Transcript ISDN - BSNL Durg SSA(Connecting India)

Project Report On
ISDN
Under the Guidance of
Mr. Sachin Khandelwal(S.D.E.)
Submitted by:
Arpit Biyani(Group leader)
Shazalee Lalani
Yogita Tripure
Devshri Sahu
Saurabh Kumar Nayak
PROJECT TEAM
Project Leader:
Arpit Biyani
Phone: 9993334143
Email: [email protected]
Project Members:
Shazalee Lalani
Phone: 9406252520
Email: [email protected]
Saurabh Kumar Nayak
Phone: 8871100992
Email:
Devshri Sahu
Phone:
Email:
Yogita Tripure
Phone:
Email:
DECLARATION
We hereby declare that the Project
entitled “Integrated Service Digital Network”
being submitted in partial fulfillment for the
certificate for vocational training to “Bharat
Sanchar Nigam Limited, Durg” is the authentic
record of our own work done under the
guidance of Mr. Sachin Khandelwal(SDE), our
project guides.
Project Members:
1.Arpit Biyani
2.Shazalee Lalami
3.Yogita Tripure
4.Devshri Sahu
5.Saurabh Kumar Nayak
ACKNOWLEDGEMENT
First & foremost, we thank Almighty God for giving me this
unique opportunity to express my heartfelt gratitude to all those
who have extended helping hands to make this study success.
We have a great pleasure in expressing my deep sense of
gratitude to Mr. Sachin Khandelwal ,S.D.E. (Computer Faculty),
without whose help we would never have achieved completion in
our work. We would heartily like to thank DGM(CFA) and all
our teachers for guiding us forth in this project.
With due regards we feel immense pleasure in expressing our
deepest gratitude to our amiable parents & friends whose filial
affection, encouragement & blessing have been a beacon light to
us in all undertakings.
Date: - 4th July 2011
Submitted by:
Arpit Biyani
Shazalee Lalani
Devshri Sahu
Yogita Tripure
Saurabh Kumar Nayak
CERTIFICATE
This is to certify that Arpit Biyani a student of
Computer Science Branch in Natinal
Institute of Technology, Raipur College has
successfully completed a project on the topic“Integrated Service Digital Network” on 4th
Jul.,2011 & submitted the report on the same,
under the guidance of our project guides Mr.
Sachin Khandelwal, S.D.E.(computer faculty)
Project GuidesMr. Sachin Khandelwal(S.D.E.)
CERTIFICATE
This is to certify that Shazalee Lalani a student
of Electronics and Telecommunication
Branch in Chhattisgarh Institute Of
Technology, Rajnandgaon College has
successfully completed a project on the topic“Integrated Service Digital Network” on 4th
Jul.,2010 & submitted the report on the same,
under the guidance of our project guides Mr.
Sachin Khandelwal, S.D.E.(computer faculty)
Project Guides-
Mr. Sachin Khandelwal(S.D.E.)
CERTIFICATE
This is to certify that Saurabh Kumar Nayak a
student of Electronics and Telecommuniction
Branch in Garv Institute of Technology
College has successfully completed a project on
the topic-“Integrated Service Digital
Network” on 4th Jul.,2010 & submitted the
report on the same, under the guidance of our
project guides Mr. Sachin Khandelwal,
S.D.E.(computer faculty).
Project GuidesMr. Sachin Khandelwal(S.D.E.)
CERTIFICATE
This is to certify that Yogita Tripure a student
of ……………………………. Branch in
……………………………… ………………… ……………………
College has successfully completed a project on
the topic- “Integrated Service Digital
Network”on 4th Jul.,2010 & submitted the
report on the same, under the guidance of our
project guides Mr. Sachin Khandelwal,
S.D.E.(computer faculty).
Project GuidesMr. Sachin Khandelwal(S.D.E.)
CERTIFICATE
This is to certify that Devshri Sahu a student of
………………………. Branch in ………………………………
…………………
……………………
College
has
successfully completed a project on the topic“Integrated Service Digital Network” on 4th
Jul.,2010 & submitted the report on the same,
under the guidance of our project guides Mr.
Sachin Khandelwal, S.D.E.(computer faculty)
Project GuidesMr. Sachin Khandelwal(S.D.E.)
