CHP: 1 INTRODUCTION
Data communication is a vital part of the information
society because it provides the infrastructure allowing the
computers to communicate with one another. An airline data
communication system uses data
communications to link
reservation offices to the computer. The space flight use data
communications systems to send data to and from the rockets
and command centers on Earth. The purpose of data
communication system is transport of user data between and
among user machines.
Evolution of Data Communication:
It came to existence shortly after the computer were
widely used in the organizations. The 1970s and 1980s saw a
merger of the fields of computer science and data
communications that profoundly c
hanged the technology,
products, and companies of the now
communications industry. Although the consequences of this
revolutionary merger are still being worked out, it is safe to say
that the revolution has occurred, and any investigatio
n of the
field of data communications must be made within this new
context. In order to obtain the services of the computer user's
simply walk to the room where the computer is located and
submitted a request for the computer to perform a service. This
equest was called a job. The
computer accepted the user’s job,
performed its operations, and returns
the results in hard copy
formats. As the
grew, it became inefficient for all
users to walk to the computer room, submit their job and return
get the results. Consequently, computer based terminals
were built and were placed in user work spaces within a
building. This approach allowed users to submit their job from
As organizations, grew and the need for the computer grew it
me necessary to share the computer with other users in
different buildings. The solution was to utilize the widely used
telephone system to transport these traffic. Even though the
telephone system was designed for voice traffic, various
techniques were em
ployed to send data through the telephone
system. This became known as remote time sharing and is still a
form of data communications.
Analog and Digital Communication
Analog data take on continuous values in some intervals.
For example voic
e and video are
of intensity. Similarly, digital data can be text or character
strings. Character form cannot be easily stored or transmitted
by data processing and communication system. Hence, such
systems are designed for
In a communication system, data are propagated from
one point to another by means of electromagnetic signals.
analog signal is a
that may be propagated over a variety of media.
A digital signal
pulses that may be transmitted over a
For example, a constant +ve voltage level may
represent binary zero and a constant negative voltage level may
represent binary 1.
nalog transmission is a means of
g analog signals without regard to th
signals may represent analog data
(eg. voice) or digital data( eg.
binary data that pass through a modem).
The analog signal will
aker after a certain distance.
Amplifiers are used to
the energy in the signal.
However, due to amplifiers the
signal become more distorted in long distance transmission.
Digital transmission is concerned with a content of the
To achieve, signal at greater distances,
the digital signal, recovers the pattern
of 1s and0s and retransmits a new signal.
Thus attenuation is
Digital transmission is more preferred due to following
1. Digital Technology:
2. Data Integrity
3. Capacity Utilization
. Security and Privacy
The fundamental purpose of a communications system is
the exchange of data between two parties. Figure presents one
particular example, which is the communication between a
workstation and a server over a public telephone network.
Another example is the
exchange of voice signals between two
telephones over the same network. The key elements of the
Source: This device generates the data to be transmitted;
examples are telephones and personal computers.
Transmitter: Usually, the data generate
d by a source system are
not transmitted directly in the form in which they were
generated. Rather, a transmitter transforms and encodes the
information in such a way as to produce electromagnetic signals
that can be transmitted across some sort of transm
system. For example, a modem takes a digital bit stream from
an attached device such as a personal computer and transforms
that bit stream into an analog signal that can be handled by the
Transmission System: This can be a sing
le transmission line or a
complex network connecting source and destination.
Receiver: The receiver accepts the signal from the transmission
system and converts it into a form that can be handled by the
destination device. For example, a modem will acce
pt an analog
signal coming from a network or transmission line and convert it
into a digital bit stream.
Destination: Takes the incoming data from the receiver.
Advantages and Disadvantages of Digital Communication:
It increases immunit
y to channel noise and external
Privacy is preserved by using data encryption.
Data from voice, video and data sources may be merged and
transmitted over a common digital transmission system.
Flexible operation of the system.
error detection and correction by the use of coding.
Long distance communication is possible due to the use of
separator where noise doesn't accumulate.
Relatively inexpensive digital circuit may be used.
Generally, more bandwidth is
required than that of analog
Synchronization is required.
High complexity due to use of analog to digital and digital to
CHP: 2 DATA TRANSMISSION
Serial and Parallel Transmission
Digital data transmission can occur in two
basic modes: serial or
parallel. Data within a computer system is transmitted via
parallel mode on
with the width of the parallel bus
matched to the word size of the computer system. Data
between computer systems is usually transmitted in
. Consequently, it is necessary to make a parallel
conversion at a computer
when sending data from a
computer system into a network and a serial
conversion at a computer interface when receiving information
from a networ
k. The type of transmission mode used may also
depend upon distance and required data rate.
In parallel transmission,
(usually 8 bits or a byte/character) are sent
simultaneously on different channels (wires, frequency
channels) within the same cable, or radio path,
to a clock. Parallel devices have a wider data
bus than serial devices and can therefore transfer data in words
of one or more bytes at a time. As a result, there is a speedup in
nsmission bit rate over serial transmission bit rate.
However, this speedup is a tradeoff versus cost since multiple
wires cost more than a single wire, and as a parallel cable gets
longer, the synchronization timing between multiple channels
sensitive to distance. The timing for parallel
transmission is provided by a constant clocking signal sent over
a separate wire within the parallel cable; thus parallel
transmission is considered
In serial transmission, bi
on the same channel (wire) which reduces
costs for wire but also slows the speed of transmission. Also, for
serial transmission, some overhead time is needed since bits
must be assembled and sent as a unit and then disassembled at
Serial transmission can be either synchronous
. In synchronous transmission, groups of bits
are combined into frames and frames are sent continuously with
or without data to be transmitted. In asynchronous
of bits are sent as independent units with
start/stop flags and no data link synchronization, to allow for
arbitrary size gaps between frames. However, start/stop bits
maintain physical bit level synchronization once detected.
Synchronous and Asynchronous
Asynchronous communication utilizes a transmitter, a
receiver and a wire without coordination about the timing of
. There is no coordination between the two end
points on just how long the
certain level to represent a single
. Each device uses a
clock to measure out
the 'length' of a
. The transmitting
device simply transmits. The receiving device has to look at the
and figure out what it is receiving and
coordinate and retime its clock to match the incoming
Sending data encoded into your
requires that the sender
and receiver are both using the same
know where to look in the
to find data.
Asynchronous systems do not send separate information to
indicate the encoding or clocking information. The
decide the clocking of the
on it's own. This means that the
receiver must decide where to look in the
stream to find
ones and zeroes, and decide for itself where each individual bit
stops and starts. This information is not in the data in
sent from transmitting unit.
