2. Integrated Services Digital Network
(ISDN)
Definition:-
• ISDN stands for Integrated Services Digital Network
• It is the name for digital telephone service that works over existing
copper telephone wiring. Thus ISDN is a high speed, fully digital
telephone service.
• ISDN was developed by ITU-T (International Telecommunication
Union, Telecommunications Standardization Sector) in 1976.
• It is a set of protocols that combine digital telephony and data
transport services.
3. • ISDN involves digitization of the telephone network, which permits
voice, data, text, graphics, music, video, and other source materials
to be transmitted over existing telephone wires.
• The purpose of ISDN is to promote fully integrated digital services
to users. These services fall in following three categories:-
1. Bearer Services:-
• Bearer services provide the mean to transfer information (voice,
data and video) between users without network manipulating the
concept of that information.
• Bearer services belong to first three layer of OSI model.
• They can be provided using circuit switching, packet switching,
frame switching or cell switching.
4. 2. Teleservices:-
• In teleservices the network may change or process the content of
data.
• These services are provided by layer 4-7 of OSI model
• Teleservices include telephony, tele box, tele fax, video fax and
teleconferencing.
5. 3. Supplementary services:-
• These services provide additional functionality to the bearer
services and teleservices.
• Examples of such services are reverse charging, call waiting and
message handling.
1. Principles of ISDN:- The various principles of ISDN as per ITU-T
recommendation are:-
• To support voice and non-voice applications:- The main feature of
the ISDN concept is the support of a wide range of voice & non-
voice applications in the same network.
• To support switched and non-switched applications:- ISDN
support both circuit & packet switching. ISDN supports non-
switched services in the form of dedicated lines.
6. • Reliance on 64-kbps connections:- ISDN provides circuit switched
and packet switched connections at 64 kbps. This is the
fundamental building block of ISDN. This rate was chosen because
at the time, it was standard rate for digitized voice.
• Intelligence in the network:- An ISDN is expected to provide
sophisticated services beyond the simple setup of circuit switched
calls. These services include maintenance and network
management functions.
• Layered protocol architecture:- A layered protocol structure
should be used for the specification of the access to an ISDN. Such
a structure can be mapped into OSI model.
7. • Variety of configuration:- Several configuration are possible for
implementing ISDN. This allows for differences in national policy, in
the state of technology and in the needs and existing equipment of
the customer base.
Transmission Modes
The term Transmission Mode defines the direction of the flow of
information between two communication devices i.e. it tells the
direction of signal flow between two devices.
There are three types of transmission modes:-
• Simplex
• Half Duplex
• Full Duplex
8. 1.Simplex Mode:-
• In simplex transmission is sent in only one direction.
• Device connected in simplex mode is either sent only or received
only i.e. one device can only send, other device can only receive.
• There is no mechanism for information to be transmitted back to
the sender.
9. • Communication is unidirectional.
Examples of Simplex Mode:-
• Keyboard and traditional moniters are the examples of simplex
device. Keyboard can only introduce input, the moniter can only
accept output.
10. • Another example of simplex transmission is loudspeaker system.
An announcer speaks into a microphone and his/her voice is sent
through an amplifier and then to all speakers.
• Many fire alarms system work the same way.
2. Half Duplex (HDX):-
• In half duplex transmission data can be sent in both the
directions, but only in one direction at a time.
• Both the connected devices can transmit and receive but not
simultaneously.
• When one device is sending the other can only receive and vice-
versa.
Examples of half duplex:-
11. A walkie-talkie operates in half duplex mode. It can only send or
receive a transmission at any given time. It cannot do both.
Computer A sends information to computer B. At the end of
transmission, computer B sends information back to computer A.
Computer A cannot send information to computer B while
computer B is transmitting data.
12. 3. Full Duplex (FDX):-
• In full duplex transmission, data can be sent in both the
directions simultaneously.
• Both the connected devices can transmit and receive at the same
time .
• Therefore it represents truly bi-directional system.
• In full duplex mode, signals going in either direction share the full
capacity of link.
• The link may contain two separate transmission paths on efor
sending and another for receiving.
• The other way is that the channel capacity is divided between
the signals traveling in both the directions.
13. • The use of full duplex line improves the efficiency as the line turn
around time required in a half duplex arrangement is eliminated.
Examples of Full duplex mode:-
Telephone networks operate in full duplex mode when two persons
talk on telephone line, both can listen and speaks simultaneously.
14. Comparison between Half duplex and Full duplex:-
HDX FDX
1. Data can be sent in both the directions but not
simultaneously
Data can be sent in both the direction simultaneously
2. In HDX, devices can transmit & receive but not at
the same time. When one device is sending, the
other is receiving and vice-versa
In FDX, devices can transmit and receive at the same
time i.e. Each station/device can send as well as
receive data simultaneously.
3. Examples of HDX system is walkie-talkie where
one person speaks and other listens & vice-versa.
Examples of FDX is the telephone system where both
the persons can speak and listen simultaneously.
4. Diagrammatically HDX is represented as: Diagrammatically FDX is represented as:
15. Multiplexing
Definition:- It is the process of sending signals from two or more
different sources simultaneously over a single communication
channel.