INDEX
CONTENTS….
PAGE NO.
ISDN
Integrated Services Digital Network (ISDN) is a
set of communications standards for
simultaneous digital transmission of voice,
video, data, and other network services over the
traditional circuits of the public switched
telephone network. It was first defined in 1988
in the CCITT red book.[1] Prior to ISDN, the phone
system was viewed as a way to transport voice,
with some special services available for data.
The key feature of ISDN is that it integrates
speech and data on the same lines, adding
features that were not available in the classic
telephone system.
ISDN is a circuit-switched telephone network
system, which also provides access to packet
switched networks, designed to allow digital
transmission of voice and data over ordinary
telephone copper wires, resulting in potentially
better voice quality than an analog phone can
provide. It offers circuit-switched connections
(for either voice or data), and packet-switched
connections (for data), in increments of 64
kilobit/s. ISDN has been specifically designed to
solve the low bandwidth problems that small
offices or dial-in users have with traditional
telephone dial-in services.
Telephone companies developed ISDN with the
intention of creating a totally digital network
whilst making use of the existing telephone
wiring system.
ISDN works very much like a telephone - When
you make a data call with ISDN, the WAN link is
brought up for the duration of the call and is
taken down when the call is completed. ISDN
allows digital signals to be transmitted over
existing telephone wiring.
This became possible when the telephone
company switches were upgraded to handle
digital signals.
ISDN is generally viewed as an alternative to
leased lines, which can be used for
telecommuting and networking small and remote
offices into LANs.
A major market application for ISDN in some
countries is Internet access, where ISDN typically
provides a maximum of 128 kbit/s in both
upstream and downstream directions. Channel
bonding can achieve a greater data rate; typically
the ISDN B-channels of 3 or 4 BRIs (6 to 8 64 kbit/s
channels) are bonded.
ISDN should not be mistaken for its use with a
specific protocol, whereby ISDN is employed as
the network, data-link and physical layers in the
context of the OSI model. In a broad sense ISDN
can be considered a suite of digital services
existing on layers 1, 2, and 3 of the OSI model.
ISDN is designed to provide access to voice and
data services simultaneously.
However, common use has reduced ISDN to be
limited and related protocols, which are a set of
protocols for establishing and breaking circuit
switched connections, and for advanced call
features for the user. They were introduced in
1986.In a videoconference, ISDN provides
simultaneous voice, video, and text transmission
between individual desktop videoconferencing
systems and group (room) videoconferencing
systems. ISDN provides several communication
channels to customers via local loop lines
through a standardized digital transmission line.
ISDN is provided in two interface formats: basic
rate (primarily for consumers) and high-speed
rate (primarily for businesses). The basic rate
interface (BRI) is 144 kbps and is divided into
three digital channels called 2B + D. The primary
rate interface (PRI) is 1.54 Mbps and is divided
into 23B + D for North America and 2.048 Mbps
and is divided into 30B + 2D for the rest of the
world. The digital channels for the BRI are carried
over a single, unshielded, twisted pair of copper
wires and the PRI is normally carried on (2)
twisted pairs of copper wire.
This diagram shows the different interfaces are
available in the integrated services digital
network (ISDN). The two interfaces shown are
BRI and PRI. These are all digital interfaces from
the PSTN to the end customer's network
termination. Network termination 1 (NT1)
equipment devices can directly connect to the
NT1 connection. Devices that require other
standards (such as POTS or data modems)
require a terminal adapter (TA). This example
shows that the NT2 interface works with the
NT1 interface to allow the application layers
(terminal intelligence) to communicate with the
ISDN termination equipment.
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.
ISDN Disadvantages
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.
Integrated services refers to ISDN's ability to
deliver at minimum two simultaneous
connections, in any combination of data, voice,
video, and fax, over a single line. Multiple devices
can be attached to the line, and used as needed.
That means an ISDN line can take care of most
people's complete communications needs (apart
from broadband Internet access and
entertainment television) at a much higher
transmission rate, without forcing the purchase of
multiple analog phone lines. It also refers to
Integrated Switching and Transmission in that
telephone switching and carrier wave
transmission are integrated rather than separate
as in earlier technology.
There are several kinds of access interfaces to
ISDN defined as Basic Rate Interface (BRI),
Primary Rate Interface (PRI) and Broadband ISDN
(B-ISDN).
