Download ISDN ( INTEGRATED SERVICES DIGITAL NETWORK)

ISDN ( INTEGRATED SERVICES DIGITAL NETWORK)
INTRODUCTION:
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ISDN was one of the largest and most ambitious projects launched in the mid 1980’s.
Rapid development in Semiconductor, Computer and Communication Technologies were
leading to a merger of various fields and, increasingly same digital techniques and digital
equipment were being employed for data, voice and video transmission. This led to the
concept of ISDN- a world wide single digital telecommunication network that can
provide all these services over the existing telephone twisted pair wire
ISDN had following objectives:
A world wide all digital Network
It will provide access to both leased and dial-up telephone services
It will be governed by a single set of standards
It will be a natural evolution of the existing Public switched Telephone Network
(PSTN) and will take a decade or more to be fully implemented
It will provide a wide variety of services including voice, data, video, monitoring
etc, using a common outlet ( RJ-45 connector) similar to telephone jack thereby
providing portability of equipment and applications.
 ISDN was designed to provide a wide variety of Telecommunication Services
through an all digital network including
Access to dial up and leased telephone lines
Data transmission capability via access to Packet Switching Network
Video Conferencing and Videotext
Alarm monitoring
Telemetry
Teletext
Fax
All modern Telephone services such as Call Forwarding, Call Waiting, Caller ID,
Call Transfer, Conference Call, Credit Call Calling and so on
 It is instructive to note that despite of all the hype about ISDN, it has yet to
achieve the universal deployment it hoped for because of cost factor. AT & T and
MCI did try to push National ISDN aggressively a few years back. However, it is
fading away in favor of more recent developments. Still, there a re a lot of
installations in Europe and North America with ISDN deployed and a basic
knowledge of ISDN operation is necessary in order to gain an appreciation of
more recent high-speed technologies. We shall look at the basic features of ISDN
and move on to more advanced Broadband technologies.
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ISDN CHARACTERISTICS AND ARCHITECTURE
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The ISDN is an all-digital Network, which means information is carried in digital form
right from the customer premises to destination including the local loop. Also important
to note is that it uses existing twisted pair telephone wiring. This implies that the
conversion from analog to digital is performed in the Telephone set.
 ISDN Provides both the Circuit Mode and Packet Mode Services
 ISDN Allows multitasking capabilities over multiple channels, all over a single pair of
twisted wire and over the existing local loop.. This allows a user to
(i)
Simultaneously make a telephone call and receive another one on the same line.
(ii)
Carry out a telephone conversation on one channel and transmit data on the 2nd
channel at the same time on a single link
 This multitasking capability of ISDN allows it to provide Integrated Services
Simultaneously including voice, data, video etc
 For example, ISDN allows a researcher to have telephone conversation with his assistant
from a remote location, see him on the screen and also discuss a document that appears in
a separate window of the screen or a specialist in North America to discuss an X-RAY of
a patient some where in Asia with his family Doctor
 These kinds of applications require very High BW, Sophisticated equipment and complex
Communication Technologies.
 ISDN uses complex time Division Multiplexing schemes to multiplex several services on
a single link and sophisticated signaling Protocols to control the link
 ISDN uses a separate Packet mode channel for Signaling functions. This channel can also
be used for special services such as Telemetry and Alarm Monitoring because of the
intermittent nature of these services.
 National ISDN called ISDN-1 provides Standards for Vendor Interoperability.
 Broadband ISDN provides Standards for very High Speed Networking with technologies
such as ATM and SONET.
ISDN INTERFACES /SERVICES AND CHANNELS:
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ISDN defines two types of Interfaces to provide the services mentioned above.
(a)
A BASIC RATE INTERFACE commonly known as BRI
(b)
A PRIMARY RATE INTERFACE commonly known as PRI.
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BASIC RATE INTERFACE (BRI): A BRI consists of 2 Bearer channels called BChannels each running at 64Kbps and used to carry Voice or Data ( in Circuit Mode and
Packet Mode ). And a CONTROL channel Called D- channel running at 16 Kbps used
primarily for Signaling and Control. The D channel only operates in the Packet Mode.
Notice also that the D-Channel can also be used for special services such as telemetry
and alarm monitoring when not used by B-channel This set up is popularly known as
2B+D running at a combined rate of 144Kbps as shown in the Diagram below. Draw the
diagram in the space provided.