When the receiver of a
carrying information has to
derive how that
is organized without consulting the
transmitting device, it is called asynchronous communication. In
short, the two ends do not always negotiate or work out the
connection parameters before communicating. Asynchronous
communication is more efficient when there is low loss and low
error rates over the
because data is not
retransmitted and no time is spent setting negotiating the
connection parameters at the beginning of transmission.
Asynchronous systems just transmit an
d let the far end station
figure it out. Asynchronous is sometimes called "best effort"
transmission because one side simply transmits, and the other
does it's best to receive.
Asynchronous communication is used on
devices such as on an IBM
COM 1, 2, 3, 4
) also uses this means
of communication. Your PS2 ports on your
communication. This is the method is also used to
communicate with an external
communication is also used for things like your
Think of asynchronous as a faster means of connecting, but less
Synchronous systems negotiate the communication
parameters at the
layer before communication begins.
Basic synchronous systems will sy
nchronize both clocks before
transmission begins, and reset their numeric counters for errors
etc. More advanced systems may negotiate things like
and compression. It is possible to have both sides try
to synchronize the connection at the same time. Usually, there
is a process to decide which end should be in control. Both sides
can go through a lengthy negotiation cycle where they
communications parameters and status information. Once a
connection is established, the transmitter sends out a
the receiver sends back
data regarding that transmission, and
what it received. This connection negotiation process takes
longer on low error
rate lines, but is highly efficient in systems
itself (an electric wire,
or laser beam) is not particularly reliable.
In telecommunications and computer networking, a
communication channel, or channel, refers either to a physical
transmission medium such as a wire, or to a logical connection
over a multiplexed medium such as a radio channel.
hannel is used to convey an information signal, for example
a digital bit stream, from one or several
transmitters) to one or several receivers. A channel has a certain
capacity for transmitting information, often measured by its
bandwidth in H
z or its data rate in
bits per second
In information theory, a channel refers to a theoretical channel
model with certain error characteristics.
In this more general view, a storage device is also a kind of
channel, which can be sent to (written) and r
232C is a long
established standard ("C" is the current
version) that describes the physical interface and protocol for
data communication between
computers and related devices.
defined by an industry trade group, the Electronic
Industries Association (EIA), originally for
232C is the interface that your computer uses to talk to and
exchange data with your modem and other serial devices.
in your PC, typically on a Universal Asynchronous
Receiver/Transmitter (UART) chip on your motherboard, the
data from your computer is transmitted to an internal or
external modem (or other serial device) from its Data Terminal
Equipment (DTE) interface.
Since data in your computer flows along parallel circuits and
serial devices can handle only one bit at a time, the UART chip
converts the groups of bits in parallel to a serial stream of bits.
As your PC's DTE agent, it also communicates with the mo
or other serial device, which, in accordance with the RS
standard, has a complementary interface called the Data
Communications Equipment (DCE) interface.
In telecommunications, an interface standard is a standard that
cribes one or more functional characteristics (such as code
conversion, line assignments, or protocol compliance) or
physical characteristics (such as electrical, mechanical, or optical
characteristics) necessary to allow the exchange of information
n two or more (usually different) systems or pieces of
An interface standard may include operational characteristics
and acceptable levels of performance.
In the military community, interface standards permit
command and control functions
to be performed using
communication and computer systems.
SIGNALS AND SYSTEM
function of independent variables which carry certain
may be function of time, temperature, pressure, distance
can be voltage or
current in electrical sense.
Types of Signals:
2. Deterministic and non
3. Periodic and aperiodic
4. Even and Odd signal
5. Energy and Power signal
Energy and Power Signals:
Energy signal has finite energy and
zero average power.
0 < E < inf and P = 0
all practical non
E = intg frm
inf to inf | x(t) |2 dt for
E = sum frm n =
inf to inf | x(n) |2 dn for discrete
Power signal has finite average power and
0 < P < inf and E = inf
almost all practical periodic signal
can exist over infinte time
eg. Periodic signal: Pulse Train
is a combination of elements, components which perform
is a set of element which produces o/p in response to i/p
Mathematically, y(n) = f[ x(n) ]
Discrete System Classification:
Causal and Non
o/p depends on the present and past value
. Linear and Non
linear if it sat
isfies principle of superposition
sum of weighted i/p is same as the sum of
variant and invariant system:
Time invariant if the i/p o/p relationship doesnt vary with time
Static and Dynamic
memory less if the o/p at any time depends only on
the value of i/p at same time.
static if its impulse response h(n) is 0 for n != 0
x(n) = del(n) and y(n) = h(n)
Stable and unstable:
stable if it produces bounded o/p from every bounded i/p
Stability of LTI system:
Consider an input x(n) i.e. is bounded in magnitude |x(n)| < m for
allvaluse of n.
The o/p of the discrete time LTI system is found by convolution sum
and is given by,
magnitude of y(n) is given by
substituting the values
k)| <m for all values of k and n,
From above equation, we can conclude that if the impulse response
h(n) is absolutely
, then the o/p of the discrete time LTI
system is bounded in magnitude and therefore the system is called
ut Bounded Output (BIBO) stable.
A sufficient and necessary condition of stability of a discrete LTI
system is expressed as
S = sum k =
annel Capacity Theorem
This gives the relationship between the channel bandwidth and
signal to noise ratio and the limitation that they impose on
Let B be the channel bandwidth and SNR be the recived signal to
Then, the channel capacity theorem states that,
B = log2(1+SNR) bit/sec
C is defined as the maximum
rate at which information may be
transmitted without error through the channel.
Nyquist Sampling Theorem:
Nyquist Sampling theorem states that if x(t) is band limited with no
components at frequencies greater than fm Hz then it is completely
by samples, taken at the uniform rate fx>2fm Hz
The minimum sampling rate or minimum sampling frequency,
fs=2fm for complete specification of the continuous time signal is
referred as Nyquist Rate or Nyquist Frequency.
Sampling of a continuous analog s
ignal is the first step of
transmission of analog signal over digital communication system.
The sampling theorem states that analog signal can be reproduced
from an appropriate set of its sample taken at some fixed point
interval of time.
1. Exponential signal:
x(t) = Ae
Growing exponential and Decaying exponential
2. Sinusoidal signal:
3. Unit step function:
u(t) = 1 for t >= 0
0 for otherwise
4. Unit Impulse Function:
del(t) = 1 for t = 0
r(t) = 0 for t < 0
t for t > 0
6. Sinc Function:
Sinc(t) = 1 for t = 0
sint/t for t != 0
OVERVIEW OF DATA COMMUNICATION
Types of Network:
LOCAL AREA NETWORK
A local area network (LAN) supplies
networking capability to a
group of computers in close proximity to each other such as in
an office building, a school, or a home.