Basic Rate Interface
The entry level interface to ISDN is the Basic(s)
Rate Interface (BRI), a 128 kbit/s service delivered
over a pair of standard telephone copper wires.
The 144 kbit/s payload rate is broken down into
two 64 kbit/s bearer channels ('B' channels) and
one 16 kbit/s signaling channel ('D' channel or
delta channel). This is sometimes referred to as
2B+D.
The interface specifies the following network
interfaces:
The U interface is a two-wire interface between
the exchange and a network terminating unit,
which is usually the demarcation point in nonNorth American networks.
The T interface is a serial interface between a
computing device and a terminal adapter, which
is the digital equivalent of a modem.
The S interface is a four-wire bus that ISDN
consumer devices plug into; the S & T reference
points are commonly implemented as a single
interface labeled 'S/T' on an Network termination
1 (NT1).
The R interface defines the point between a
non-ISDN device and a terminal adapter (TA)
which provides translation to and from such a
device.
BRI-ISDN is very popular in Europe but is
much less common in North America. It is also
common in Japan - where it is known as INS64.
Primary Rate Interface
The other ISDN access available is the Primary
Rate Interface (PRI), which is carried over an E1
(2048 kbit/s) in most parts of the world. An E1 is
30 'B' channels of 64 kbit/s, one 'D' channel of 64
kbit/s and a timing and alarm channel of 64 kbit/s.
In North America PRI service is delivered on
one or more T1 carriers (often referred to as
23B+D) of 1544 kbit/s (24 channels). A PRI has
23 'B' channels and 1 'D' channel for signalling
(Japan uses a circuit called a J1, which is similar
to a PRI). Inter-changeably but incorrectly, a PRI
is referred to as T1 because it uses the T1
carrier format. A true T1 or commonly called
'Analog T1' to avoid confusion uses 24 channels
of 64 Kbit/s of in band signaling. Each channel
uses 56 kb for data and voice and 8 kb for
signaling and messaging. PRI uses out of band
signaling which provides the 23 B channels with
clear 64 kb for voice and data and one 64 kb 'D'
channel for signaling and messaging. In North
America allows two or more PRIs to be
controlled by a single D channel, and is
sometimes called "23B+D + n*24B". D-channel
backup allows for a second D channel in case
the primary fails. NFAS is commonly used on a
T3.
PRI-ISDN is popular throughout the world,
especially for connecting PBXs to PSTN.
While the North American PSTN can use PRI or
Analog T1 format from PBX to PBX, the POTS or
BRI can be delivered to a business or residence.
North American PSTN can connect from PBX to
PBX via Analog T1, T3, PRI, OC3, etc...
Even though many network professionals use the
term "ISDN" to refer to the lower-bandwidth BRI
circuit, in North America BRI is relatively
uncommon whilst PRI circuits serving PBXs are
commonplace.
In the 1980s the telecommunications industry
expected that digital services would follow
much the same pattern as voice services did
on the public switched telephone network,
and conceived a grandiose end-to-end circuit
switched services, known as Broadband
Integrated Services Digital Network (B-ISDN).
This was designed in the 1990s as a logical
extension of the end-to-end circuit switched
data service, Integrated Services Digital
Network (ISDN).
Before B-ISDN, the original ISDN attempted to
substitute the analog telephone system with a
digital ISDN system which was appropriate for
both voice and non voice traffic. Obtaining
worldwide agreement on the Basic rate
interface standard was expected to lead to a
large user demand for ISDN equipment, hence
leading to mass production and inexpensive
ISDN chips. However, the standardization
process took years while the technology in
this area moved rapidly. Once the standard
was finally agreed upon it was already
obsolete.
For home use the largest demand for new
services was video and voice transfer, but the
ISDN basic rate lacks the necessary channel
capacity. For business, ISDN's 64 kbit/s data rate
compared unfavorably to 10 Mbit/s LANs. This
led to introduction of B-ISDN. Services included
video telephone and video conferencing.
The designated technology for B-ISDN was
Asynchronous Transfer Mode (ATM), which was
intended to carry both synchronous voice and
asynchronous data services on the same
transport.
