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Oct 23, 2013 (3 years and 8 months ago)

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INTRODUCTION

Def: Network


A network is a set of devices(nodes or terminals) connected by

communications link(channels). OR


A network is interconnected nodes.


A node can be a
computer or printer or any other device capable of

sending and/or receiving
data
generated by other nodes on the

network.


The nodes are computers and are called
computer networks.


The computer networks are used to establish communication between

set of

nodes.


When the nodes are communicated, sharing/exchange of

data/information is possible.


sharing/exchange of data/information is called
data communication.

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INTRODUCTION


The data communication can be either local/remote.


Loca
l means face to face. OR


Communicating nodes are in the same location.


Remote communication takes place over the distance. OR


Communicating nodes are not in the same location.

Note:
the term
telecommunication
, which includes telephony ,

telegraphy

and television , means communication at a distance.(
tele
is

greek word, it means
far
)


Basically, in telecommunication data in the form of
voice.


But data communication refers data in whatever form agreed upon by

communication parties.


In data communication data in may forms like Text, numbers, audio

and video etc.

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INTRODUCTION


Data Communication are the exchange of data between set of devices

using some transmission media.


The transmission medium can be
either
air or wire(Cable).


For data communication, communicating devices must be part of the

communication system.


The Communication system made with
Hardware & Software

components.

Characteristics of Data Communication


Delivery

Deliver data to the
correct destination.


Accuracy

data delivered to the destination without change during

the transmission.


Timeliness

Data must be delivered with in the specified time.


Jitter

Jitter refers to the variation in the packet arrival time, jitter

must
be uniform.

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INTRODUCTION

Components in DC

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Rule 1

Rule 2

----

----

Rule N

Rule 1

Rule 2

----

----

Rule N

Sender

Receiv er

Message

Protocol Protocol


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2

INTRODUCTION

Message:
it is the information/data to be
communicated.

Sender :
it is the device that sends the message, it can be computer,

workstation, mobile phone etc.

Receiver :
it is the device that accept the message.

Transmission medium:
this is the path, which is used to transmit

message from sender to
receiver.


Transmission medium is either air or wire.

Protocol :
it is a set of rules that governs the data communication.

OR

It is an agreement between communicating parties.


With out protocol two devices are connected, but not communicated.

Note: Protocol is a Data Communication Software.

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INTRODUCTION


Computer networks uses Client/Server Architecture and it exists with

distributed processing.


In distributed processing, task is divided into multiple sub
-
tasks and

distributed to multiple nodes.


Distributed processing supports parallel processing and it minimizes

the response time.


In C/S architecture data communication takes place on requestresponse

basis.


In distributed processing, C/S architecture exists wit
h more than one

server.

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Cl i ent Network Server

Request

Response

INTRODUCTION

Uses of Computer Networks


Business Applications

1. Resource Sharing ( data exchange, printer etc.)

2. Communication medium(e
-
mail)

communication among

emp
loyees.

3.e
-
Commerce( e
-
business).

Business Applications of Networks

Client Server Model :
A network with two clients and one server:

Employees accessing company’s Information System

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INTRODUCTION

Business Applications of Networks

The
client
-
server model involves requests and replies.

Peer
-
to
-
PeerModel of Communication

In peer
-
to
-
peer system there are no fixed clients and servers.

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3

INTRODUCTION


Home Applications.

1.Access to remote information(WWWetc)

2.Person
-
t
o
-
person communication(Email, Instant Messaging etc,

Phone)

3.Interactive entertainment(Remote operated or online Games)

4.Electronic commerce

Mobile Users


Mobile computers, such as notebook computers, PDAs etc.


Mobile computers are mainly used to work

from home/ away from

office.


Wireless networking & mobile computing are often related.


The following table shows the combinations of mobile computing &


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wireless networking.

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INTRODUCTION

Wireless & Mobile

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INT
RODUCTION

Network Hardware


Computer networks is a collection of H/W& S/WComponents.


H/WComponents includes


Computer H/W


Connecting devices like switch, bridges, router etc


Transmission medium like Twisted pair, Coax
-
Cable, Fiber Optic

cable etc.