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PRIMARY RATE SERVICE (PRI):
(a)
Provides 23 B channels and a D- Channel each running at 64Kbps
(b)
B-Channels are used for to carry voice or data and can operate in either the
circuit mode or packet mode.
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D- Channel operates only in the packet mode and is used for Call set-up and
Clearing, Signaling and control of B-Channels on time multiplexed basis
(d)
D- Channel can also be used for special services such as Telemetry, file transfer,
email, fax etc when idle
(e)
This set-up is commonly known as (23B+D) and offers a combined rate of 1.544
Mbps, which coincides with the T1 or DS1 rate. Thus ISDN was designed to be
compatible with all existing technological infrastructure in order to allow a
natural evolution.
Fig below shows BRI applications. Draw these pictures in the space provided. (Refer to
the pictures in your notes written for Telecommunications Institute).
Fig below shows PRI applications. Notice that prior to implementing ISDN, a large
business organization may have been using separate connections for the various services
such as :
A bunch of analog trunks for its switched public Telephone requirements
A separate analog Tie trunk for leased lines
A separate Packet mode line for Data
A separate Centrex line etc
With ISDN implementation ( as shown in the fig), a business organization gets the same
range of services through an integrated digital access.
MegaLink PRI is providing the access to various Networks such as PSTN, Public
Switched Data networks (such as FR or ATM) and other services
Notice also that with ISDN, BW can be shared between applications, for example, if
outgoing channels are idle, they can be used to carry in coming traffic and vice versa.
Common PRI CPES include LAN Servers, HOSTS, Concentrators and PBXS
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The Table shows ISDN channels and their applications ( HIOKI)
H-channels are used for High Speed transmissions such as High Quality Audio,
Teleconferencing, Video conferencing and high Speed Data
ISDN INTERFACES, FUNCTIONAL GROUPINGS AND REFERENCEPOINTS:
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A key element of ISDN is a small set of compatible multipurpose user to Network
Interfaces developed to support a wide range of applications. Fig below shows a complete
ISDN Network Architecture. Draw the picture in the space provided.
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As shown in the fig, the ISDN reference configuration consists of functional groupings
and reference points at which physical interfaces may exist.
The functional groupings are a set of functions that may be required at an Interface,
whereas, the reference points are employed to divide the groupings into distinct entities.
Following functional groupings have been defined:
TE ( Terminal Equipment): TE1 and TE2
NT (Network Termination): NT1 and NT2
Reference Points: R, S, T, U
We shall now consider each one of these functional groupings and Reference points
separately and try to understand what they really mean.
TE FUNCTIONAL GROUPING: This simply means what kind of user equipment
can be connected to an ISDN Network. ISDN defines two kinds-TE1 and TE2
TE1: These are ISDN compatible equipment with 4-wire digital interface built in. These
Equipment can be connected directly to an ISDN Network with out requiring any
physical or Protocol conversion. These include ISDN digital telephones, data terminals
and integrated voice and data workstations. They connect directly to the “S” interface.
Thus “S” interface in ISDN compatible Interface.
TE2: These are non-ISDN compatible equipment such as regular analog telephone and
workstations that use RS-232, 422, 423, V.35 interfaces. These equipment must be
connected through a device called TERMINAL ADAPTER TA . The TA performs
physical and protocol conversions to enable regular devices to communicate over the
ISDN Network. The Interface between the TE2and TA is called “R” interface. Thus
“R” stands for any interface that is not compatible with “s” interface. The function
of TA is to convert “R” interface to “S” interface.
NT1: (Network termination 1): is a device that makes the 4-Wire ISDN internal
wiring compatible with the 2-Wire local loop. It performs physical layer ( Layer 1)
functions such as physical and electrical termination of the loop, line monitoring, timing
and bit multiplexing etc. It interfaces the CPE 4-Wire digital ISDN wiring to the 2-Wire
ISDN Network Subscriber’s loop. A BRI connection would need an NT1.
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NT2: (Network Termination 2): include devices that perform switching and data
concentration functions equivalent to layer1, 2, and 3 of the OSI model ( physical, data
link, and Network). These include PBXS, LAN Servers, Terminal controllers,
concentrators, Multiplexers etc. Typically a PRI line would require an NT2 device.
