CHP: 1 INTRODUCTION

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1


CHP: 1 INTRODUCTION


Data Communication
:

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
-
combined computer
-
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
r
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
computers

grew, it became inefficient for all
users to walk to the computer room, submit their job and return
to
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
each office.


As organizations, grew and the need for the computer grew it
beca
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
prevalent

form of data communications.


Analog and Digital Communication
:


Analog data take on continuous values in some intervals.
For example voic
e and video are
continuously

varying patterns
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
binary data.



In a communication system, data are propagated from
one point to another by means of electromagnetic signals.


An
analog signal is a
continuously

varying
electromagnetic

wave
that may be propagated over a variety of media.


A digital signal

is a
sequence

of
voltage

pulses that may be transmitted over a
wired medium.


For example, a constant +ve voltage level may
represent binary zero and a constant negative voltage level may
represent binary 1.

A
nalog transmission is a means of
t
ransmittin
g analog signals without regard to th
e
i
r content.


The
signals may represent analog data

(eg. voice) or digital data( eg.
binary data that pass through a modem).


The analog signal will
becom
e

w
e
aker after a certain distance.


Amplifiers are used to
2


boost
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
signal.


To achieve, signal at greater distances,
repeaters

are
used.


A
repeate
r

receives

the digital signal, recovers the pattern
of 1s and0s and retransmits a new signal.


Thus attenuation is
overcome
.

Digital transmission is more preferred due to following
reasons:

1. Digital Technology:

2. Data Integrity

3. Capacity Utilization

4
. Security and Privacy

5. Integration


Communication Model:


SOURCE>>TRANSMITTER>>TRANSMISSION
SYSTEM>>RECIEVER>>DESTINATION



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
model are:


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
ission
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
telephone network.


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:


Advantages:

-

It increases immunit
y to channel noise and external
interference.

-

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.

-

Easy to
error detection and correction by the use of coding.

3


-

Long distance communication is possible due to the use of
separator where noise doesn't accumulate.

-

Relatively inexpensive digital circuit may be used.


Disadvantages:

-

Generally, more bandwidth is
required than that of analog
communication system.

-

Synchronization is required.

-

High complexity due to use of analog to digital and digital to
analog.

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

buses

with the width of the parallel bus
matched to the word size of the computer system. Data
between computer systems is usually transmitted in

bit serial
mode
. Consequently, it is necessary to make a parallel
-
to
-
serial
conversion at a computer

interface

when sending data from a
computer system into a network and a serial
-
to
-
parallel
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.

Parallel Transmission
:
In parallel transmission,
multiple

bits

(usually 8 bits or a byte/character) are sent
simultaneously on different channels (wires, frequency
channels) within the same cable, or radio path,
and

synchronized

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
parallel tra
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
becomes more
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

synchronous
.

Serial Transmission
:
In serial transmission, bi
ts are
sent

sequentially

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
the receiver.

Serial transmission can be either synchronous
or

asynchronous
. In synchronous transmission, groups of bits
are combined into frames and frames are sent continuously with
or without data to be transmitted. In asynchronous
transmission, groups
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
Communication:


Asynchronous communication utilizes a transmitter, a
receiver and a wire without coordination about the timing of
individual

bits
. There is no coordination between the two end
points on just how long the
transmitter

leaves the

signal

at a
certain level to represent a single

digital

bit
. Each device uses a
clock to measure out

the 'length' of a

bit
. The transmitting
4


device simply transmits. The receiving device has to look at the
incoming

signal

and figure out what it is receiving and
coordinate and retime its clock to match the incoming

signal
.

Sending data encoded into your

signal

requires that the sender
and receiver are both using the same
encoding
/decoding
method, and
know where to look in the

signal

to find data.
Asynchronous systems do not send separate information to
indicate the encoding or clocking information. The
receiver must
decide the clocking of the

signal

on it's own. This means that the
receiver must decide where to look in the

signal

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
the

signal

sent from transmitting unit.

When the receiver of a

sign
al

carrying information has to
derive how that

signal

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

transmission

medium

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.

EXAMPLES:


Asynchronous communication is used on

RS
-
232

based serial
devices such as on an IBM
-
compatible

computer's

COM 1, 2, 3, 4
ports.

Asynchronous

Transfer

Mode

(
ATM
) also uses this means
of communication. Your PS2 ports on your

computer

also use
serial

communication. This is the method is also used to
communicate with an external
modem
. Asynchronous
communication is also used for things like your

computer's

keyboard

and

mouse
.

Think of asynchronous as a faster means of connecting, but less
reliable.