The B-ISDN vision has been overtaken by the
disruptive technology of the Internet. The ATM
technology survives as a low-level layer in most
Digital Subscriber Line (DSL) technologies, and as
a payload type in some wireless technologies
such as WiMAX.
Components of ISDN
While individual operating companies and
ministries will define the specific services,
within the ISDN architecture the ITU standards
define a number of component parts and
functions:
ISDN CHANNELS
ACCESS TYPES
DEVICES
INTERFACES
PROTOCOLS
•ISDN Channels
A CHANNEL is the basic unit of ISDN service.
The ISDN Standards define three basic types of
channels:
Bearer channels (B channels)
Delta (or "Demand") channels (D channels)
High-capacity channels (H channels)
B Channel
A B channel is a 64-Kbps unit of clear digital
bandwidth. Based on the data rate required to
carry one digital voice conversation, a B channel
can carry any type of digital information (voice,
data, or video) with no restrictions on format or
protocol imposed by the ISDN carrier.
D Channel
A D channel is a signalling channel. It carries the
information needed to connect or disconnect
calls and to negotiate special calling parameters
(i.e., automatic number ID, call waiting, data
protocol). The D channel can also carry packetswitched data using the X.25 protocol.
The D channel is not a clear channel. It operates
according to a well-defined pair of layered
protocols:
•Q.921 (LAPD) at the Data Link Layer (Layer 2)
•Q.931 at the upper layers (Layers 3 and above)
The data rate of a D channel varies according to
the type of access it serves: a Basic Rate Access
D channel operates at 16 Kbps and a Primary
Rate Access D channel operates at 64 Kbps.
H Channel
An H channel is a special, high-speed clear
channel. H channels, designed primarily for fullmotion color video, are not yet in common use.
There are currently three kinds of H channel:
•H0 ("H-zero")
•H11 ("H-one-one")
•H12 ("H-one-two")
An H0 channel operates at 384 Kbps (roughly
one fourth of a North American Primary Rate
Access or one fifth of a European Primary Rate
Access). An H1 channel operates at 1.536 Mbps
and occupies one whole North American Primary
Rate Access. An H12 channel occupies an entire
European Primary Rate Access.
ISDN Devices
In the context of ISDN standards, STANDARD
DEVICES refers not to actual hardware, but to
standard collections of functions that can
usually be performed by individual hardware
units. The ISDN Standard Devices are:
•Terminal Equipment (TE)
•Terminal Adapter (TA)
•Network Termination 1 (NT1)
•Network Termination 2 (NT2)
•Exchange Termination (ET)
Terminal Equipment (TE)
A TE is any piece of communicating equipment
that complies with the ISDN standards.
Examples include: digital telephones, ISDN data
terminals, Group IV Fax machines, and ISDNequipped computers.
In most cases, a TE should be able to provide a
full Basic Rate Access (2B+D), although some
TEs may use only 1B+D or even only a D
channel.
Terminal Adapter (TA)
A TA is a special interface-conversion device that
allows communicating devices that don't
conform to ISDN standards to communicate
over the ISDN.
The most common TAs provide Basic Rate
Access and have one RJ-type modular jack for
voice and one RS-232 or V.35 connector for data
(with each port able to connect to either of the
available B channels). Some TAs have a separate
data connector for the D channel.
Network Termination (NT1 and NT2)
The NT devices, NT1 and NT2, form the physical
and logical boundary between the customer's
premises and the carrier's network. NT1
performs the logical interface functions of
switching and local-device control (local
signalling). NT2 performs the physical interface
conversion between the dissimilar customer
and network sides of the interface.
In most cases, a single device, such as a PBX or
digital multiplexer, performs both physical and
logical interface functions. In ISDN terms, such a
device is called NT12 ("NT-one-two") or simply
NT.
Exchange Termination (ET)
The ET forms the physical and logical boundary
between the digital local loop and the carrier's
switching office. It performs the same functions
at the end office that the NT performs at the
customer's premises.
In addition, the ET:
1. Separates the B channels, placing them on
the proper interoffice trunks to their
ultimate destinations.
2. Terminates the signalling path of the
customer's D channel, converting any
necessary end-to-end signalling from the
ISDN D-channel signalling protocol to the
carrier's switch-to- switch trunk signalling
protocol.