A LAN is useful for sharing resources like files, printers, games
or other applications.
A LAN in turn often connects to
other LANs, and to the Internet
or other WAN.
Most local area networks are built with relatively inexpensive
hardware such as Ethernet
cables, network adapters, and hubs.
Wireless LAN and other more advanced LAN hardware options
perating system software may be used to
configure a local area network.
For example, most flavors of Microsoft Windows provide a
software package called Internet Connection Sharing (ICS) that
supports controlled access to LAN resources.
WIDE AREA NETWOR
A WAN spans a large geographic area, such as a state, province
or country. WANs often connect multiple smaller networks, such
as local area networks (LANs) or metro area networks (MANs).
The world's most popular WAN is the Internet. Some segments
the Internet, like VPN
based extranets, are also WANs in
Finally, many WANs are corporate or research networks that
utilize leased lines.
WANs generally utilize different and much more expensive
networking equipment than do LANs. Key techn
found in WANs include SONET, Frame Relay, and ATM.
METROPOLITAN AREA NETWORK
A Metropolitan Area Network (MAN) is one of a number of
types of networks.
A MAN is a relatively new class of network, it serves a role
similar to an ISP, but
for corporate users with large LANs. There
are three important features which discriminate MANs from
LANs or WANS:
The network size falls intermediate between LANs and
WANs. A MAN typically covers an area of between 5 and 50 km
diameter. Many MANs cov
er an area the size of a city.
A MAN (like a WAN) is not generally owned by a single
. The MAN, its communications links and equipment
are generally owned by either a consortium of users or by a
single network provider who sells the service
to the users. This
level of service provided to each user must therefore be
negotiated with the MAN operator, and some performance
guarantees are normally specified.
A MAN often acts as a high speed network to allow sharing of
regional resources (sim
ilar to a large LAN). It is also frequently
used to provide a shared connection to other networks using a
link to a WAN.
In this type of network topology, all the nodes of a network are
connected to a common
transmission medium having two
All the data that travels over the network is transmitted
through a common transmission medium known as the bus or
the backbone of the network.
When the transmission medium has exactly two endpoints, the
k topology is known by the name, ‘linear bus topology'.
In case the transmission medium, also called as the network
backbone, has more than two endpoints, the network is said to
have a distributed bus topology.
Bus topology is easy to handle and implem
ent and is best
suited for small networks.
But the downside of this topology is that the limited cable
length limits the number of stations, thus limiting the
performance to a less number of nodes.
In a ring topology, every node in the
network is connected to
two other nodes and the first and the last nodes are connected
to each other.
The data that are transmitted over the network pass through
each of the nodes in the ring until they reach the destination
In a ring network, t
he data and the signals that pass over the
network travel in a single direction.
The dual ring topology varies in having two connections
between each of the network nodes.
The data flow along two directions in the two rings formed
The ring t
opology does not require a central server to manage
connectivity between the nodes and facilitates an orderly
But, the failure of a single station in the network can render
the entire network inoperable.
Changes and moves in the stat
ions forming the network affect
the network operation.
In a full mesh network, each network node is connected to
every other node in the network.
Due to this arrangement of nodes, it becomes possible for a
simultaneous transmission of
signals from one node to several
In a partially connected mesh network, only some of the
network nodes are connected to more than one node.
This is beneficial over a fully connected mesh in terms of
redundancy caused by the point
links between all the
The nodes of a mesh network require possessing some kind of
routing logic so that the signals and the data traveling over the
network take the shortest path during each of the
In this type of
network topology, each node of the network is
connected to a central node, which is known as a hub.
The data that is transmitted between the network nodes
passes across the central hub.
A distributed star is formed by the interconnection of two or
e individual star networks.
The centralized nature of a star network provides a certain
amount of simplicity while also achieving isolation of each
device in the network.
However, the disadvantage of a star topology is that the
network transmission is
largely dependent on the central hub.
The failure of the central hub results in total network
It is also known as a hierarchical topology and has a central
root node that is connected to one or more nodes of a lower
In a symmetrical hierarchy, each node in the network has a
specific fixed number of nodes connected to those at a lower
>>> Apart from these basic types of network topologies, there
are hybrid network topologies, which are composed of a
mbination of two or more basic topologies.
These network mappings aim at harnessing the advantages of
each of the basic topologies used in them.
Network topologies are the physical arrangements of network
nodes and wires. What is interesting is that th
e inanimate nodes
and wires turn 'live' for the transmission of information!
is the layered structure of hardware and software that
supports the exchange of data between systems and supports
a as electronic
mail and file transfer.
The key features of protocol are":
> syntax: concerns the format of the data blocks
> semantics: Includes control information for coordination and
>Timing: Includes speed matching and sequencing
for Open System Interconnection.
The communications concern are partitioned into
set of layers.
Each layer performs a related subset of the functions with
A physical layers covers the physical interface
and the rules by which bits passed from one to another.
It relates to the physical properties of the interface to a
For example, connector that joins one or more cir
Electrical part of physical layer relates to the representation of
Functional parts of physical layer specifies the function
performed by individual circuits between a system and the
Similarly, procedural part of phy
of events by which bit streams are exchanged across
the physical medium.
Data Link Layer:
Data link layer attempts to make the physical link
provides the means to activate, maintain, and deactivate the
It provides for the
transfer of information across
It sends blocks with the necessary synchronization, error
control and flow control.
A computer system engage in the dialog with the network to
specify the destination address and to
It provides the mechanism for the
of data between
t service ensures that data
are delivered error free, in sequence with no loss or duplication.
It provides the mechanism for controlling the dialog between
in and systems.
It defines the format of
the date to be exchanged between
It defines the syntax used between application and provides
for the selection and subsequent modification of the
Application Layer provides a means for applicati
on programs to
access the OSI environment.
It contains management functions and general purpose
applications such as file transfer, electronic mail and terminal
access to remote computers.
More efficient transmission scheme
call control signal
ing is carried on separate logical connection
from user data.
Intermediate nodes need not to maintain state tables or
process messages relating to call control
Multiplexing and switching of logical connections takes place at
instead of layer 3, eliminating one entire layer of
There is no hop
by hop flow control and error control. End to
end flow control and error control are the responsibility of a
higher layer, if they are employed at all.