SYNCHRON
OUS

Synchronous systems negotiate the communication
parameters at the

data

link

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

error

correction

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
exchange
communications parameters and status information. Once a
connection is established, the transmitter sends out a
signal
, and
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
where the

transmission

medium

itself (an electric wire,
radio

signal

or laser beam) is not particularly reliable.


Transmission Channel:


-

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.

5


-

A c
hannel is used to convey an information signal, for example
a digital bit stream, from one or several

senders (or
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
eceived from
(read).


RS 232C:


-

RS
-
232C is a long
-
established standard ("C" is the current
version) that describes the physical interface and protocol for
relatively low
-
speed serial

data communication between
computers and related devices.

-

It was
defined by an industry trade group, the Electronic
Industries Association (EIA), originally for

teletypewriter devices.


-

RS
-
232C is the interface that your computer uses to talk to and
exchange data with your modem and other serial devices.


-

Somewhere
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
dem
or other serial device, which, in accordance with the RS
-
232C
standard, has a complementary interface called the Data
Communications Equipment (DCE) interface.


Interface Standards:


-

In telecommunications, an interface standard is a standard that
des
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
betwee
n two or more (usually different) systems or pieces of
equipment.


-

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.












6


CHP: 3

SIGNALS AND SYSTEM


Signal:


-

function of independent variables which carry certain
information.

-

may be function of time, temperature, pressure, distance

-

can be voltage or
current in electrical sense.


Types of Signals:


1.
Continuous

and Discrete

2. Deterministic and non
-
Deterministic

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

-

almost

all practical non
-

periodic signal

-

time limited

-

rectangular pulse

-

E = intg frm
-

inf to inf | x(t) |2 dt for
continuous

-

E = sum frm n =
-
inf to inf | x(n) |2 dn for discrete


-

Power signal has finite average power and
infinite

energy

-

0 < P < inf and E = inf

-

almost all practical periodic signal

-

can exist over infinte time

-

eg. Periodic signal: Pulse Train

System:


-

is a combination of elements, components which perform
some task.

-

is a set of element which produces o/p in response to i/p

-

Mathematically, y(n) = f[ x(n) ]


Discrete System Classification:


1.
Causal and Non
-
Causal
:

-

o/p depends on the present and past value


2
. Linear and Non
-
linear
:

-

linear if it sat
isfies principle of superposition

-

sum of weighted i/p is same as the sum of
weighted

o/p


3.
Time

variant and invariant system:

-

Time invariant if the i/p o/p relationship doesnt vary with time

-

Shift Invariance


4.
Static and Dynamic
:

-

static or
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)


5.
Stable and unstable:

-

stable if it produces bounded o/p from every bounded i/p



7


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

of |x(n
-
k)| <m for all values of k and n,

-

From above equation, we can conclude that if the impulse response
h(n) is absolutely
summabl e
, then the o/p of the discrete time LTI
system is bounded in magnitude and therefore the system is called
Bounded Inp
ut Bounded Output (BIBO) stable.

-

A sufficient and necessary condition of stability of a discrete LTI
system is expressed as

S = sum k =
-



to


|h(k)| <




Ch
annel Capacity Theorem
:


-

This gives the relationship between the channel bandwidth and
signal to noise ratio and the limitation that they impose on
communication.

-

Let B be the channel bandwidth and SNR be the recived signal to
Noise Ratio.

-

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
specified
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.


Elementary sig
nals:


1. Exponential signal:

-

x(t) = Ae
bt

-

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


0 otherwise

5. Unit
Ramp function:

-

r(t) = 0 for t < 0


t for t > 0


6. Sinc Function:

-

Sinc(t) = 1 for t = 0


sint/t for t != 0


8


CHP:
5

OVERVIEW OF DATA COMMUNICATION
NETWORKING


Types of Network:


1.


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
also exist.

-

Specialized o
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.


2.

WIDE AREA NETWOR
K


-

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
of

the Internet, like VPN
-
based extranets, are also WANs in
themselves.

-

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
ologies often
found in WANs include SONET, Frame Relay, and ATM.


3.

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:


A.

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.

B.

A MAN (like a WAN) is not generally owned by a single
organization
. 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.

C.


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.

9


Network Top
o
logies:


1.

Bus Topology:


-

In this type of network topology, all the nodes of a network are
connected to a common
transmission medium having two
endpoints.

-

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
networ
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.


2.

Ring Topology:


-

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
node.

-

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
thereby.

-

The ring t
opology does not require a central server to manage
connectivity between the nodes and facilitates an orderly
network operation.