ISDN Protocols
The ISDN protocols are signalling protocols that govern the
exchange of data on the D channel. The two ISDN signalling
protocols make up a layered protocol stack, with the Link
Access Protocol for the D Channel (LAPD, also known as Q.921)
providing Layer 2 data-link services and the Q.931 protocol
providing higher-layer services.
LAPD is a simple, bit-oriented data-link protocol similar in
structure and operation to HDLC and SDLC. The Q.931
signalling protocol is one of the most complex and feature-rich
communication protocols ever designed.
LAPD (Q.921)
The LAPD protocol operates between TE and NT over the D
channel of an ISDN S interface. In traditional data
communications terms, the TE acts as DTE and the NT acts as
DCE.
The unit of LAPD transmission is a FRAME. As in other bitoriented protocols, frames are demarcated from an idle circuit
and from other frames by a FLAG pattern. Like HDLC, LAPD can
operate with either a Modulo 8 or a Modulo 128 frame
window.
A LAPD frame contains the following fields:
•ADDRESS
•COMMAND/RESPONSE BIT
•CONTROL
•INFORMATION (only in frames carrying higher-layer data)
•FRAME CHECK SEQUENCE
Refer to the INTERVIEW Technical Manual for information
about decoding LAPD frames.
LAPD vs. Other Bit-Oriented Protocols
The principal differences between LAPD
and other bit-oriented protocols are the
structure of the address field and the
optional exchange of Sequenced
Information (SI0 and SI1) frames.
LAPD Address Field
A LAPD address is 16 bits long and
contains two parts: the SERVICE
ACCESS POINT IDENTIFIER (SAPI)
and the TERMINAL ENDPOINT
IDENTIFIER (TEI). The SAPI identifies
the specific service (i.e., voice, circuitswitched data, network management,
etc.) to which the frame refers. The TEI
identifies the TE itself, especially in
situations such as Primary Rate Access
or the Basic Rate Access short passive
bus, where a single physical link might
terminate at more than one TE.
Sequenced INFORMATION
Frames
For applications that require a quicker
response to frame errors than the
normal MOD 8 or MOD 128 sequence
numbering offers, LAPD provides a
Sequenced Information service which
uses a MOD 2 "frame window."
Sequenced Information frames
traveling in the same direction alternate
between SI0 and SI1, reducing the
LAPD frame window to one outstanding
frame for special situations.
Q.931: The ISDN D-Channel
Signalling Protocol
In fulfilling the ISDN goal of Integrated Services
over common facilities, the Q.931 D-channel
signalling protocol does much of the
integrating. The principal job of Q.931 is to
carry signalling information about the nature
of the ISDN service required for specific calls
(or data sessions) between the end user's
terminal equipment and the ISDN carrier's end
office.
The following is a short list of
some critical information
theQ.931 protocol MUST
convey:
SERVICE INFORMATION
Information on the nature of the service
requested for the call: voice, D-channel packet
switched data, B-Channel packet switched data,
circuit-switched data, electronic mail, facsimile,
video, or others
TERMINAL CAPABILITIES
Information on the capabilities of the terminal
equipment originating and receiving the call:
the type of signalling required (i.e., stimulus
signalling for simple digital telephones or
functional signalling for full- featured ISDN
terminals) and the terminal's ability to handle
special features and services [e.g., Automatic
Number Identification (ANI), ANI blocking, 800service, call screening, call forwarding, data rate
adaptation, conference calling].
Testing ISDN
Like any digital communications facility, ISDN can
be tested at any of several levels. ISDN tests can
operate strictly at the physical level, at the level
of the logical transmission path, and at the
higher levels of logical protocol. All of these tests
can provide valuable information in testing ISDN
circuits and equipment.
Physical Testing
PAIR QUALIFICATION is the most common reason
for testing ISDN at the physical level. ISDN
circuits must often use pre-ISDN local-loop
facilities designed to carry more-robust analog
transmissions. A high-speed digital transmission
technique is sensitive to signal degradation from
such common local-loop features as bridge taps
and echo cancellers.
Before installing ISDN, carrier craftspeople must
qualify the wire pairs to handle the ISDN signal.
Purely physical parameters such as continuity,
impedance, and electrical loading are especially
important.
