up to 1.544

Mbps (same as T1)

Europe, Australia, and other parts of the world – PRI service

has 30 64 Kbps B channels, one 64 Kbps D channel, and 64

Kbps of framing and synchronization for a total bit rate of up to

2.048 Mbps (same as E1)

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Integrated Services Digital Network

Each B channel to be used for separate applications including

voice, data and Internet

Multiple B channels can be Multilinked together

A PRI connection can assign various 64 Kbps channels to both ISDN

and analog modem connections

North America and Japan – PRI service has 23 64 Kbps B channels,

one 64 Kbps D channel, and 8 Kbps of synchronization and framing

for a total bit rate of up to 1.544 Mbps (same as T1)

Europe, Australia, and other parts of the world – PRI service has 30 64

Kbps B channels, one 64 Kbps D channel, and 64 Kbps of framing and

synchronization for a total bit rate of up to 2.048 Mbps (same as E1)

Each B channel to be used for separate applications including voice,

data and Internet

Multiple B channels can be Multilinked together

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Integrated Services Digital Network

(FIGURE OF PRI SERVICE)

4.3 B Channels

The B channels can be used for relatively high-speed data transport.

In this mode, the information is carried in frame format, using either

HDLC or PPP as the Layer 2 protocol.

PPP is more robust than HDLC because it provides a mechanism for

authentication and negotiation of compatible link and protocol

configuration.

4.4 D Channel

When a TCP connection is established, there is an exchange of

information called the connection setup.

This information is exchanged over the path on which the data

will eventually be transmitted.

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Integrated Services Digital Network

Both the control information and the data share the same

pathway.

This is called in-band signaling.

ISDN however, uses a separate channel for control information, the D

channel.

This is called out-of-band signaling.

The D channel carries signaling messages, such as call setup and

teardown, to control calls on B channels.

Traffic over the D channel employs the Link Access Procedure on the

D Channel (LAPD) protocol.

LAPD is a data link layer protocol based on HDLC.

5. ISDN Specifications

This section describes the various ISDN specifications for Layer 1, Layer 2,

and Layer 3.

5.1 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). Both physical layer interfaces are shown in Figure 12-2. 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

Figure illustrates ISDN Physical Layer Frame Formats Differ Depending

on Their Direction

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Integrated Services Digital Network

(Figure of Layer 1 of ISDN)

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. This simple technique ensures

that only one terminal can transmit its D message at one time. 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

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Integrated Services Digital Network

priority until all other devices on the same line have had an opportunity to

send a D message. Telephone connections have higher priority than all other

services, and signaling information has a higher priority than nonsignaling

information.

5.2 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) (see Chapter 16, "Synchronous Data

Link Control and Derivatives," and Chapter 17, "X.25," for more information

on these protocols). 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. The LAPD frame format (see

Figure 12-3) is very similar to that of HDLC; like HDLC, LAPD uses

supervisory, information, and unnumbered frames. The LAPD protocol is

formally specified in ITU-T Q.920 and ITU-T Q.921.

Figure illustrates LAPD Frame Format Is Similar to That of HDLC and

LAPB

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Integrated Services Digital Network

(Figure of Layer 1 of ISDN)

The LAPD Flag and Control fields are identical to those of HDLC. The LAPD

Address field can be either 1 or 2 bytes long. If the extended address bit of the

first byte is set, the address is 1 byte; if it is not set, the address is 2 bytes. The

first Address-field byte contains the service access point identifier (SAPI),

which identifies the portal at which LAPD services are provided to Layer 3.

The C/R bit indicates whether the frame contains a command or a response.

The Terminal Endpoint Identifier (TEI) field identifies either a single terminal

or multiple terminals. A TEI of all ones indicates a broadcast.

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Integrated Services Digital Network

5.3 Layer 3

Two Layer 3 specifications are used for ISDN signaling: ITU-T (formerly

CCITT) 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. These messages are functionally

similar to those provided by the X.25 protocol Following figure , from ITU-T

I.451, shows the typical stages of an ISDN circuit-switched call.

Figure illustrates n ISDN Circuit-Switched Call Moves Through Various

Stages to Its Destination

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(Figure of Layer 3 of ISDN)

6. ISDN Characteristics

The probably most important keywords concerning ISDN are:

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Integrated Services Digital Network

end to end digital connection

integration of multiple services (voice-, data-, video-, multimedia

transmission)

standard terminal interface

(FIGURE OF ISDN CHARACTERISTICS)

7. ISDN 3-layer model and protocols:-

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Integrated Services Digital Network

(Figure Of ISDN 3-layer model and protocols)

ISDN utilizes a suite of ITU-T standards spanning the physical, data link, and

network layers of the OSI reference model.