-

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.


3.

Mesh Topology:


-

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
other nodes.

-

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
-
to
-
point

links between all the
nodes.

-

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
transmissions.


4.

Star Topology:

-

In this type of

network topology, each node of the network is
connected to a central node, which is known as a hub.

10


-

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
mor
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
inoperability.


5.

Tree Topology:


-

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
hier
archy.

-

In a symmetrical hierarchy, each node in the network has a
specific fixed number of nodes connected to those at a lower
level.


>>> Apart from these basic types of network topologies, there
are hybrid network topologies, which are composed of a
co
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!



Protocol Architecture:


-

is the layered structure of hardware and software that
supports the exchange of data between systems and supports
applications such

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
error handling

>Timing: Includes speed matching and sequencing


OSI :


-

OSI stands

for Open System Interconnection.

-

The communications concern are partitioned into
hierarchical

set of layers.

-

Each layer performs a related subset of the functions with
another system.


1.
Physical Layer
:


-

A physical layers covers the physical interface
between

devices
and the rules by which bits passed from one to another.

-

It relates to the physical properties of the interface to a
transmission medium.

-

For example, connector that joins one or more cir
cuits.

-

Electrical part of physical layer relates to the representation of
bits.

-

Functional parts of physical layer specifies the function
performed by individual circuits between a system and the
transmission medium.

11


-

Similarly, procedural part of phy
sical layer
species

the
sequence

of events by which bit streams are exchanged across
the physical medium.


2.
Data Link Layer:


-

Data link layer attempts to make the physical link
reliable

and
provides the means to activate, maintain, and deactivate the
link.

-

It provides for the
reliable

transfer of information across
physical link.

-

It sends blocks with the necessary synchronization, error
control and flow control.


3.
Network Layer:


-

A computer system engage in the dialog with the network to
specify the destination address and to
request

network facilities.


4.
Transport Layer:


-

It provides the mechanism for the
exchange

of data between
and system.

-

The connection
oriented

transpor
t service ensures that data
are delivered error free, in sequence with no loss or duplication.


5.
Session Layer:


-

It provides the mechanism for controlling the dialog between
application

in and systems.


6.
Presentation Layer:


-

It defines the format of

the date to be exchanged between
applications.

-

It defines the syntax used between application and provides
for the selection and subsequent modification of the
presentation used.


7.
Application Layer:


-

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.


Frame Relay:


-

More efficient transmission scheme
than

X.25

-

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
layer 2
instead of layer 3, eliminating one entire layer of
processing.

-

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
available


12


-

Frame Relay is designed to provide
efficient

transmission than
X.25.

-

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
frame
and
acknowledgement

frame.

-

At each intermed
i
ate node, state tables must be maintained
for each virtual circuit to deal with cost manage
ment and
flow/error, control as
pects of X.25 protocol.

-

All

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
i
a
ble digital transmission
t
echnology over high quality relia
ble digital transmission
t
echnology over high qua
lity rel
i
a
ble transmi
ssion links such as
optical fiber
.

-

In this environment, the overhead of X.25 is not only
unnecessary

but degrades the effe
ctive utilization of the
availa
ble high data rates.

-

Frame Relay is designed to eliminate much of the overhead

that X.25 imposes on end user systems.


LLC/MAC:


-

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
-
layer

OSI
reference mo
del.

-

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
-
l
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

identifier.

-

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
cation protocol
sub
-
layer, also known as the Medium Access Control, is a sub

layer of the Data Link Layer specified in the seven
-
layer OSI
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).
-

The
hardware that implements the MAC is referred to as a Medium
Access Controller.

The MAC sub
-
layer acts as an
interface between the Logical Link
Control (LLC) sub

layer and the network's physical layer.
-

The
MAC layer emulates a full
-
duplex logical communication channel
in a multi
-
point ne
twork. This channel may provide
unicast,
multicast or broadcast communicati
on service.



Routing
:

-

Routing is the process of selecting paths in a network along
which to send network traffic.

13


-

Routing is performed for many kinds of networks, including the
telephone network (Circuit switching) , electronic data networks
(such as
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.
-

General
-
purpose computers
can also forward packets and perform routing, thou
gh they are
not specialized hardware and may suffer from limited
performance.
-

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
alternative paths.

-

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
in localized
environments.


IEE 802 Reference Model


-

This a
rchitecture w
as developed by the IEEE 802 co
m
mittee

and has been adopted by all organizations working on the
specification of LAN standards.

-

It is generally referred to as IEEE 802
Reference

Model working
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.