Frame relay used acc
ess speed up to 2Mbps Frame relay
service at even higher data rates are now
Frame Relay is designed to provide
The X.25 approach results in considerable overhead at each
hop through the network.
The data l
ink control protocol involves the
exchange of a data
At each intermed
ate node, state tables must be maintained
for each virtual circuit to deal with cost manage
flow/error, control as
pects of X.25 protocol.
these overhead may be justified when there is significant
probability of error in any of the links in the network.
Today's network employee rel
ble digital transmission
echnology over high quality relia
ble digital transmission
echnology over high qua
ssion links such as
In this environment, the overhead of X.25 is not only
but degrades the effe
ctive utilization of the
ble high data rates.
Frame Relay is designed to eliminate much of the overhead
that X.25 imposes on end user systems.
The Logical Link Control (LLC) data communication protocol
layer is the upper sub
layer of the Data Link Layer (which is itself
layer 2, just above the Physical Layer) in the seven
It provides multiplexing mechanisms that make it possible for
several network protocols (IP, IPX) to coexist within a multipoint
network and to be transported over the same network media,
and can also provide flow control mechanisms.
The LLC sub
ayer acts as an interface between the Media
Access Control (MAC) sub
layer and the network layer.
As the Ether
type in an Ethernet II framing formatted frame is
used to multiplex different protocols on top of the Ethernet
MAC header it can be seen as LLC
The LLC sub
layer is primarily concerned with:
> Multiplexing protocols transmitted over the MAC layer (when
transmitting) and decoding them (when receiving).
> Providing flow and error control
The Media Access Control (MAC) data communi
layer, also known as the Medium Access Control, is a sub
layer of the Data Link Layer specified in the seven
model (layer 2).
It provides addressing and channel access control mechanisms
that make it possible for several ter
minals or network nodes to
communicate within a multi
point network, typically a local area
network (LAN) or metropolitan area network (MAN).
hardware that implements the MAC is referred to as a Medium
The MAC sub
layer acts as an
interface between the Logical Link
Control (LLC) sub
layer and the network's physical layer.
MAC layer emulates a full
duplex logical communication channel
in a multi
twork. This channel may provide
multicast or broadcast communicati
Routing is the process of selecting paths in a network along
which to send network traffic.
Routing is performed for many kinds of networks, including the
telephone network (Circuit switching) , electronic data networks
the Internet), and transportation networks.
This article is concerned primarily with routing in electronic
data networks using packet switching technology.
In packet switching networks, routing directs packet
forwarding, the transit of logically addres
sed packets from their
source toward their ultimate destination through intermediate
nodes, typically hardware devices called routers, bridges,
gateways, firewalls, or switches.
can also forward packets and perform routing, thou
gh they are
not specialized hardware and may suffer from limited
The routing process usually directs forwarding
on the basis of routing tables which maintain a record of the
routes to various network destinations.
Thus, constructing routin
g tables, which are held in the router's
memory, is very important for efficient routing.
Most routing algorithms use only one network path at a time,
but multipath routing techniques enable the use of multiple
Routing, in a more nar
row sense of the term, is often
contrasted with bridging in its assumption that network
addresses are structured and that similar addresses imply
proximity within the network.
Because structured addresses
allow a single routing table entry to represent t
he route to a
group of devices, structured addressing (routing, in the narrow
sense) outperforms unstructured addressing (bridging) in large
networks, and has become the dominant form of addressing on
the Internet, though bridging is still widely used with
IEE 802 Reference Model
as developed by the IEEE 802 co
and has been adopted by all organizations working on the
specification of LAN standards.
It is generally referred to as IEEE 802
from the bottom of the lowest layer of IEEE 802 corresponds to
the physical layer of the OSI model and includes functions as
encoding, decoding of signals, preamble generation/removal
and bit transmission/reception.
encoding and decoding of signals
able generation/removal (for synchronization)
MAC(Medium Access Control)
on transmission assemble data into a frame with error
detection and address fields
frame and perform address
recognition and error detection
access to LAN transmission medium
LLC(Logical Link Co
provide an interface to higher layers and perform flow and
Three services are provided un
der LLC services:
Unacknowledged Connectionless Service:
It is a very simple service that does not involve any of the flow
and error control mechanisms.
Delivery of data is not guaranteed.
There will be some higher layer of software that deals wit
Connection Mode services:
A logical connection is set up between two users
data and flow and error control are provided.
It provides acknowledgement but no logical connection
Ethernet is a family of frame
based computer networking
technologies for local area networks (LANs).
The name came from the physical concept of the ether.
It defines a number of wiring and signaling standards for the
Physical Layer of the OSI networking model as well as a common
addressing format and Media Access Control at
Ethernet is standardized as IEEE 802.3.
The combination of
the twisted pair versions of Ethernet for connecting end systems
to the network, along with the fiber optic versions for site
backbones, is the most widespread wired LAN technology.
CSMA/CD (Carrier Sense Multiple Access / Collision Detection
is the protocol used in
to ensure that only
one network node is transmitting on the network wire at any
Carrier Sense means that every Ethernet device listens to the
Ethernet wire before it attempts to transmit.
If the Eth
ernet device senses that another device is
transmitting, it will wait to transmit.
Multiple Access means that more than one Ethernet device can
be sensing (listening and waiting to transmit) at a time.
Collision Detection means that when multiple Ether
accidentally transmit at the same time, they are able to detect
How Collisions Occur under CSMA/CD:
Imagine a very simple Ethernet network with only two nodes.
Each node, independently, decides to send an Ethernet frame
Both nodes listen to the Ethernet wire and sense that no
carrier is present.
Both nodes transmit simultaneously, causing a collision.
Both nodes detect the collision and each node waits a random
amount of time before transmitting again.
Collisions are normal on an Ethernet network.
A small amount of collisions are expected in the protocol
If too many nodes are transmitting on an Ethernet network the
number of collisions can rise to an unacceptable level.
This can reduce th
e amount of available bandwidth on an
Ethernet network because so much bandwidth is lost in
Ethernet switches greatly reduce the already minor difficulties
experienced with the CSMA/CD protocol.
The X.25 protocol, adopted as a s
tandard by the Consultative
Committee for International Telegraph and Telephone (CCITT), is
used network protocol.
The X.25 protocol allows computers on different public
networks (such as CompuServe, Tymnet, or a TCP/IP network) to
through an intermediary computer at the network
X.25's protocols correspond closely to the data
layer protocols defined in the Open Systems
Interconnection (OSI) communication model.
physical interface between and attached station(computer
terminal and Packet Switching mode.
transfer of data across
It is referred as Link Access protocol
rtual circuit service
enables any subscriber to the network to setup logical
Following are the key features of X:25:
call control packets, used for setting up and cleaning virtual
circuits are carried on same channel and same virtual circuit as
multiplexing of virtual circuits takes place at layer 3
Both layer 2 and layer 3 include flow control and err
It is a streamlined packet transfer interface.