There are two types of transmission technology/network connections

1. point
-
point link

2. Broadcast link/Multipoint link.


Point to point link provides dedicated link between two connecting

devices.


In multipoint link, one link/ channel shared by two or

more nodes.

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INTRODUCTION

Point to point link

Multi
-
point link

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Sender

Receiv er

Poi nt
-
Point l ink

stati on

stati on

stati on

stati on

Mul ti Point Li nk

4

INTRODUCTION

Modes of Operation


A sender send a data to

exactly to one receiver is called
unicasting.


It generally happens in point
-
point links and also in
multipoint links.


A sender send a data to few nodes in the network is called

multicasting.


A sender send a data to all nodes in the network is called

broadcasting.

Data Flow


Communication between devices can be simplex, half
-
duplex or fullduplex(

duplex).

Simplex:


Communication is unidirectional and is fixed.


For Ex: keyboard to CPU.

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INTRODUCTION

Half
-
Duplex:


Communication

is bidirectional, but not at the same time.


For Ex: one
-
line road, traffic allowed in both the directions, but not at

the same time.


walkie
-
talkie

Full
-
Duplex(Duplex):


Communication is bidirectional, at the same time(simultaneously ).


For Ex:
two
-
line road, traffic allowed in both the directions.


Ex: telephone conversation, chatting .

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INTRODUCTION

Based the geographical area occupied by the networks are divided into

following categories or types.

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INTRODUCTION


Based on the physical/logical arrangement, networks are divided into

the following categories.


The physical/logical arrangement of nodes in a network are called

topology.

1. Mesh topology

2. Star topology

3. Ring topology

4. Bus topology

5
. Hybrid (Tree) topology.

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INTRODUCTION

Mesh Topology


Every node has a dedicated point
-
point link to every other device.


If a N/W with N nodes, the number of links are n(n
-
1)/2.

Adv

1. Each connection carries its own load, no tra
ffic problem.

2. It is robust.

3. Privacy & security is high.

4. Fault identification & fault isolation is easy.

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s1 s2

s4 s3

INTRODUCTION

DisAdv

1. It requires large number of cables & i/o ports.

2. Installation & reconnection are
difficult.

3. It requires more space.

4. This topology is more expensive

Some implementations

1. Connecting backbone networks.

2. Each telephone regional office connected to all other regional offices.

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INTRODUCTION

Star Topology


Each node has dedicated point
-
point link up to central controller / hub.


Nodes are not directly connect to other.


In this topology data flow between the devices only through hub.


The hub acts as exchange point.


The hub at the Center and all nodes c
onnected to it resemble like a

star.

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s1 s2

s4 s3

CE

INTRODUCTION

Adv

1. It is less expensive than mesh topology.

2. Each node needed one link & one I/O port only.

3. The above factors supports easy to install & configure.

4. It is
also robust.

5. Fault identification & fault isolation is easy.

DisAdv

1. If the hub/CE goes down, the whole n/w is dead.

2. It requires more cabling than other topologies (bus, ring).

It implantations

1. Most LANs are star topology.

2. Few MANs are star
topology.

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INTRODUCTION

Bus Topology


It uses multipoint / broadcast link.


One long cable acts a backbone, all the nodes connected to the

backbone cable.


All the nodes are connected serially and resemble like a bus.


Nodes conn
ected to the bus cable(backbone) using drop lines & taps.

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stati on1 station2 station3

Dr op cable

Tap

Cabl e End

Cabl e End

INTRODUCTION

Adv


Easy of installation & configure.


It requires less cable than star topology.

DisAdv

1.
Difficult to reconnect & fault detection.

2. Difficult to add the new nodes.

3. Backbone cable damages, entire network dead.

It implementations

1. Early LANs uses bus topology.

2. Ethernet LANs can use a bus topology.

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INTRODUCTION

Ring Topology


In this topology, each device has a dedicated point
-
point connection

with only adjacent devices of it.


A message transmit only in one direction from device to device until

reaches to the destination.