1.
Physical Layer:


-

encoding and decoding of signals

-

pre

able generation/removal (for synchronization)

-

bit transmis
sion
/reception


2.
MAC(Medium Access Control)


-

on transmission assemble data into a frame with error
detection and address fields

-

On reception,
dissemble

frame and perform address
recognition and error detection

-

Goren

access to LAN transmission medium


3.
LLC(Logical Link Co
ntrol)


-

provide an interface to higher layers and perform flow and
error control


4.
LLC Services:


-

Three services are provided un
der LLC services:

14



>
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
h
reliability

issues.


>
Connection Mode services:

-

A logical connection is set up between two users
exchanging

data and flow and error control are provided.


>
Acknowledged

Connectionless Services:

-

It provides acknowledgement but no logical connection
is set
up.


Ethernet CSMA
-
CD:


-

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
the Data

Link
Layer.

-

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:


-

CSMA/CD (Carrier Sense Multiple Access / Collision Detection
)
is the protocol used in
Ethernet networks

to ensure that only
one network node is transmitting on the network wire at any
one time.

-

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
net devices
accidentally transmit at the same time, they are able to detect
this error.


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
to the

other node.

-

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
design.

-

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
retransmission.

15


-

Ethernet switches greatly reduce the already minor difficulties
experienced with the CSMA/CD protocol.


X.25:


-

The X.25 protocol, adopted as a s
tandard by the Consultative
Committee for International Telegraph and Telephone (CCITT), is
a commonly
-
used network protocol.

-

The X.25 protocol allows computers on different public
networks (such as CompuServe, Tymnet, or a TCP/IP network) to
communicate

through an intermediary computer at the network
layer level.
-

X.25's protocols correspond closely to the data
-
link
and physical
-
layer protocols defined in the Open Systems
-

Interconnection (OSI) communication model.


-

Three levels:


>
Physical Layer:

-

physical interface between and attached station(computer
terminal and Packet Switching mode.


>
Link Level:

-

provides
reliable

transfer of data across
physical

link

-

It is referred as Link Access protocol
-

Balanced(LABP)


>

Packet Level:


-

provides vi
rtual circuit service

-

enables any subscriber to the network to setup logical
conditions



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
data packets

-

multiplexing of virtual circuits takes place at layer 3

-

Both layer 2 and layer 3 include flow control and err
or control
mechanisms


ATM:


-

Asynchronous

Transfer Mode

-

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
scheme for
transmission

over hig
h speed networks.

-

data rate range from 25.6 Mbps to 622.08 Mbps

-

Physical layer specifies
transmission

medium and signal
encoding scheme

-

ATM layer defines
transmission

of data in fixed size cells and
defines the use of logical connection.

-

ATM adapta
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,
err
or control)

-

Control plane provides call control and connection control
functions.

-

Management plane performs coordination between all the
planes and layers management.



16


CHP: 6 TRANSMISSION MEDIA


Guided and Unguided Media
:


-

Guided media are those that
provides

physical conduction
from one device to another which includes twisted pairs, co
-
axial cables and
fiber
-
optic cables.

-

Unguided Media transports
electromagnetic

waves
without

using a physical conductor.

-

This type of

communication is often referred to as wireless
communication.


Transmission Media:


-

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
electromagnetic waves.

-

With guided media, the waves are guided along a solid
medium, such as copper twisted pair, copper coaxial cable, and
optical fiber.

-

The atmosphere and outer space are examples of unguided
media that

provide a means of transmitting

Electromagnetic

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
the characteristics

o
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

transmission.

-

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
directionality.

-

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:


>>
Bandwidth:

-

All other factors remaining constant, the greater the
bandwidth of a signal, the higher the data rate

that can be
achieved.


>>
Transmission impairments:

-

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
optical fiber.


>>
Interferenc
e:

-

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.

17


-

For guided media, interference can be caused

by emanations
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

problem.


>> Number of receivers:

-

A guided medium can be used to construct a point
-
to point

link
or a shared link with multiple attachments.

-

In the latter case, each attachment introduces some
attenuation and distortion on the line, limiting

d
istance

and/or
data rate.


Wired Pairs:


-

Wires are described by
their

size.

-

Higher gauge number
indicates

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
communication path.

-

At higher transmission frequencies, the
signal tends

to travel
on the outside surface of the wire.

-

A small wire provides less total surface for the
radiating

signal,
resulting

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
-
gauge wires.

-

Several Hundred of these wires are packaged into one cables.

-

The wires are paired
and

twisted around each other to
decrease certain
electromagnetic

problems.