ATM makes use of a fixed size packets called cells.
The use of fixed size and fixed formats results an efficient
h speed networks.
data rate range from 25.6 Mbps to 622.08 Mbps
Physical layer specifies
medium and signal
ATM layer defines
of data in fixed size cells and
defines the use of logical connection.
tion layer maps higher layer information into ATM
cells to be transported over an ATM network.
User plane provides user information into ATM cells to be
transported over an ATM network
user plane provides user information transfer(eg. flow control,
Control plane provides call control and connection control
Management plane performs coordination between all the
planes and layers management.
CHP: 6 TRANSMISSION MEDIA
Guided and Unguided Media
Guided media are those that
from one device to another which includes twisted pairs, co
axial cables and
Unguided Media transports
using a physical conductor.
This type of
communication is often referred to as wireless
Transmission medium is the physical path between transmitter
and in a data transmission system.
Transmission media can be
classified or unguided.
In both cases, com
munication is in the form of
With guided media, the waves are guided along a solid
medium, such as copper twisted pair, copper coaxial cable, and
The atmosphere and outer space are examples of unguided
provide a means of transmitting
signals but do not guide them; this form of
transmission is usually referred to as wireless transmission.
The characteristics and quality of a data transmission are
determined both by
f the medium and the
characteristics of the signal.
In the case of guided media, the medium itself is more
important in determining the limitations of
For unguided media, the bandwidth of the signal produced by
the transmitting antenna i
s more important than the medium in
determining transmission characteristics.
One key property of signals transmitted by antenna is
In general, signals at lower frequencies are omnidirectional;
that is, the signal propagates
in all dir
ections from the antenna.
At higher frequencies, it is possible to focus
the signal into a directional beam.
In considering the design of data transmission systems, a key
concern, generally, is data rate and distance: the greater the
data rate and dist
ance, the better.
A number of design factors relating to the transmission
medium and to the signal determine the data rate and distance:
All other factors remaining constant, the greater the
bandwidth of a signal, the higher the data rate
that can be
Impairments, such as attenuation, limit the distance.
For guided media, twisted pair generally suffer more
impairment than coaxial cable, which in turn suffers more than
Interference from competing signals in overlapping frequency
bands can distort or wipe out a signal. Interference is of
particular concern for unguided media, but it is also a problem
with guided media.
For guided media, interference can be caused
from nearby cables.
For example, twisted pair are often bundled together, and
conduits often carry multiple cables.
Interference can also be
experienced from unguided transmissions.
Proper shielding of a guided medium can minimize this
>> Number of receivers:
A guided medium can be used to construct a point
or a shared link with multiple attachments.
In the latter case, each attachment introduces some
attenuation and distortion on the line, limiting
Wires are described by
Higher gauge number
thinner wire size.
The smaller the diameter of the wire, the greater is resistance
to the propagation of a signal.
Increased resistance results in a
decreased bit rate across the
At higher transmission frequencies, the
on the outside surface of the wire.
A small wire provides less total surface for the
in increased signal loss.
The local subscriber loops ( of the telephone system) and
usually to 22
26 gauge wire.
Trunk and toll lines typically employ 19
Several Hundred of these wires are packaged into one cables.
The wires are paired
twisted around each other to
The most common twisted pair cable used in communications
to as unshielded twisted pair(UTP) cable.
STP cable(shielded Twisted Pair) has a metal foil or braided
mesh covering each pair of
Although metal casing improves the
of cable by
preventing the penetration of noise or cross
talk, it is bulkier
and more expensive.
Twisted pair cables are used in telephone lines to provide voice
and data channels.
The line that connects subscribers to the central telephone
office is most commonly
twisted pair cables.
Local area networks such as 10 Base
T and 100 Base
T also use
twisted pair cables.
Microwave is a directed line of sight(LOS) radio transmission.
It is used for wide band communication systems and is quite
common in the telephone
Television transmission also uti
lizes microwave transmission
because microwave transmission is above the 1 GHz and
provides the capacity
for video transmission.
The high bandwidth gives small wavelength and the smaller
the wavelength, the smaller one can design the microwave
The antenna size has significant implications for distributed
The transmitting towers are spaced 20
30 m apart.
Transmitted radio bean is focused to the
axial cables carries sign
als of higher
twisted pair cable.
Instead of having two wires, co
axial cable has a central core
conductor of solid or standard wire(usually copper) enclosed in
an insulating sheath which in turn is encased in an outer
metal foil or combination of two.
The outer metallic wrapping serves both as a shield against
noise and as a second conductor.
The whole cable is protected by a plastic cover.
The use of co
axial cable is diverse but nowadays it is
due to fibre optic cable.
axial cables are used in analog telephone networks and
cable Tv networks.
optic cable is made of glass or plastic and transmits
signals in the form of lights.
Lights travels in a st
raight line as long as it is moving through a
single uniform substance.
If a ray of light travelling through one substance suddenly
enters another(more or less dense the ray changes direction).
As the above figure, if the angle of incidence is less th
critical angle, the ray
and move to closer to the surface.
If angle of incidence is equal to the critical angle, the light
bends along the interface and refraction occurs.
If the angle is greater than the critical angle, the ray reflec
and travels again in the denser substances.
Optical fibres use reflection to guide light through a channel.
A glass or plastic core is surrounded by cladding of less dense
glass or plastic.
The difference in density of the two materials must be s
that a beam of light moving through the core is reflected off the
cladding instead of being reflected into it.
Multimode step index:
in multimode step
, the density of the core remains
constant from the center of
A beam of light moves through these constant densities in a
straight line until it reaches the interface of the core and the
As the interface, there is an abrupt change to a lower density
that alters the angle of the beam motion.
The term step index refers to the suddenness of this change.
Multimode graded index
A second type of
called multimode graded index
one with varying densities.
Density is highest at center of the core and decreases gradually
lowest at the edge.
Single mode uses step index fiber and a highly focused source
of light that limits beams to a small range of angles, all close to
The single mode fiber is manufactured with a much smaller
that that of multimode fiber.
cable is found in backbone
its wide bandwidth and is cost effective.
cable TV companies use a combination of optical
axial cable, thus creating a hybrid netw
Less signal attenuation
Resistance to corrosive materials
waves used e propagation characteristics
of wireless channels are highly dependent on
Since, electromagnetic waves doesn't need any medium to
transmit the signal specially in wireless communication system
we often use the atmo
Here, interference and propagation condition are strongly
dependent upon the
Types of Electromagnetic waves:
Ground Wave Propagation
Dominant mode of propagation for
below 2 mHz.
gnetic waves are guided by the conducting surface
of the earth, along which they are propagated.