The ISDN BRI and PRI physical layer specifications are defined in ITU-T

I.430 and I.431, respectively.

The ISDN data link specification is based on LAPD and is formally specified

in the following, ITU-T Q.920, ITU-T Q.921, ITU-T Q.922, ITU-T Q.923

The ISDN network layer is defined in ITU-T Q.930, also known as I.450 and

ITU-T Q.931, also known as I.451.

These standards specify user-to-user, circuit-switched, and packet-switched

connections.

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7.1 BRI Physical Layer

If the frame is outbound, it is sent from the terminal to the network.

– Outbound frames use the TE frame format.

If the frame is inbound, it is sent from the network to the terminal.

– Inbound frames use the NT frame format.

(B1, B2, D and Framing Bits)

(Figure Of ISDN BRI Physical layer)

ISDN BRI frames contain 48 bits.

Four thousand of these frames are transmitted every second, 4,000 x 48 =

192,000 bps.

Each B channel, B1 and B2, have a capacity of 2(8*4000) = 64 kbps,

128 kbps for both B channels (B1 and B2)

The D channel has a capacity of 4*4000 = 16 kbps (D)

Framing and overhead 12*4,000 = 48,000 kbps. (F, L, E, A, S)

The overhead bits of an ISDN physical layer frame are used as follows:

Framing bit – Provides synchronization

Load balancing bit – Adjusts the average bit value

Echo of previous D channel bits – Used for contention resolution when several

terminals on a passive bus contend for a channel

Activation bit – Activates devices

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Spare bit – Unassigned

7.2 ISDN Data Link Layer

(Figure Of ISDN Data Link layer)

The LAPD flag and control fields are identical to those of HDLC.

The LAPD address field is 2 bytes long.

Service access point identifier (SAPI), which identifies the portal at which

LAPD services are provided to Layer 3.

The command/response bit (C/R), indicates whether the frame contains a

command or a response.

The second byte contains the terminal endpoint identifier (TEI).

Each piece of terminal equipment on the customer premises needs a

unique identifier.

The TEI may be statically assigned at installation, or the switch may

dynamically assign it when the equipment is started up.

Statically assigned TEIs range from 0 to 63.

Dynamically assigned TEIs range from 64 to 126.

A TEI of 127, or all 1s, indicates a broadcast.

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8. ISDN TYPICAL NETWORK

(Figure Of ISDN Typical Network)

Among the kinds of data that can be moved over the 64 kbit/s channels are

pulse-code modulated voice calls, providing access to the traditional voice

PSTN. This information can be passed between the network and the user end-

point at call set-up time.

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In North America, ISDN is now used mostly as an alternative to analog

connections, most commonly for Internet access. Some of the services

envisioned as being delivered over ISDN are now delivered over the Internet

instead. In Europe, and in Germany in particular, ISDN has been successfully

marketed as a phone with features, as opposed to a POTS phone with few or

no features.

Meanwhile, features that were first available with ISDN (such as Three-Way

Calling, Call Forwarding, Caller ID, etc.) are now commonly available for

ordinary analog phones as well, eliminating this advantage of ISDN.

Another advantage of ISDN was the possibility of multiple simultaneous calls

(one call per B channel), e.g. for big families, but with the increased

popularity and reduced prices of mobile telephony this has become less

interesting as well, making ISDN unappealing to the private customer.

However, ISDN is typically more reliable than POTS, and has a significantly

faster call setup time compared with POTS, and IP connections over ISDN

typically have some 30–35ms round trip time, as opposed to 120–180ms (both

measured with otherwise unused lines) over 56k or V.34/V.92 modems,

making ISDN more reliable and more efficient for telecommuters.

Where an analog connection requires a modem, an ISDN connection requires

a terminal adapter (TA). The function of an ISDN terminal adapter is often

delivered in the form of a PC card with an S/T interface, and single-chip

solutions seem to exist, considering the plethora of combined ISDN- and

ADSL-routers.

ISDN is commonly used in radio broadcasting. Since ISDN provides a high

quality connection this assists in delivering good quality audio for

transmission in radio.

In ISDN, there are two types of channels, B (for "bearer") and D (for "data").

B channels are used for data (which may include voice), and D channels are

intended for signaling and control (but can also be used for data).

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There are two ISDN implementations. Basic Rate Interface (BRI), also called

basic rate access (BRA) consists of two B channels, each with bandwidth of

64 kbit/s, and one D channel with a bandwidth of 16 kbit/s.