In this, each node acts as repeater,

it means each node re
-
generate the

signal and forward to its neighbor.

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A

B C

D

F E

Repeater

INTRODUCTION


Flow of data controlled using token, and is also called
token ring n/w.

Adv:


It is easy to install & configure.


To add or
delete a node only modify two connections.


Fault identification is simple.

DisAdv:


Only one node can send at an instant of time.


Any one of the node is damaged, entire n/w can be dead.

Its implementations are


It is initially implemented IBM for LAN
(Token Ring).


It is also used in high speed LANs.

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INTRODUCTION

Hybrid(Tree) topology


A n/w with more than one topologie is called
hybrid topology.


For Ex: Backbone star topology and each branch connecting several

stations

in a bus topology.

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11 12 13

21 22 23

31 32 33

Hybrid topology with star & bus

INTRODUCTION


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N/W Software


In computer networks, communication occurs between entities in

different systems.


An entity is anything capable of sending
and/or receiving information.


Two entities can simply exchange data/ Information.


To exchange data, two entities must agree on
protocol.


A protocol is a set of rules/agreement that governs data

communication.


A protocol defines


How it is communica
ted.


What is communicated.


When it is communicated.


The key elements of the protocol are syntax, semantics & timing.

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INTRODUCTION

Syntax:
the term syntax refers to the structure or format of the data.


In computer networks
exchange of data always in fixed format.


Exchange of data in terms of packets and frames.

Semantics:
it refers, meaning of each field in the packet.

Timing:
it refers two characteristics

1. when data should be sent.

2. how fast they can be sent.

NOTE:
Protocols are the network software.

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INTRODUCTION

Protocol Hierarchies


To reduce the complexity, network s/w are organized in the form of

layers or levels.


Each layer built upon the below it.


The number of layers, name of the lay
er and functions of the each

layer differ from network to network.


Each layer offering certain services to its immediate upper layer.


Functions of each layer are abstract to outside world.


For Ex: to consider 5 layer network.

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INTRODUCTION


The following diagram shows the communication in 5 layer

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Layer 5 Layer 5

Layer 4 Layer 4

Layer 3 Layer 3

Layer 2 Layer 2

Layer 1 Layer 1

Transmission Medium

Host 1 Host 2

Layer 5 protocol

Layer 4protocol

Layer 3
protocol

Layer 2 protocol

Layer 1 protocol

Layer 4/5 interface

Layer 3/4 interface

Layer 2/3 interface

Layer 1/2 interface

INTRODUCTION


In reality, no data are directly transferred from layer N one machine to

layer N on another machine.


Instead, each
layer passes data & control information to the layer

immediately below it until the lowest layer reached.


Below the lowest layer is the transmission medium through which

actual communication occurs.


In the above diagram virtual communication shows dott
ed lines and

physical communication shows solid lines.

Interface


Between each pair of adjacent layers is an
interface.


Passing of data & control information down through the layers of the

sending device and receiving up through, layers of the receiving

side.


The interface defines which primitive operations and the services the


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lower layer makes available to the upper layer.

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INTRODUCTION

Real Life Example of Layered Structure


Let us consider two friends communicate through
postal mail.


The process of sending a letter to a friend would be complex if there

were no services available from the post office.

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The letter is written, put in an envelope

and dropped in mail box.

The letter is carried from mail b
ox to post

office.

The letter is delivered to a carrier by the

post office

The letter is picked up, removed from the

envelope and read

The letter is carried from post office to

mail box.

The letter is delivered from a carrier to

the post office

sender Rece
iver

Parcel is carried from source to destination

INTRODUCTION

Control Information(header & trailer)


In computer networks, exchange of information consists of data &

control information.


Data is the actual information sent by the sender.


Control
information is nothing but additional information added by

sending host and/or intermediate host to deliver accurate information

to the receiver.


At the sender side, each layer add its own header and deliver to its

lower layer through its interface.


At

the receiver side, each layer remove its own header and deliver to

its upper layer through its interface.

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INTRODUCTION

The following diagram shows the flow of information.