Diffraction of the wave causes it to propagate where this
propagation mode is used in AM broadcasting.
For efficient radiation, the antenna needs to be lo
1/10 th of the wave length.
Sky Wave Propagation:
Dominant mode of propagation for
in between 2
to 30 Mhz.
Long distance coverage is obtained by the reflecting the wave
at the ionosphere and at the earth boundaries.
is caused due to reflection.
Line of sight(LOS) or Space Wave propagation
mode of propagation for
above 30 Mhz.
Here, electromagnetic wave propagates in a straight line.
Very little reflection by the ionosphere,
used for satellite communication.
Its maximum range is limited to line of sight due to nature of
For communication between two each stations, the signal path
has to be above the horizon otherwise they will block the LOS
Thus antennas need to be placed on tall towers so that
antenna can see the
Radio waves and Microwaves:
1. 3 KHz to 1 GHz
1 GHz to 300 GHz
2. for the most part are omnidirectional
Omnidirectional antennas are generally used
line of sight propagation, unidirectional antennas are used
cellular phones, satellite networks
Satellite communications are comprised of 2 main
The satellite itself is also known as the space segment, and is
composed of three separate units, namely the fuel system, the
satellite and telemetry controls, and the transponder.
The transponder includes the receiving antenna to pick
nals from the ground station, a broad band receiver, an input
multiplexer, and a frequency converter which is used to reroute
the received signals through a high powered amplifier for
The primary role of a satellite is to reflect electronic si
In the case of a telecom satellite, the primary task is to receive
signals from a ground station and send them down to another
ground station located a considerable distance away from the
This relay action can be two
way, as in the case o
f a long
distance phone call.
Another use of the satellite is when, as is the case with
television broadcasts, the ground station's uplink is then
over a wide region, so that it may be received by many
different customers possessing compatibl
Still another use for satellites is observation, wherein the
satellite is equipped with cameras or various sensors, and it
merely downlinks any information it picks up from its
The Ground Station
This is the earth segm
The ground station's job is two
fold. In the case of an uplink, or
transmitting station, terrestrial data in the form of baseband
signals, is passed through a baseband processor, an up
converter, a high powered amplifier, and through a parabolic
dish antenna up to an orbiting satellite.
In the case of a downlink, or receiving station, works in the
reverse fashion as the uplink, ultimately
converting signals received through the parabolic antenna to
base band signal.
Cellular Telephony System:
A cellular telephone system provides a wireless connection to
telephone network( PSTN: Public switch
Telephone Network) for any user
the radio range
of the system.
a large number of users over a
large geographic area, within a limited
radio system provide high quality service that is often
comparable to that of the landline telephone systems.
High capacity is achieved by limiting the
coverage of each base
station transmitter to a small geographic area called a cell so
that the same radio channels may be reused by another base
station located some distance away.
A sophisticated switching technique called a handoff enables a
call to p
roceed uninterrupted when the user moves from one
cell to another.
The basic structure of cellular system is as below:
The basic cellular system consists mobile stations, base
stations and a mobile
The mobile switching center
is sometimes called a mobile
telephone switching office (MTSO), since it is responsible for
connecting all mobiles to the PSTN via central office(CO).
Each user communicates via radio from a cellular telephone set
to the cell site base station.
base station is connected via telephone lines or
microwave link to the mobile
The MSC connects the user to the called party if the called
party is land
, the connection is via the central office (CO)
is the terrestrial telephone ne
If the called party is mobile, the connection is made to the
cellular site that covers the area in which the third party is
located, using an
radio channel in the cell
with the called party.
If more channels are needed, the
existing cell sizes are
decreased, and additional small cells are inserted, so that
existing channels can be reused more efficiently.
consideration is to design the cells for acceptable
levels of a Co channel interference.
As the mobile us
er travels from one cell to another, the MSC
automatically switches the user to an available channel in the
new cell and the telephone continues un
has following advantages:
> large subscriber capacity
> Efficient use
of the radio spectrum
> Service to hand held portables, as well as vehicles.
> High Quality telephone and data service to the mobile user at
relatively low cost.
Parallel Transmission is a method of data tra
nsmission in which
the bits of a data character are transmitted simultaneously over
a number of channels/ports.
In parallel transmission, coded information are transmitted via
a system with multiple ports/channels.
The port 1 is used to
transport the f
irst MSB (Most Significant Bit) and the second
port carries the second MSBs, so on and so forth.
Synchronous and asynchronous transmission
Given the problems that arise with a parallel
serial connections are normall
y used. However, since a single
wire transports the information, the problem is how to
the transmitter and receiver, in other words, the
receiver can not necessarily distinguish the characters (or more
generally the bit sequences) because the b
its are sent one after
the other. There are two types of transmission that address this
An asynchronous connection, in which each character is sent at
irregular intervals in time (for example a user sending characters
entered at the keyboard in r
eal time). So, for example, imagine
that a single bit is transmitted during a long period of silence...
the receiver will not be able to know if this is 00010000,
10000000 or 00000100...
To remedy this problem, each character is preceded by some
on indicating the start of character transmission (the
transmission start information is called a START bit) and ends by
transmission information (called STOP bit, there
may even be several STOP bits).
In a synchronous connection, the transm
itter and receiver are
paced by the same clock. The receiver continuously receives
(even when no bits are transmitted) the information at the same
rate the transmitter send it. This is why the transmitter and
receiver are paced at the same speed. In additi
supplementary information is inserted to guarantee that there
are no errors during transmission.
During synchronous transmission, the bits are sent successively
with no separation between each character, so it is necessary to
ments; this is called character
The main disadvantage of synchronous transmission is
the data at the receiver, as there may be differences
between the transmitter and receiver clocks. That is why each
data transmission m
ust be sustained long enough for the
receiver to distinguish it. As a result, the transmission speed can
not be very high in a synchronous link
may differ from that is transmitted due to
for analog signals, these impairments can degrade the signal
for digital, bit errors may be introduced.
A binary 1 is transformed into binary 0 and vice versa.
of a signal falls off with
distance over any transmission
signal must have sufficient
can detect the signal.