Therefore in multiplexing, single communication line carries several
transmission or conversation at the same time.
Multiplexing is done by using a device called multiplexer that
combines n input lines to generate one output line i.e. many to
one. Therefore multiplexer has several inputs and outputs.
At the receiving end, a device called DE multiplexer is used that
separate signal into its components signals.
17. Multiplexer takes 4 input lines and diverts them to single output
line. The signal from 4 different devices is combines and carried by
this single line. At the receiving side, a DE multiplexer takes signal
from a single line & breaks it into the original signals and passes
them to the 4 different receivers.
Advantages of Multiplexing:-
If no multiplexing is used between the users at two different sites
that are distance apart, then separate communication lines would
be required.
This is not only costly but also become difficult to manage. If
multiplexing is used then, only one line is required. This leads to the
reduction in the line cost and also it would be easier to keep track of
one line than several lines.
18. Types of Multiplexing:-
There are three techniques used for multiplexing:
1. Frequency Division Multiplexing (FDM)
2. Wave Division Multiplexing (WDM)
3. Time Division Multiplexing (TDM)
19.
20. 1. Frequency Division Multiplexing:-
• It is analog technique.
• In FDM, signals of different frequencies are combined into a signal
composite signal and are transmitted on the single link.
• FDM requires that the bandwidth of a link should be greater than
the combined bandwidths of the various signals to be
transmitted.
21. • Thus each signal having different frequency forms a particular
logical channel on the link and follows this channel only.
• These channels are then separated by the strips of unused
bandwidth called guard bands. These guards bands prevent the
signals from overlapping.
Applications of FDM:-
1. FDM is used for FM & AM radio broadcasting. Each AM & FM
radio stations uses a different carrier frequency. In AM
broadcasting, these frequencies use a special band from 530 to
1700 KHz. All these signals/frequencies are multiplexed and are
transmitted in air. A receiver receives all these signals but tunes
only one which is required. Similarly FM broadcasting uses a
bandwidth of 88 to 108 MHz.
22. 2. FDM is used in television broadcasting.
3. First generation cellular telephone also uses FDM.
4. Wave Division Multiplexing:-
• WDM is the analog multiplexing technique. WDM is conceptually
similar to FDM, in the sense that it combines different signals of
different frequencies into single composite signal and transmit it
on a single link.
• In WDM the different signals are optical or light signals that are
transmitted through optical fiber. Wavelength of a wave is
reciprocal of its frequency. Therefore, if wavelength goes up, the
frequency goes down and vice-versa.
• This combining and the splitting of light waves is done by using a
prism.
23. • Thus in WDM, various light waves from different sources are
combined to form a composite light signal that is transmitted
across the channel to the receiver.
• At the receiver side, this composite light signal is broken into
different light wave by DE multiplexer.
• One prism is used at the sender side to perform multiplexing and
another prism is used at receiver side that performs DE multiplexer.
• The basic principle behind the usage of prisms is that, the prism
bends a beam of light based on the angle of incidence and the
frequency of light wave.
24. Applications of WDM:-
WDM is used in SONET (Synchronous Optical Network). It makes use
of multiple optical fiber lines which are multiplexed and DE
multiplexed.
25.
26. 3. Time Division Multiplexing (TDM):-
• TDM is the digital multiplexing technique.
• In TDM, the channel/link is not divided on the basis of frequency
but on the basis of time.
• Total time available in the channel is divided between several
users.
• Each user is allotted a particular a time interval called time slot or
time slice during which the data is transmitted by that user.
• Thus each sending device takes control of entire bandwidth of
the channel for fixed amount of time.
• In TDM the data rate capacity of the transmission medium should
be greater than the data rate required by sending or receiving
devices.
28. Synchronous TDM (STDM):-
• In synchronous TDM, each device is given same time slot to
transmit the data over the link, irrespective of the fact that the
device has any data to transmit or not. Hence the name
Synchronous TDM. Synchronous TDM requires that the total
sped of various input lines should be exceed the capacity of
path.
• Each device places its data onto the link when its time-slot
arrives i.e. each device is given the possession of line turn by
turn.
• If any device does not have data to send then its time slot
remains empty.
29. • The various time slots are organized into frames and each frame
consists of one or more time slots dedicated to each sending
device.
• If there are n sending devices, there will be n slots in frame i.e.
one slot for each device.
30. Asynchronous TDM:-
• It is also known as statistical time division multiplexing.
• Asynchronous TDM is called so because in this type of
multiplexing, time slots are not fixed i.e. the slots are flexible.
• In synchronous TDM, if we have n input lines then there are n
slots in one frame. But in asynchronous it is not so.
• In asynchronous TDM, if we have n inputs lines then the frame
contains not more than m slots, with m less than n.
• In asynchronous TDM, the number of time slots in a frame is
based on a statistical analysis of number of input lines.
• The multiplexer scans the various input lines, accepts the data
from the lines that have data to send, fills the frame and then
sends the frame across the link.
31. • If there are not enough data to fill all the slots in a frame, then
the frames are transmitted partially filled.