At a slightly higher level, digital tests such as Bit
Error Rate and Error-Free Seconds Rate can be
used to qualify the local loop.
Protocol Testing
There are four basic reasons for performing
protocol tests on ISDN circuits:
•CONFORMANCE TESTING
•INTEROPERABILITY TESTING
•PERFORMANCE TESTING
•TROUBLESHOOTING
Conformance Testing
Conformance testing is designed to prove
whether a given device, service, feature, or
implementation of ISDN conforms to a specific
standard. The standard may be the ITU I- and Qseries references or may be a carrier's or
manufacturer's own technical reference.
Conformance tests are usually run automatically
in long series of short, very specific tests with
pass/fail results provided in stages along the
way. Many ISDN providers, especially
telecommunications ministries, require
conformance testing before a given product or
service can be operated on their networks. A
given product or service is usually tested once
for conformance.
Interoperability Testing
Interoperability testing is designed to prove
whether two ISDN products or services (i.e., one
vendor's terminal and another vendor's switch)
can perform together according to specification.
Any ISDN product needs to be tested for
interoperability with any other ISDN product with
which it may communicate.
A maxim in interoperability testing is that, "The
commutative law does not apply." In other
words, if A interoperates with B and B
interoperates with C, A does not necessarily
interoperate with C. ISDN products must be
tested for conformance and interoperability at
every major revision.
Performance Testing
Performance testing requires the gathering and
display of statistics on the numbers of protocol
units (i.e., frames, packets, messages)
transmitted and received over time between
units. The goal of performance testing is to
discover deviations (from a specification or from
normal operation) that point to underlying
problems in the terminal or switching equipment
or in the operation of the protocols themselves.
For ISDN, degrading performance of the D
channel protocols (such as longer and longer Call
Setup times) can indicate a number of protocol
problems that ranges from user error at the
terminal to traffic overloading on the carrier's
network. Degrading data communications
performance on a B channel might point to a
failure to negotiate Calling parameters on the D
channel.
In general, performance testing uncovers
operational problems that might otherwise pass
interoperability testing.
Troubleshooting
Once the user has determined that a problem
has occurred on a circuit, troubleshooting finds
the problem's cause. For ISDN circuits to date,
the principal cause of circuit problems has been
user error.
ISDN defines many new ways of performing
familiar tasks (e.g., making a telephone call).
Practices that were once common sense can
now cause protocol problems.
Failure of terminal and switching equipment to
interoperate properly despite passing
interoperability tests is another major ISDN
worry, especially in end-to-end signalling
between similar but not identical terminals.
ISDN also adds a new layer of complexity to
straightforward protocol testing of data
communications over the B channels. Users
must now look for subtle effects of D-channel
Call Setup procedures, such as failure to
complete the call over the D channel before link
startup begins on the B channel.
Multichannel Protocol Monitoring
These descriptions of ISDN problems and testing
techniques illustrate the need for multichannel
protocol testing on ISDN circuits. Protocols on
the D channel control much of what happens on
the B channels, and events on the B channels
can highlight protocol problems on the D
channel.
In order to test ISDN properly, a protocol
analyzer must be able to monitor at least the D
channel and one of the B channels
simultaneously.
Monitoring and Emulation
An ISDN protocol analyzer should also be able
to monitor on one channel and emulate on
another.
Monitoring a B channel while simulating a Call
Setup on the D channel allows an operator to
see the intended (or unintended) results of Dchannel actions on the B channel under
control. Monitoring the D channel while
emulating on a B channel can illustrate
important signalling events, such as how the D
channel responds to an abnormal termination
on the B channel.
Multichannel Emulation
Emulating a switch or terminal device on both
the D channel and a B channel allows the
protocol analyzer to control an ISDN
communication completely, both to verify
normal operation and to test the effects of
abnormal conditions. All conformance and
interoperability testing of ISDN protocols
should be performed as dual-channel
emulations.
ISDN Backup
Backup systems are used in specific cases, when
you need to maintain a connection, even if a fault
occurs. For example, if someone cuts the wires, the
router can automatically connect to a different
interface to continue its work. Such a backup is
based on an utility that monitors the status of the
connection - netwatch, and a script, which runs the
netwatch.