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Sender

H5 M

H4 M

H3 M

H2 M

H1 M H1 M

H2 M

H3 M

H4 M

H5 M

Receiver

Message carried from source to destination

INTRODUCTION

Design Issues for the Layers


Some important points to consider at the time of designing network

software.

1. Addressing

2. Error Control

3. Flow Control

4. Multiplexing

5. Routing.

Services


A layer can offer services to layer above it.


Generally there are two types of services.

1. Connection oriented services.

2. Connectionless services.

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INTRODUCTION


Connection oriented system is modeled based
on telephone system.


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
In this system, establish the connection, use the connection and release

the connection.


Connectionless service is modeled based on postal system.


Each message(letter) carried with full destination address.


Each message is rout
ed independently.


Each service is characterized by a QOS.


Quality of service(QOS) specifies service is reliable (or) un
-
reliable.


Reliable service can be implemented by the receiver acknowledge the

receipt of each message.


Reliable connection orien
ted service has two major variations,

message sequence & byte stream.

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INTRODUCTION


Connection oriented is appropriate for
file transfer.


Connectionless service is often called
datagram service.


Datagram service is un
-
reliable.



A variant of connectionless service is
acknowledged datagram

service.


Ex: Registered Post.


Another service is
request
-
reply.


In this service, sender transmit a single datagram containing a request,

the reply contains the answer.

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NETWORKS

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INTRODUCTION

The following table shows the types of services.

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INTRODUCTION

Service Primitives


A service is formally specified by a set of primitives (operations)

available to a user process to access the service.


These

primitives tell the service to perform some action or report taken

by peer entity.


If protocol stack(layered structure) in the part of OS, primitives are

called
system calls.


The set of primitives varies from service to service.


For Ex: service prim
itives for reliable byte stream in C/S.

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INTRODUCTION

The primitives are used as follows

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INTRODUCTION

The relationship of services & protocols(interface to protocols)


Primary thing is protocol and service are di
fferent concepts.


Protocol always deals peer entities on two communicating machines.


A service is a set of primitives that a layer provides to the layer above

it.


A service relates to an interface between two layers.


Lower layer is service provider

& upper layer is service user.


The following diagram shows the relationship of services to protocols

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INTRODUCTION

The relationship between a service and a protocol.

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INTRODUCTION

Principles to design layers
in network software.

1. Each layer should be created where a different abstraction is needed.

2. Each layer should perform well defined function.

3. Each layer function should meet internationally standardized

protocol.


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4. The layer boundaries should be ch
osen to minimize the information

flow across the interface.

5. The number of layer should be minimum.

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INTRODUCTION

Reference Models


There are various reference models to develop efficient network

software.

1. OSI ReferenceModel

2. T
CP/IP Reference Model(DoDModel)

3. Hybrid / Internet/TCP/IPModel

4. ATM Reference Model (Asynchronous transfer mode)

5. ISDN reference Model (Integrated Services digital network) etc.

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INTRODUCTION

OSI Reference Model


The OSI model
is based on proposal developed by the International

Standard Organization (ISO).


It is also called ISO OSI (open system interconnection) model.


It supports communication between heterogeneous systems.


The OSI model has 7 layers.


Each of which defin
es a part of the process of moving information

across a network.


It was standardized in year 1983.


The following diagram shows the organization of 7 layers.

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INTRODUCTION

The OSI reference model.

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INTRODUCTI
ON

Functions of each layer

Physical Layer


It deals functions required to carry a bit stream over a physical

medium.


The physical layer is concerned with following

1. Physical characteristics of interface & medium:
it defines the

interface

between device and medium.

it also defines type of medium.

2. Representation of bits: specify 0s & 1s are in the form of signals.

3. Data Rate: No. of bits per second.

4. Synchronization of Bits: the sender & receiver clocks must sync.

5. Line
configuration: point
-
point or multipoint

6. Physical topology and transmission mode.

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INTRODUCTION

Data Link Layer


It is responsible for moving frames from one hop(node) to another

hope.


It makes physical layers error free to the u
pper layer(n/w layer).


All the layers add its own header to message and forwarded to lower

layer, where as data link layer add both header & tail to the message.