-

The most common twisted pair cable used in communications
is
referred

to as unshielded twisted pair(UTP) cable.

-

STP cable(shielded Twisted Pair) has a metal foil or braided
-
mesh covering each pair of
insulated

conductors.

-

Although metal casing improves the
quality

of cable by
preventing the penetration of noise or cross
-
talk, it is bulkier
and more expensive.


>
>
Applications:

-

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
unshielded

twisted pair cables.

-

Local area networks such as 10 Base
-
T and 100 Base
-
T also use
twisted pair cables.


Micro
-
waves:


-

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
system
.

-

Television transmission also uti
lizes microwave transmission
because microwave transmission is above the 1 GHz and
provides the capacity
required

for video transmission.

-

The high bandwidth gives small wavelength and the smaller
the wavelength, the smaller one can design the microwave
antenna.

-

The antenna size has significant implications for distributed
processing systems.

18


-

The transmitting towers are spaced 20
-
30 m apart.

-

Transmitted radio bean is focused to the
receiving

antenna.



COAXIAL Cables:


-

Co
-
axial cables carries sign
als of higher
frequency

ranges than
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
conductor of
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.


Applications:

-

The use of co
-
axial cable is diverse but nowadays it is
shrinking
due to fibre optic cable.

-

Co
-
axial cables are used in analog telephone networks and
cable Tv networks.


Fiber
optic Cables:


-

A
fiber

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
an the
critical angle, the ray
diffracts

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
ts
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
uch
that a beam of light moving through the core is reflected off the
cladding instead of being reflected into it.


Propagation Modes:


a.

Multimode step index:

-

in multimode step
-
index
fiber
, the density of the core remains
constant from the center of
the edges,

-

A beam of light moves through these constant densities in a
straight line until it reaches the interface of the core and the
cladding.

-

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.


b.

Multimode graded index
:

-

A second type of
fiber

called multimode graded index
fiber

is
one with varying densities.

-

Density is highest at center of the core and decreases gradually
to its

lowest at the edge.


19


c.

Single mode
:

-

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 horizontal.

-

The single mode fiber is manufactured with a much smaller
diameter
that that of multimode fiber.



Applications
:


-

Optical
fiber

cable is found in backbone
networks

because of
its wide bandwidth and is cost effective.

-

cable TV companies use a combination of optical
fibers

and co
-
axial cable, thus creating a hybrid netw
ork.


Advantages:


-

Higher bandwidth

-

Less signal attenuation

-

Immunity to
electromagnetic

interference

-

Light weight

-

Resistance to corrosive materials


Disadvantages:


-

Installation/
maintenance

-

Unidirectional

-

Cost


Electromagnetic waves:

-

The

electromagnetic

waves used e propagation characteristics
of wireless channels are highly dependent on
frequency
.

-

Since, electromagnetic waves doesn't need any medium to
transmit the signal specially in wireless communication system
we often use the atmo
sphere for
transmission

of channel.

-

Here, interference and propagation condition are strongly
dependent upon the
frequency
.



Types of Electromagnetic waves:


a.

Ground Wave Propagation
:


-

Dominant mode of propagation for
frequency

below 2 mHz.

-

Electroma
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
nger than
1/10 th of the wave length.


b.

Sky Wave Propagation:


-

Dominant mode of propagation for
frequencies

in between 2
to 30 Mhz.

-

Long distance coverage is obtained by the reflecting the wave
at the ionosphere and at the earth boundaries.

-

This

is caused due to reflection.


c.

Line of sight(LOS) or Space Wave propagation
:


-

Dominant

mode of propagation for
frequencies

above 30 Mhz.

-

Here, electromagnetic wave propagates in a straight line.

20


-

Very little reflection by the ionosphere,

-

This is
used for satellite communication.

-

Its maximum range is limited to line of sight due to nature of
propagation.


Disadvantages
:


-

For communication between two each stations, the signal path
has to be above the horizon otherwise they will block the LOS
pa
th.

-

Thus antennas need to be placed on tall towers so that
receiver

antenna can see the
transmitting

antenna.


Radio waves and Microwaves:


1. 3 KHz to 1 GHz

-

1 GHz to 300 GHz


2. for the most part are omnidirectional

-

unidirectional


3.
Omnidirectional antennas are generally used

-

line of sight propagation, unidirectional antennas are used


4.AM,FM radio

-

cellular phones, satellite networks


Satellite Communications:


-

Satellite communications are comprised of 2 main
components:


The S
atellite


-

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
-
up
sig
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
downlink.

-

The primary role of a satellite is to reflect electronic si
gnals.