This is an example of how to make simple router
backup system. In this example we'll use an ISDN
connection for purpose to backup a standard
Ethernet connection. You can, however, use instead
of the ISDN connection anything you need - PPP, for
example. When the Ethernet fail (the router nr.1
cannot ping the router nr.2 to 2.2.2.2 (see picture) the
router nr.1 will establish an ISDN connection, socalled backup link, to continue communicating with
the nr. 2.
You must keep in mind, that in our case there are just
two routers, but this system can be extended to
support more different networks.
The backup system example is shown in the
following picture:
Consumer and industry
perspectives
There are two points of view into the ISDN
world. The most common viewpoint is that of
the end user, who wants to get a digital
connection into the telephone network from
home, whose performance would be better
than a 20th century analog 56K modem
connection. Discussion on the merits of various
ISDN modems, carriers' offerings and tariffs
(features, pricing) are from this perspective.
Since the principal consumer application is for
Internet access, ISDN was mostly superseded by
DSL in the early 21st century. Inexpensive ADSL
service offers speeds up to 384 kbps, while
more expensive versions are improving in speed
all the time. As of fall 2005, standard ADSL
speeds are in millions of bits per second.
There is a second viewpoint: that of the
telephone industry, where ISDN is a core
technology. A telephone network can be
thought of as a collection of wires strung
between switching systems.
The common electrical specification for the
signals on these wires is T1 or E1. Between
telephone company switches, the signaling is
performed via SS7. Normally, a PBX is
connected via a T1 with robbed bit signaling to
indicate on-hook or off-hook conditions and MF
and DTMF tones to encode the destination
number. ISDN is much better because messages
can be sent much more quickly than by trying
to encode numbers as long (100 ms per digit)
tone sequences. This results in faster call setup
times. Also, a greater number of features are
available and fraud is reduced.
ISDN is also used as a smart-network
technology intended to add new services to the
public switched telephone network (PSTN) by
giving users direct access to end-to-end circuitswitched digital services and as a backup or
failsafe circuit solution for critical use data
circuits.
ISDN and broadcast industry
ISDN is used heavily by the broadcast industry as
a reliable way of switching low latency, high
quality, long distance audio circuits. In
conjunction with an appropriate codec using
MPEG or various manufacturers proprietary
algorithms, an ISDN BRI can be used to send
stereo bi-directional audio coded at 128kbps
with 20 Hz-20 kHz audio bandwidth, although
commonly the G.722 algorithm is used with a
single 64 kbps B channel to send much lower
latency mono audio at the expense of audio
quality. Where very high quality audio is
required multiple ISDN BRIs can be used in
parallel to provide a higher bandwidth circuit
switched connection. BBC Radio 3 commonly
makes use of three ISDN BRIs to carry 320 kbps
audio stream for live outside broadcasts. ISDN
BRI services are used to link remote studios,
sports grounds and outside broadcasts into the
main broadcast studio. ISDN via satellite is used
by field reporters around the world. It's also
common to use ISDN for the return audio links to
remote satellite broadcast vehicles.
In many countries, such as the UK and Australia,
ISDN has displaced the older technology of
equalized analogue landlines, with these circuits
being phased out by telecommunications
providers. IP based streaming codecs are
starting to gain a foothold in the broadcast
sector, using broadband internet to connect
remote studios. However reliability and latency
is crucially important for broadcasters and the
quality of service offered by ISDN has not yet
been matched by packet switched alternatives.
India
Bharat Sanchar Nigam Limited, Reliance
Communications and Bharti Airtel, are the
largest communication service providers, offers
both ISDN BRI and PRI services across the
country. Reliance Communications and Bharti
Airtel uses the DLC technology for providing
these services. With the introduction of
broadband technology, the load on bandwidth
is being absorbed by ADSL. ISDN continues to be
an important backup network for point-to-point
leased line customers such as banks, Eseva
Centers , Life Insurance Corporation of India,
and SBI ATMs.
Types of communications
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. 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 Call, 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 ISDNand 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. Most radio studios are
equipped with ISDN lines as their main form of
communication with other studios or standard
phone lines. Equipment made by companies
such as Telos/Omnia , Comrex, Tieline and
others are used regularly by radio broadcasters.
Almost all live sports broadcasts on radio are
backhauled to their main studios via ISDN
connections.