The tail for error detection & correction

Functions of DLL

1. Frames : it divides received

data into manageable frames.

2. Physical addressing: it specify address of receiving node on the n/w.

3. Flow control: sender must be slower than the receiver.

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INTRODUCTION

Error Control:
it provides the detection of errors,
retransmit damaged

frames and lost frames.


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It also correct error up to some extent.

Access Control :
when two or more devices connected to the same link,

DLL determines which device has access control over the link at an

instant of time.

Network Layer:


i
t is responsible for source
-
destination delivery of packet across

multiple networks(links).


DLL delivery of the frame with in the same network.


Source & Destination on the same network, n/w layer is optional, but

source &

destination on different networks n/w layer is essential.


It deliver the packet from true source node to true destination node.

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INTRODUCTION

Its functions are

1. Logical addressing:


if a packet transmitting across network, ano
ther addressing scheme

is needed.


Logical addressing, which is unique & universal in the internet.


The n/w layer adds header, it consist of source & destination logical

address.

2. Routing:


When independent n/w’s are connected to create internetworks(

networks of networks).


Its connecting devices are router or switches.


One of the major functionality of the n/w layer is routing.


Routing defines the optimal path from multiple paths.

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INTRODUCTION

Transport Layer


It is responsibl
e for process
-
process delivery of the message.


It also ensures that whole message arrives and in order.


Its functionality are


Service point addressing:


Computer often run several programs at the same time.


For this reason source
-
destination means
delivery not only from

one host to another host but also from process on sending host to

process on receiving host.


Transport layer header includes the service
-
point address or process

address or port number.


Segmentation & Reassembly:


Divide the
message into transmittable units is called segments and

each segment contains sequence number.

JPR COMPUTER NETWORKS

INTRODUCTION


Reconstruct the message from the segments at receiver side is

reassembly.


Connection Control:
transport layer can be either

connection

oriented or connection less.


Flow Control:
it performs end to end flow control rather than across

link.


Error Control:
it performs end


end error control rather than across

link.

Session Layer:


It is the n/w dialog controller.


Its
functions are

1. Dialog Control:
it allows the communication b/w two process in

either simplex, half duplex or duplex.

2. Synchronization:
it allows a process to add
check points or sync

points.

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INTRODUCTION

Presentation Layer


It is

concerned with the syntax and semantics of the information.


Its functions are

1. Translation

2. Encryption & Decryption

3. Compression & Expansion.

Application Layer


It enables the user, whether human or s/w to access the network.

or


It is a tool to

access the network s/w. (or)


It acts as interface between the n/w user & n/w software.


It is responsible for providing services to the user.

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INTRODUCTION

Its functions are

1. n/w virtual terminal : it is a s/w version of physi
cal terminal and it

allows a user to log on to a remote host.

2. File transfer.

3. Mail services( Microsoft outlook, Lotus Notes etc).

TCP/IPModel


this model was proposed by DoD of USA in 1974.


This is also called DoD Model.


This layered structure
later became known as TCP/IP reference

model, because of its two primary protocols.


One is transmission control protocol(TCP) and other is Internet

protocol (IP).


DoD initially designed a layer structure to proved efficient

communication between their
forces.

JPR COMPUTER NETWORKS

INTRODUCTION


Due to its efficiency, TCP/IP ReferenceModel became popular.


TCP/IP Reference Model exits with only 4 layer instead of 7 layers.


Layer in the TCP/IP ReferenceModel are

JPR COMPUTER NETWORKS

Application Layer

Presentation Layer

Session Layer

Transport Layer

Network Layer

Data Link Layer

Physical Layer

Process/Application

Layer

Transport Layer

Network Layer

Host
-
Network

Layer

INTRODUCTION

Comparison of OSI & TCP/IP


Application layer, Session Layer and Present
ation Layer combined

together is called Application or process
-
application layer.


Transport layer is similar in TCP/IP


Network layer is called Internet Layer.


Data Link layer & Physical layer together is called host
-
host layer.