This interface is called a “T” interface. This device may be just an extra card installed in
a PBX or a Multiplexer or a LAN Server. In North America, a 3rd party and not the
communication Carrier provide this device.
Notice that the point between NT2 and NT1 is called the “T” interface. For BRI service,
NT2 wouldn’t exist. In that case the point between the TE and NT1 is called an S/T
interface.
Fig shows that NT1 interfaces 4-Wire digital wiring of the customer to 2-Wire street
wiring. This 2-Wire loop terminates at the Local Termination (LT) inside the Local
Exchange (LE). The Communication Carrier will provide NT1. The Street wiring is
called the “U” interface.
Fig shows an interface point “V” inside the ISDN local exchange, between LT and ET.
ET is called the Exchange Termination. The LT complements the functions of NT1 on
the LE side. ET is the ISDN circuit mode Switch and PH (Packet Handler) is the ISDN
packet Switch which interfaces with the public data switching Network such as Frame
Relay or ATM or X.25 etc.
The ISDN BRI Physical Layer Protocol : Defines the physical interfaces, electrical
connector, data encoding scheme, bit timing and synchronization techniques, signaling
capabilities for activation and deactivation of terminals as well as procedures to gain an
orderly access to D-channel
The BRI has two modes of operation- point –to-point and multipoint. In point – to –
point operation the maximum distance between the TE and NT is 1000 meters.
The BRI is a serial, Synchronous, full duplex connection.
The BRI electrical interface between the NT and TE is an 8 pin interface called RJ-45
connector similar to our telephone jack. The fig shows this connector as well as the pin
layout and pin function
ISDN BRI runs at 192 kbps. But the user BW is only 144 kbps. Rest of the BW is used
for framing.
ISDN BRI uses PSEUDO-TERNARY line encoding over the S/T interface between TE
and NT. This Scheme is shown below. (Draw picture in the space below). A “1” is coded
as 0V and a “0” is coded as +1 or –1 alternately. Why? __________________________
The ISDN BRI uses Two Binary one Quaternary (2B1Q) encoding over the “u” interface
(that is over the street wiring) between the NT and LE. As shown in the fig below. Draw
fig in the space provided. It encodes two bits as follows: 00= -3V, 01= -1V, 10= +3V,
11= +1V. Encode the bit pattern 100111100010101 in the space below.
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It is a FDX, 160Kbps, 4 Wire, synchronous link between an NT and LE. It uses echo
cancellers to provide FDX operation. User BW is still 144kbps. Maximum distance
between NT and LE is 18KM.
ISDN PRI physical layer protocol:
The PRI provides a point to point connection between the ISDN Network and a device
that requires a large amount of BW such as a PBX, Communication controller, a LAN
Server, a concentrator or Multiplexer and Video conferencing system.
PRI provides 23-B channels for user application and a D- channel for control. In north
America, PRI operates at 1.544MBPS and can be carried over a T1 circuit. Thus T1 is
the physical layer of PRI. Frame structure is exactly the same as T1 line-193 bit frame
including the framing bit (248 +1) transmitted 8000 times per second. Frame duration
is 125 microseconds Uses AMI- B8ZS encoding. Notice that NT1 functions are built in
and therefore a PRI typically stops at NT2. It is usually a card inserted into the devices
mentioned above.
Since 23 channels share the PRI trunk BW, ISDN defines layer 1,2,3 functions for the
operation of the D channel in order to provide control of these devices. We shall look at
the layer 2 and layer 3 functions of the D channel in the next section. Remember that
both BRI and PRI - B channels require only physical layer services, but D- channel (BRI
and PRI both) require layer 1,2 and 3 services. In Europe, PRI is transmitted over E lines
at 2.048 MBPS with 32 and provides 32 channels. Time slot “0” is used for error control
and frame alignment and time slot 16 is used for D-channel operation.
ISDN Data link Control Protocol for D-channel: ISDN uses LAP-D (Link Access
Procedure for D Channel) to provide Data link services. Lap-D is very similar to HDLC
studied in earlier semester and so it will not be elaborated here. Please consult your notes
for Principles of Telecom course.
ISDN Network Layer Protocol for the D channel: ISDN network layer services are
used for call establishment and termination and to control the call in progress. It uses
SS#7 call control messages as shown in the table below. The procedure is similar to
X.25 and TCP network layer functions. Study the table shown. It is called ISUP (ISDN
User Part).
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