The signal must maintain a level sufficiently higher than noise
Attenuation is an increasing concern of
Hence, amplifiers must be used that amplify high
more than lower
occurs because of the
propagation of a signal
through guided medium varies with
For a band limited signal, the velocity tends to be highest near
and falls off towards the two edges of the
components of a signal
will arrive at
times resulting in phase shifts between
occurs due to delay distortion
unwanted man made or natural random signal that adds to
l and degrades the performances.
Such type of unwanted random signal is noise.
due to the thermal agitation of electrons in the conductor.
due to the flow of current at the junction of semi conductor.
due to the sudden high amplitude making the signal change its
value such as lightning, electrical ignition
due to non linearity’s in the transmitter, receiver and
> Cross talk Noise:
due to coupling of nearby line.
> Flicker Noise:
Transit time No
DATA LINK CONTRO
Flow control is a technique for assuring that a transmitting
entity does not overwhelm a receiving
entity with data. The
receiving entity typically allocates a data buffer of some
maximum length for a transfer.
When data are received, the receiver
must do a certain amount of processing before passing the data
to the higher
In the ab
sence of flow control, the receiver's buffer may fill up
while it is processing old data.
Wait Flow Control:
The simplest form of flow control, known as stop
control, works as
A source entity transmits a
frame. After reception, the
indicates its willingness to accept another frame by sending back
an acknowledgment to the frame just received.
must wait until it receives the acknowledgment before sending
the next frame.
destination can thus stop the flow of data by simply
This procedure works fine and,
indeed, can hardly be improved upon when a message is sent in
a few large frames.
However, it is often the case that a source will break up
block of data into smaller blocks and transmit the data in many
This is done for the following
> The buffer size of the receiver may be limited.
> The longer the transmission, the more likely that there will be
an error, necessit
ating retransmission of the entire frame. With
smaller frames, errors are
detected sooner, and a smaller amount of data needs to be
> On a shared medium, such as a LAN, it is usually desirable not
to permit one
station to occupy the medium f
or an extended period, as this
delays at the other sending stations.
Window Flow control:
Allows multiple frames to be in transit
Sender maintains list of
number of anticipated frame
Sender maintains list of
numbers it can send,
maintains list of
numbers it can
) supplemented with RNR (
The essence of the problem desc
ribed so far is that only one
frame at a time can be in transit.
In situations where the bit length of the link is greater than the
length (a > I), serious inefficiencies result.
Efficiency can be greatly improved by
allowing multiple frames to b
e in transit at the same time.
Error control refers to mechanisms to detect and correct errors
that occur in the transmission of frames.
The model that we will use, which covers the typical case.
As before, data are sent as a sequence
of frames; frames arrive
in the same order in which they are sent; and each transmitted
frame suffers an arbitrary and variable amount of delay before
reception. In addition, we admit the possibility of two types of
A frame fails
to arrive at the other side.
For example, a noise burst may damage a frame to the extent
that the receiver is not aware that a frame has been
: A recognizable frame does arrive, but some
of the bits are in error (have been al
tered during transmission).
The most common techniques for error control are based on
some or all of the
. As discussed in the preceding section.
. The destination returns a positive
to successfully received, error
a Retransmission after timeout
. The source retransmits a
frame that has not
been acknowledged after a predetermined amount of time.
Negative acknowledgment and retransmission
eturns a negative acknowledgment to frames in
which an error is detected.
The source retransmits such frames.
Collectively, these mechanisms are all referred to as automatic
(ARQ); the effect of ARQ is to turn an unreliable data link into
Three versions of ARQ have been standardized:
Stop or Wait ARQ:
wait ARQ is based on the stop
The source station transmits a single
frame and then must
await an acknowledgment (ACK).
No other data frames can be sent
until the destination station's reply arrives at the source station.
Two sorts of errors could occur. First, the frame that arrives at
could be damaged
; the receiver detects this by using the error
detection technique referred to earlier and simply discards the
To account for this possibility, the
source station is equipped with a timer.
After a frame is
transmitted, the source station waits f
or an acknowledgment.
If no acknowledgment is received by the time the timer
same frame is sent again.
Note that this method requires
that the transmitter maintain a copy of a transmitted frame
until an acknowledgment
is received for
The second sort of error is a damaged acknowledgment.
Go back N ARQ
The form of error control based on sliding
window flow control
that is most commonly used is called go
N ARQ, a station may send a series of fra
modulo some maximum value.
The number of unacknowledged frames outstanding is
determined by window size, using the sliding
While no errors occur, the destination will acknowledge (RR =
incoming frames as usual.
If the destination station detects an error in a frame, it sends a
negative acknowledgment (REJ = reject) for that frame.
The destination station will discard that frame and all future
incoming frames until the frame in
error is correctly received.
Thus, the source station, when it receives an REJ, must
retransmit the frame in error plus all succeeding frames that
were transmitted in the interim.
Real time data transmission is provided by the direct
Dial up delay can be eliminated by using leased lines
can occur in
case busy signal is returned to
Transmissions are point to point
nnection is established, any
is invisible to the connected components
Connection is not a direct physical interface as in circuit
variable slots if TDM is
Messages are stored onto disk, tape before transmission, real
time processing is usually not feasible
messages can be broadcast to all nodes in the network or
subset of nodes
are allowed in the message traffic
combination of circuit and message switching
packet contains user and control data
provides stastical multiplexing
fast response to all users
of the network to all users.
There are n inputs to a
multiplexer. The multiplexer is
connected by a single data link to a demultiplexer.
The link is able to carry n separate channels of data.
combines (multiplexes) data from the n input
lines and transmits over a higher capacity data link
The demultiplexer accepts the multiplexed data stream,
) the data according to channel, and
delivers them to the appropriate output lines.
(Frequency Division Multiplexing)
FDM is possible when the useful bandwidth of t
medium exceeds the required bandwidth of signals to be
A number of signals can be carried
simultaneously if each signal is modulated onto a different
and the carrier frequencies are sufficiently separated th
bandwidths of the signals do not overlap.
Six signal sources are fed into a multiplexer, which modulates
each signal onto a different frequency (fi, . . . , f6).
Each modulated signal requires a certain bandwidth centered
around its carrier freq
uency, referred to as a channel.
To prevent interference, the
channels are separated by guard bands, which are unused
portions of the spectrum.
division multiplexing is possible when the
achievable data rate
(sometimes, unfortunately, called bandwidth) of the medium
exceeds the data rate of digital signals to be transmitted.
Multiple digital signals (or analog signals carrying digital data)
can be carried on a single transmission path by interleaving
s of each signal in time.
The interleaving can be at the bit level or in blocks of bytes or
For example, the multiplexer has six inputs which might each
be, say, 9.6 kbps.