OSI model makes the
clear distinction between services, interfaces &

protocols where as TCP/IP did not clearly distinguish between these.


Protocols in the OSI model developed based on the specifications

where as in TCP/IP, protocols came first and the model was just a

description of the existing protocols.

JPR COMPUTER NETWORKS

INTRODUCTION


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A Critique(Review) of the OSI Model & Protocols

Bad Timing: the time at which a standard is established is absolutely

critical to its success.

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15

INTRODUCTION

Bad Technology:


OSI model exists with seven layers


Session & Presentation layers are nearly empty, where as two

other layers( data link & network) are overfull.

Bad Implementation


Protocols implemented using this model are complex and slow.


In
other side, first implementation of TCP/IP was part of Berkeley

UNIX and was quite good.

Bad Politics


TCP/IP was in the part of UNIX, and in 1980s in academic point of

view UNIX is not quite good.


Other hand European & USA telecommunication ministries a
re not

interested in OSI implementations.

JPR COMPUTER NETWORKS

INTRODUCTION

A Critique of the DOD ReferenceModel


It is also having its own problems.


It does not clearly distinguish the concepts of service, interface &

protocols.


DOD model is not
good guide for designing new networks using new

technologies.


Host


Network layer is not really a layer at all, it just an interface.


it does not distinguish physical & data link layers.

Hybrid Model/Internet Model


In spite of its problems, OSI mode
l has proven to be exceptionally

useful for discussing computer networks.(except presentation &

session layer.)


OSI protocols have not became popular, TCP/IP protocols was so

popular.

JPR COMPUTER NETWORKS

INTRODUCTION


Computer scientists developed a
new model based on the OSI Model &

TCP/IP protocols and is called hybrid model or Internet model.


It exists with five layers.


Rest of the book, discussion continued on Internet Model.


It exists with all the layers of OSI model, except presentation &
session

layers


Functionality of each layer is similar to OSI Model, except Application

layer.


Application layer performs its own functionality and presentation &

session layers functionality.


Lay er in the Internet model are

JPR COMPUTER NETWORKS

Appl
ication Layer

Transport Layer

Network Layer

Data link Layer

Physical Layer

PHYSICAL LAYER


Physical layer is the bottom most layer in all the reference models.


Physical layer directly controls the transmission medium.


A transmission medium can be
anything that can carry information

from source to destination.


Data must be converted into electromagnetic signals to be transmitted

from device to device.


Signals can travel through a vacuum, air, or other media


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
May be in the form of power, voice,
radio waves, infrared light, and X,

gamma, and cosmic rays

JPR COMPUTER NETWORKS

16

PHYSICAL LAYER


Theoretical basics for data communication


Data must be transformed to electromagnetic signals to be transmitted.


Data at the source can be either
digital or analog.


Analog data : human voice, chirping of birds etc , converted to


Analog or digital signals


Digital : data stored in computer memory, converted to


Analog or digital signals

Examples


Analog data as analog signal : Human voice from o
ur houses to the

telephone exchange.

JPR COMPUTER NETWORKS

PHYSICAL LAYER


Analog data as digital signal : most of the systems today : Say Human

voice, images sent on digital lines .. New telephone system (digital

exchanges)


Digital data as analog
signal : computer data sent over internet using

analog line .. Say telephone line ( say our house to the exchange)


Digital data as digital signal : say from one digital exchange to

another.


Analog signal has infinitely many levels of intensity (infinit
ely many

values, continuous values) over a period of time.


Digital signal has only a limited number of defined values(discrete

values) say, 0,1.

JPR COMPUTER NETWORKS

PHYSICAL LAYER

Comparison of analog and digital signals


If a signal does not change
at all, its frequency is zero.


If it changes instantaneously, its frequency is infinite.


An analog signal is best represented in the frequency domain.

JPR COMPUTER NETWORKS

PHYSICAL LAYER


Single
-
frequency sine wave is not useful for data communicatio
n


A single sine wave can carry electric energy from one place to another.

For eg., the power company sends a single sine wave with a frequency

of say 60Hz to distribute electric energy to our houses.