-

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
first.

-

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
down
linked

over a wide region, so that it may be received by many
different customers possessing compatibl
e equipment.

-

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
vantage
point
.




The Ground Station
:


-

This is the earth segm
ent.

21


-

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
the
terrestrial

telephone network( PSTN: Public switch
Telephone Network) for any user
location within

the radio range
of the system.

-

Cellular
systems accommodate

a large number of users over a
large geographic area, within a limited
frequency

spectrum.

-

Cellular

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
switching

center(MSC).

-

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.

-

This
base station is connected via telephone lines or
microwave link to the mobile
switching

center.

-

The MSC connects the user to the called party if the called
party is land
based
, the connection is via the central office (CO)
is the terrestrial telephone ne
twork.

-

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
available

radio channel in the cell
associated

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.

-

The
critical

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
-
interrupted.


-

The cellular
concept

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.



22


Transmission Modes:


Parallel Transmission:


-

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.


Serial
Transmission
:


Synchronous and asynchronous transmission


Given the problems that arise with a parallel
-
type connection,
serial connections are normall
y used. However, since a single
wire transports the information, the problem is how to
synchronies

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
problem:


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
informati
on indicating the start of character transmission (the
transmission start information is called a START bit) and ends by
sending end
-
of
-
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
on,
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
insert
synchronization

ele
ments; this is called character
-
level
synchronization
.


The main disadvantage of synchronous transmission is
recognizing

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










23


Transmission Impairments:


-

The signal
received

may differ from that is transmitted due to
various
transmission

impairments.

-

for analog signals, these impairments can degrade the signal
quality

-

for digital, bit errors may be introduced.

-

A binary 1 is transformed into binary 0 and vice versa.


1.
Attenuation:


-

strength

of a signal falls off with
distance over any transmission
medium.

-

Hence, a
received

signal must have sufficient
strength

so that
the electronic
circuitry

in the
receiver

can detect the signal.

-

The signal must maintain a level sufficiently higher than noise
to be
received

without

error.

-

Attenuation is an increasing concern of
frequency
.

-

Hence, amplifiers must be used that amplify high
frequencies

more than lower
frequencies
.


2.
Delay Distortion:


-

occurs because of the
velocity of

propagation of a signal
through guided medium varies with
fr
e
quency
.

-

For a band limited signal, the velocity tends to be highest near
the
central

frequency

and falls off towards the two edges of the
band.

-

Thus, various
frequency

components of a signal

will arrive at
the
receiver

at
different

times resulting in phase shifts between
the
different

frequencies.

-

Intersymbol
Interference

occurs due to delay distortion


3.
Noise:


-

unwanted man made or natural random signal that adds to
the
received

signa
l and degrades the performances.

-

Such type of unwanted random signal is noise.


>
Thermal noise
:

-

due to the thermal agitation of electrons in the conductor.


>
Shot Noise:

-

due to the flow of current at the junction of semi conductor.


>
Burst

Noise:

-
due to the sudden high amplitude making the signal change its
value such as lightning, electrical ignition
system
.


>
Intermodulation Noise
:

-

due to non linearity’s in the transmitter, receiver and
interviewing

transmission media.


> Cross talk Noise:

-

due to coupling of nearby line.


> Flicker Noise:

-

low
frequency

Noise


>
Transit time No
ise:

-

High
frequency

Noise




24


DATA LINK CONTRO
L


Flow

control:


-

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
-
level software.

-

In the ab
sence of flow control, the receiver's buffer may fill up
and overflow

while it is processing old data.



Stop
-
and
-
Wait Flow Control:


-

The simplest form of flow control, known as stop
-
and
-
wait flow
control, works as

follows.

-

A source entity transmits a
frame. After reception, the
destination entity

indicates its willingness to accept another frame by sending back
an acknowledgment to the frame just received.
-

The source
must wait until it receives the acknowledgment before sending
the next frame.

-

The
destination can thus stop the flow of data by simply
withholding acknowledgment.

-

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
a large
block of data into smaller blocks and transmit the data in many
frames.

-

This is done for the following

reasons:

> 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
retransmitted.

> 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
causes long

delays at the other sending stations.



Sliding

Window Flow control:


-

Allows multiple frames to be in transit

-

Receiver

sends
acknowledgement

with
sequence

number of
anticipated frame

-

Sender maintains list of

sequence

number of anticipated frame

-

Sender maintains list of
sequence

numbers it can send,
receiver

maintains list of
sequence

numbers it can
receive
.

