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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Integrated Services Digital Network
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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Integrated Services Digital Network
(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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Integrated Services Digital Network
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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Integrated Services Digital Network
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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Integrated Services Digital Network
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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Integrated Services Digital Network
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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Integrated Services Digital Network
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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Integrated Services Digital Network
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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