A single line with a capacity of at least 57.6
kbps (plus o
verhead capacity) could accommodate all six
Private Branch Exchange:
A PBX (private branch exchange) is a telephone system within
that switches calls between enterprise users on
local lines while allowing all users to share a cer
tain number of
external phone lines.
The main purpose of a PBX is to save the cost of requiring a line
for each user to the telephone company's central office.
The PBX is owned and operated by the enterprise rather than
the telephone company (which may
be a supplier or service
Private branch exchanges used analog technology originally.
Today, PBXs use
(digital signals are
converted to analog for outside calls on the local loop using
plain old telephone service)
A PBX includes:
> Telephone trunk (multiple phone) lines that terminate at the
> A computer with memory that manages the switching of the
calls within the PBX and in and out of it
> The network of lines within the PBX
> Usually a console or switchboa
rd for a human operator
In some situations, alternatives to a PBX include centrex
service (in which a pool of lines are rented at the phone
company's central office), key telephone systems, and, for very
small enterprises, primary rate Integrated Servic
digital version of analog switched line
data rates up to 56 Kbps
both parties must subscribe
subscribes do not need modem
digital service unit (DSU) is needed to change the rate to 56
Kbps and encode
them in the format of service providers.
, multimedia etc
DATA ENCODING AND MODULATION
Definition of digital signal encoding formats:
0 = high level
1 = low level
Nonreturn to Zero
0 = no transition at beginning of interval (one bit time)
1 = transition at beginning of interval
0 = no line signal
1 = positive or negative level, alternating for successive ones
0 = positive or negative
level, alternating for successive zeros
1 = no line signal
0 = transition from high to low in middle of interval
1 = transition from low to high in middle of interval
Always a transition in middle of interval
0 = tran
sition at beginning of interval
1 = no transition at beginning of interval
Same as bipolar AMI, except that any string of eight zeros
is replaced by a string with
two code violations
Same as bipolar AMI, except that any string of four zeros
s replaced by a string with
one code violation
In combining the processes of sampling and quantizing, the
specification of a
band signal becomes limited
to a discrete set of values but not in the form best suited for
transmission over a line or a radio path or optical
To exploit the advantages of sampling and quantizing, we
require the use of an encoding process t
o translate the discrete
set of sample values to a more appropriate form of signal.
Any plan for representing each member of this discrete values
as a particular arrangement of discrete elements is called
Suppose, in a binary code each code w
ord consists of n bits.
a code we may represent a total of 2^n
Amplitude shift Keying
The most basic form of ASK involves the process of switching
the carrier on or off, in
pulses that constitute the information signal.
binary digit 1
by the presence of a carrier and the
digit 0 represented by the absence of a carrier.
s(t) = A
t for binary 1
The basic form of FSK involves the process of varying the
of a carrier wave by choosing one of two
in correspondence of digital pulses that constitute the
Two binary digits 0
and 1 are
by two frequencies
around the carrier
Phase Shift Keying:
The most basic form of PSK involves the process of shifting the
phase of a carrier wave in
digital pulses that constitute the
The two binary digits 0 and 1 are
in which the
carrier phase f
or each symbol is differ by 180
Amplitude remains fixed.
for symbol 1
for symbol 0
Delta Modulation is 1 bit ( or
level) version of DPCM.
In delta Modulation the difference between the original
sample and its approximation is
in one of the
possible levels + /
and each level is converted in to 1 bit
Thus, the delta modulation uses only one bit to represen
sampled i/p m(nTs)
> 1 bit quantizer
> DM wave
Delay Ts <
Quantization Noise in DM:
Delta Modulation systems are subjected to two types of
If the slope of the signal is so high then the step
size may not
be sufficient to follow the rate of change of the signal.
In this case, the condition is called slope overload distortion
and the resulting quantizing error is called
distortion or noise.
Slope Overload distortion can be reduced by filtering the
signal to limit its maximum rate of changes or by increasing the
The condition for no
slope over distortion is:
del/Ts >= max(d(m(t)/dt)
When the slope of the signal is low that is signal is almost
constant w.r.t time and /
step size is relatively high, the
approximation starts to swing from
to + /
causing high noise
level called the granular noies.
This noise can be
minimized by reducing the step size /
ulse code Modulation:
PCM is an method of converting an analog signal to digital
In this method, the analog message signal is sampled
(sampling) and the amplitude of a each sampled signal is
off (quantizing) to the nearest one of the finites set of
This discrete signal is then converted into binary digital signalor
digital codeword (encoding)
PCM is not modulation in conventional sense.
to the variation of some
of carrier waves accordance with the information
AM and FM
AM >> low quality than FM and highly susceptible to noise
FM >> High Quality (or good) because noise highly effect the
ude rather than frequency so FM has high noise immunity
AM >> Bandwidth requirement for AM is less i.e. BW Am = 2 *
FM >> Bandwidth requirement for FM is greater than AM i.e BW
Fm = 2(beta+1)
AM>> Ground Wave
FM >> Ground Vertical
AM >> Required high SNR
FM >> low SNR is sufficient for transmission
AM>> used in long distance transmission
process amplification is
FM>> Low coverage area and LOS communication is
power amplification is difficult
The digitally oriented computers and terminals often
communicate with one another through the analog telephone
Therefore the digital messages must be translated into a
suitable for transmission across the analog network.
The term modem is derived from the process of accepting
digital bits and changing them into a form suitable for analog
the signal at other station and
transforming it bac
k to original digital representation.
i.e. first modulation and then demodulation
Modems are derived from these two words.
Modems are designed around the use of carrier
Digital Modulation Methods:
Three basic modulation methods exists.
Some modems use more than one of the methods.
Each method impresses the digital data signal onto the analog
Amplitude Modulation alter the carrier signal amplitude in
accordance with the modulating digital
and phase of the carrier are held constant and
the amplitude is raised or lowered to represent a 0 or 1.
In its simplest form, the carrier signal can be switched on or off
to represent the binary state.
AM modulation is not
often used by itself due to transmission
power problems and sensitivity due to distortion.
However it is commonly used with phase
to yield a
method superior to either FM or AM.
This method changes the frequency of the
accordance with the digital bit stream.
The amplitude and phase are held constant.
In its simplest form, a binary 1 is represented by a certain
and a binary 0 by another.
Phase modulation Modems interrupts th
e continuous wave
form and alter the phase of the signal to represent a 1 0r 0.
the common approach today is to compare the phase of the
cycle in a current time period to the phase of in a previous time
This approach is called
e shift keying(DPSK)