-

ACk (
acknowledgement
) supplemented with RNR (
receiver

not
ready)


25


-

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
frame

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:


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
errors:


>
Lost frame
.

-

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
transmitted.


>
Damaged frame
: 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

following ingredients:


>
Error detection
. As discussed in the preceding section.


>
Positive acknowledgment
. The destination returns a positive
ack
nowledgment

to successfully received, error
-
free frames.


>
a Retransmission after timeout
. The source retransmits a
frame that has not

been acknowledged after a predetermined amount of time.


>
Negative acknowledgment and retransmission
. The
destination r
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
repeat request

(ARQ); the effect of ARQ is to turn an unreliable data link into

a
reliable one.

-

Three versions of ARQ have been standardized:

> Stop
-
and
-
wait ARQ

> Go
-
back
-
N ARQ

> Selective
-
reject ARQ


Stop or Wait ARQ:


-

Stop
-
and
-
wait ARQ is based on the stop
-
and
-
wait flow
-
control
technique

-

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.

26


-

Two sorts of errors could occur. First, the frame that arrives at
the destination

could be damaged
; the receiver detects this by using the error
detection technique referred to earlier and simply discards the
frame.

-

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
expires,
then the

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
that frame.

-

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
-
back
-
N ARQ.

-

In go
-
back
-
N ARQ, a station may send a series of fra
mes
sequentially numbered

modulo some maximum value.

-

The number of unacknowledged frames outstanding is
determined by window size, using the sliding
-
window flow
control

technique.

-

While no errors occur, the destination will acknowledge (RR =
receive
ready
)

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.


























27


MULTIPLEXING

AND SWITCHING


Switching:


1.
Circuit Switching:


-

Real time data transmission is provided by the direct
connection

-

Dial up delay can be eliminated by using leased lines

-

Blockage

can occur in
which

case busy signal is returned to
sender

-

Transmissions are point to point

-

Once co
nnection is established, any
subsequent

overload of
the
switch

is invisible to the connected components


2.
Message Switching:


-

Connection is not a direct physical interface as in circuit
switching

-

Data connections
use

variable slots if TDM is
employed

-

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

-

priorities

are allowed in the message traffic


3.

Packet
Switching
:

-

combination of circuit and message switching

-

packet contains user and control data

-

provides stastical multiplexing

-

provides

fast response to all users

-

provides high
availability

of the network to all users.

Multiplexing:


-

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.

-

The multiplexer

combines (multiplexes) data from the n input
lines and transmits over a higher capacity data link
.

-

The demultiplexer accepts the multiplexed data stream,
separates

(
demultiplexer
) the data according to channel, and
delivers them to the appropriate output lines.



FDM
(Frequency Division Multiplexing)
:


-

FDM is possible when the useful bandwidth of t
he transmission
medium exceeds the required bandwidth of signals to be
transmitted.

-

A number of signals can be carried

simultaneously if each signal is modulated onto a different
carrier frequency

and the carrier frequencies are sufficiently separated th
at the
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.


28


Synchronous TDM:


-

Synchronous time
-
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
portion
s of each signal in time.

-

The interleaving can be at the bit level or in blocks of bytes or
larger quantities.

-

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
sources.


Private Branch Exchange:


-

A PBX (private branch exchange) is a telephone system within
an enterprise

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
provider, however).

-

Private branch exchanges used analog technology originally.

-

Today, PBXs use
digital technology

(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
PBX

> 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
es Digital
Network.


Switched 56/
Service
:


-

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.

-

supports video
conferencing
, multimedia etc











29



DATA ENCODING AND MODULATION


Definition of digital signal encoding formats:


Nonreturn
-
to
-
Zero
-
Level (NRZ
-
L):

0 = high level

1 = low level


Nonreturn to Zero

Inverted (NRZI):

0 = no transition at beginning of interval (one bit time)

1 = transition at beginning of interval


Bipolar
-

AM1:

0 = no line signal

1 = positive or negative level, alternating for successive ones


Pseudo ternary

0 = positive or negative
level, alternating for successive zeros

1 = no line signal


Manchester:

0 = transition from high to low in middle of interval

1 = transition from low to high in middle of interval


Differential Manchester:

Always a transition in middle of interval

0 = tran
sition at beginning of interval

1 = no transition at beginning of interval


BIZS:

-

Same as bipolar AMI, except that any string of eight zeros
is replaced by a string with

two code violations


HDB3:

-

Same as bipolar AMI, except that any string of four zeros
i
s replaced by a string with

one code violation


Encoding:


-

In combining the processes of sampling and quantizing, the
specification of a
continuous

base
-
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
fiber
.