Together these three channels can be designated as 2B+D. Primary Rate

Interface (PRI), also called primary rate access (PRA) in Europe contains a

greater number of B channels and a D channel with a bandwidth of 64 kbit/s.

The number of B channels for PRI varies according to the nation: in North

America and Japan it is 23B+1D, with an aggregate bit rate of 1.544 Mbit/s

(T1); in Europe, India and Australia it is 30B+1D, with an aggregate bit rate of

2.048 Mbit/s (E1).

Broadband Integrated Services Digital Network (BISDN) is another ISDN

implementation and it is able to manage different types of services at the same

time. It is primarily used within network backbones and employs ATM.

Another alternative ISDN configuration can be used in which the B channels

of an ISDN BRI line are bonded to provide a total duplex bandwidth of 128

kbit/s.

This precludes use of the line for voice calls while the internet connection is in

use. The B channels of several BRIs can be bonded, a typical use is a 384K

videoconferencing channel.

Using bipolar with eight-zero substitution encoding technique, call data is

transmitted over the data (B) channels, with the signaling (D) channels used

for call setup and management.

Once a call is set up, there is a simple 64 kbit/s synchronous bidirectional data

channel (actually implemented as two simplex channels, one in each direction)

between the end parties, lasting until the call is terminated.

There can be as many calls as there are bearer channels, to the same or

different end-points. Bearer channels may also be multiplexed into what may

be considered single, higher-bandwidth channels via a process called B

channel BONDING, or via use of Multi-Link PPP "bundling" or by using an

H0, H11, or H12 channel on a PRI.

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9.ISDN Applications

Photo Telephone Calling - where called party can see each other's pictures on

screen.

Desk Top Video Conferencing - on dial-up basis using a single ISDN line at

128 Kbps.

High Quality Video Conferencing  - on dial-up basis between any two ISDN

subscribers by using three ISDN lines at 384 Kbps.

Teleconferencing - which facilitates the transmission of pictures, documents

and drawings etc., apart from voice & video images of the participants, white

board sharing & document sharing is also possible.

High speed data transmission at 128 Kbps - Very high quality in digital

mode on account of very high immunity from the noise.

High speed facsimile - Time taken to send FAX messages reduced to 1/4th

compared to a conventional old telephone line provides. Subscriber can see his

metering pulses at his premises.

ISDN supports all Phone-Plus services - available to conventional old

telephone line. In addition the following additional facilities are available.

Calling Line Identification Presentation(CLIP) - When an ISDN subscriber

receives a call, the calling subscriber number will be displayed on his ISDN

telephone revealing identity of the caller giving option to the called party to

accept the call or not during conversation . It is possible to hold two more

incoming calls and ISDN subscriber can switch between the incoming calls.

Calling Line Identification Restriction (CLIR) - By means of this service,

the calling subscriber will be able to prevent the presentation of his number to

the called subscriber (Prevention of CLIP).

Multiple Subscriber Number (MSN) - Up to 8 terminals can be connected in

parallel at the subscriber premises. To call a specific terminal separate

numbers can be allotted to each terminal to facilitate when call is received

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from a normal (Analog) subscriber. In case the call is received from an ISDN

subscriber ,the terminal Selection will be made automatically.

10.ISDN Encapsulation

HDLC (High-Level Data Link Control)

PPP (Point to Point Protocol)

LABP (Link Access Procedure Balance)

10.1 HDLC (High-Level Data Link Control)

High-Level Data Link Control (HDLC) is a bit-oriented code-transparent synchronous

data link layer protocol developed by the International Organization for

Standardization (ISO). The original ISO standards for HDLC are:

ISO 3309 – Frame Structure

ISO 4335 – Elements of Procedure

ISO 6159 – Unbalanced Classes of Procedure

ISO 6256 – Balanced Classes of Procedure

The current standard for HDLC is ISO 13239, which replaces all of those standards.

HDLC provides both connection-oriented and connectionless service.

HDLC can be used for point to multipoint connections, but is now used almost

exclusively to connect one device to another, using what is known as Asynchronous

Balanced Mode (ABM). The original master-slave modes Normal Response Mode

(NRM) and Asynchronous Response Mode (ARM) are rarely used.

10.2 PPP (Point to Point Protocol)

In networking, the Point-to-Point Protocol (PPP) is a data link protocol

commonly used in establishing a direct connection between two networking

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nodes. It can provide connection authentication, transmission encryption

(using ECP, RFC 1968), and compression.

PPP is used over many types of physical networks including serial cable,

phone line, trunk line, cellular telephone, specialized radio links, and fiber

optic links such as SONET.