-

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
encoding.

-

Suppose, in a binary code each code w
ord consists of n bits.

-

Then using
such a

a code we may represent a total of 2^n
distinct numbers

L=2^n



Amplitude shift Keying


-

The most basic form of ASK involves the process of switching
the carrier on or off, in
correspondence

to a
sequence

of digital
pulses that constitute the information signal.

30


-

binary digit 1
represented

by the presence of a carrier and the
binary

digit 0 represented by the absence of a carrier.

-

Frequency

remains fixed.

s(t) = A
c

cos2
π
f
c
t for binary 1


0 for

binary 0


Frequency

Shift

Keying:


-

The basic form of FSK involves the process of varying the
frequency

of a carrier wave by choosing one of two
frequencies

in correspondence of digital pulses that constitute the
information signal.

-

Two binary digits 0

and 1 are
represented

by two frequencies
around the carrier
frequency
.

-

Amplitude

remains fixed.


s(t) =
A
c

cos2
π
f
1
t
for 1




A
c

cos2
π
f
2
t

for 0



Phase Shift Keying:


-

The most basic form of PSK involves the process of shifting the
phase of a carrier wave in
correspondence

to a
sequence

of
digital pulses that constitute the
information

signal.

-

The two binary digits 0 and 1 are
represented

in which the
carrier phase f
or each symbol is differ by 180
frequency

and
Amplitude remains fixed.


s(t) =
A
c

cos2
π

f
c

t

for symbol 1


A
c

cos(2
π

f
c

t
+

π)

for symbol 0


Delta

Modulation:


-

Delta Modulation is 1 bit ( or
two

level) version of DPCM.

-

In delta Modulation the difference between the original
sample and its approximation is
quantized

in one of the

two
possible levels + /
\

or
-
/
\

and each level is converted in to 1 bit
codeword.

-

Thus, the delta modulation uses only one bit to represen
t each
sampled value.


sampled i/p m(nTs)
---
> Sum
---
> 1 bit quantizer
-------
> DM wave


sum


Mq(nTs
-
Ts)
--------------
Delay Ts <
-------

Mq(nTs)


Quantization Noise in DM:

-

Delta Modulation systems are subjected to two types of
quantizing

error:


Slope over

load distortion:


-

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
slope overload
distortion or noise.

-

Slope Overload distortion can be reduced by filtering the
signal to limit its maximum rate of changes or by increasing the
step noise.

-

The condition for no
-
slope over distortion is:


del/Ts >= max(d(m(t)/dt)

31




Granu
lar Noise:


-

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 /
\
.

P
ulse code Modulation:


-

PCM is an method of converting an analog signal to digital
signal.

-

In this method, the analog message signal is sampled
(sampling) and the amplitude of a each sampled signal is
rounded
off (quantizing) to the nearest one of the finites set of
discrete levels.

-

This discrete signal is then converted into binary digital signalor
digital codeword (encoding)

-

PCM is not modulation in conventional sense.

-

The term
modulation

usually

refers

to the variation of some
characteristics

of carrier waves accordance with the information
bearing signal.


AM and FM


1.
Quality

AM >> low quality than FM and highly susceptible to noise

FM >> High Quality (or good) because noise highly effect the
amplit
ude rather than frequency so FM has high noise immunity
than AM.


2.
Bandwidth:

AM >> Bandwidth requirement for AM is less i.e. BW Am = 2 *
fm

FM >> Bandwidth requirement for FM is greater than AM i.e BW
Fm = 2(beta+1)


3.
Propagation:


AM>> Ground Wave
propagation

FM >> Ground Vertical
Propagation


4.

SNR
:


AM >> Required high SNR

FM >> low SNR is sufficient for transmission


6.

Distance
:


AM>> used in long distance transmission


process amplification is
easier


FM>> Low coverage area and LOS communication is

required


power amplification is difficult










32


MODEMS


Modems:


-

The digitally oriented computers and terminals often
communicate with one another through the analog telephone
facilities.

-

Therefore the digital messages must be translated into a
form
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
transmission and
receiving

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
frequency
.


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
carrier signal.



Amplitude Modulation:


-

Amplitude Modulation alter the carrier signal amplitude in
accordance with the modulating digital
bit stream.

-

The
frequency

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
modulation

to yield a
method superior to either FM or AM.


Frequency

Modulation:


-

This method changes the frequency of the

carrier in
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
frequency

and a binary 0 by another.


Phase Modulation:


-

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
period.

-

This approach is called
differential

phas
e shift keying(DPSK)