PPP is also used over Internet access connections (now marketed as

"broadband"). Internet service providers (ISPs) have used PPP for customer

dial-up access to the Internet, since IP packets cannot be transmitted over a

modem line on their own, without some data link protocol.

Two encapsulated forms of PPP, Point-to-Point Protocol over Ethernet

(PPPoE) and Point-to-Point Protocol over ATM (PPPoA), are used most

commonly by Internet Service Providers (ISPs) to establish a Digital

Subscriber Line (DSL) Internet service connection with customers.

PPP is commonly used as a data link layer protocol for connection over

synchronous and asynchronous circuits, where it has largely superseded the

older Serial Line Internet Protocol (SLIP) and telephone company mandated

standards (such as Link Access Protocol, Balanced (LAPB) in the X.25

protocol suite).

PPP was designed to work with numerous network layer protocols, including

Internet Protocol (IP), TRILL, Novell's Internetwork Packet Exchange (IPX),

NBF and AppleTalk.

10.3 LABP (Link Access Procedure Balance)

Link Access Procedure, Balanced (LAPB) implements the data link layer as

defined in the X.25 protocol suite. LAPB is a bit-oriented protocol derived

from HDLC that ensures that frames are error free and in the right sequence.

LAPB is specified in ITU-T Recommendation X.25 and ISO/IEC 7776. It can

be used as a Data Link Layer protocol implementing the connection-mode

data link service in the OSI Reference Model as defined by ITU-T

Recommendation X.222.

LAPB is used to manage communication and packet framing between data

terminal equipment (DTE) and the data circuit-terminating equipment (DCE)

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devices in the X.25 protocol stack. LAPB is essentially HDLC in

Asynchronous Balanced Mode (ABM).

LAPB sessions can be established by either the DTE or DCE. The station

initiating the call is determined to be the primary, and the responding station is

the secondary

(Figure Of Link Access Procedure Balance )

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11. ISDN Uses

11.1 Remote Access (Telecommuters)

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11.2 Remote Nodes (Voice and Data)

11.3 SOHO Connectivity (Small Branches)

11.1 Remote Access (Telecommuters)

11.2 Remote Nodes (Voice and Data)

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11.3 SOHO Connectivity (Small Branches)

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12. FEATURES OF ISDN

Increased productivity and accessibility

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Two phone numbers on a single line

Multiple Devices

Can transport many types of Network traffic (Voice, Data, Video, Text, Graphics etc)

Faster Data transfer rate than modem

Faster Call setup than Modems

Carries a variety of user traffic, such as digital video, data, and telephone network services, using the normal phone circuit-switched network

Offers much faster call setup than modems by using out-of-band signaling (D channel)

Provides a faster data transfer rate than modems by using the 64-kbps bearer channel (B channel)

Can combine multiple B channels to bandwidth of 128 kbps Can negotiate PPP links Carries a variety of user traffic, such as digital video, data, and telephone

network services, using the normal phone circuit-switched network Offers much faster call setup than modems by using out-of-band signaling (D

channel) Often less than one second

Provides a faster data transfer rate than modems by using the 64-kbps bearer channel (B channel)

Can combine multiple B channels to bandwidth of 128 kbps Can negotiate PPP links

13.ISDN ADVANTAGES

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Integrated Services Digital Network

The basic advantage of ISDN is to facilitate the user with multiple digital

channels. These channels can operate concurrently through the same one

copper wire pair.

The digital signals broadcasting transversely the telephone lines.

ISDN provides high data rate because of digital scheme which is 56kbps.

ISDN network lines are able to switch manifold devices on the single line

such as faxes, computers, cash registers credit cards readers, and many

other devices. These all devices can work together and directly be

connected to a single line.

ISDN takes only 2 seconds to launch a connection while other modems take

30 to 60 second for establishment.

14.ISDN DISADVANTAGEES

The disadvantage of ISDN lines is that it is very costly than the other

typical telephone system.

ISDN requires specialized digital devices just like Telephone Company.

15. CONCLUSION

ISDN is replacing our old analog phones and offers a lot of new services.

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Integrated Services Digital Network

Easy way to transmit voice and data simultaneously at the same time using

advantages of a digital communication.

Due to its easy accessibility it is widely used.

16. REFERENCES

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Integrated Services Digital Network

Thiagarajan Viswanathan, Telecommunication Switching Systems and

Networks by, PHI Learning Pvt. Ltd., New Delhi.

Communication Networks, A Leon-Garcia and Indra Widiaja, TMH,

New Delhi

Data and Computer Communications by W Stallings, Pearson Education

www.google.com

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