If a single sine wave was used to convey conversatio
n over the phone,

we would always hear just a buzz.


If we sent one sine wave to transfer data, we would always be sending

alternating 0’s and 1’s, which does not have any communication value.

Composite Signals


If we want to use sine wave for communicat
ion, we need to change

one or more of its characteristics. For eg., to send 1 bit, we send a

maximum amplitude, and to send 0, the minimum amplitude.

JPR COMPUTER NETWORKS

17

PHYSICAL LAYER


When we change one or more characteristics of a single
-
frequency

signal, it becomes a composite signal made up of many frequencies.


For Ex: Composite signal with Three harmonics

JPR COMPUTER NETWORKS

PHYSICAL LAYER

Adding first three harmonics

Fourier Analysis


In early 1900s, French Mathematician Jean
-
Baptiste

Fourier showed

that any composite signal can be represented as a combination of

simple sine waves with different frequencies, phases and amplitudes.


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
More is the number of components included better is the

approximation.


For eg., let us consider the sq
uare wave …

JPR COMPUTER NETWORKS

PHYSICAL LAYER

Time on x
-
axis in msec, Voltage on y
-
axis


The first trace in the above figure is the sum of 2 sine waves with

amplitudes chosen to approximate a 3 Hz square wave (time base is

msec). One sine wave has a fr
equency of 3 Hz and the other has a

frequency of 9 Hz.


The second trace starts with the first but adds a 15 Hz sine wave and a

21 Hz sine wave. It is clearly a better approximation.

JPR COMPUTER NETWORKS

PHYSICAL LAYER

Composite Signal and
TransmissionMedium


A signal needs to pass thru a transmission medium. A transmission

medium may pass some frequencies, may block few and weaken

others.


This means when a composite signal, containing many frequencies, is

passed thru a transmission mediu
m, we may not receive the same

signal at the other end.

JPR COMPUTER NETWORKS

18

PHYSICAL LAYER

Bandwidth of a channel


The range of frequencies that a medium can pass without loosing onehalf

of the power contained in that signal is called its bandwidth.

JPR COMPUTER NETWORKS

PHYSICAL LAYER

Representing data as Digital Signals


1 can be encoded as a positive voltage say 5 volts, 0 as zero voltage

(or negative voltage say

5 volts)


Most digital signals are aperiodic. Thus we use


Bit interval (instead
of period) : time required to send one bit = 1/ bit

rate.


Bit rate (instead of frequency) :number of bits per second.

JPR COMPUTER NETWORKS

PHYSICAL LAYER

Digital signal as Composite Signal


Digital signal is nothing but a composite analog signal with a
n infinite

bandwidth.


A digital signal theoretically needs a bandwidth between 0 and infinity.

The lower limit 0 is fixed. The upper limit may be compromised.

Relationship b/w bit rate and reqd. channel b/w.


Imagine that our computer creates 6bps


In
1 second, the data created may be 111111, no change in the value,

best case


In another, 101010, maximum change in the values, worst case


In another, 001010, change in between the above two cases


We have already shown .. More the changes higher are th
e frequency

components.

JPR COMPUTER NETWORKS

PHYSICAL LAYER

Digital versus analog

JPR COMPUTER NETWORKS

19

PHYSICAL LAYER

Using single harmonic


just to get the awareness


The signal 111111 (or 00000 ) can be simulated by sending a singlefrequency

signal with frequency 0.


The signal 101010 (010101) can be simulated by sending a singlefrequency


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signal with frequency 3 Hz. (3 signals or sine waves per

second)


All other cases are between the best and the worst cases. We can

simulate other cases wit
h a single frequency of 1 0r 2 Hz (using

appropriate phase).


I.e. to simulate the digital signal at data rate 6bps, sometimes we need

to send a signal of frequency 0, sometimes 1,sometimes 2 and

sometimes 3. We need that our medium should be able to pass

frequencies of 0
-
3 Hz.

JPR COMPUTER NETWORKS

PHYSICAL LAYER

Generalizing the example above


Bit rate = n bps


Best case
----

frequency 0 Hz,Worst case
-----

frequency n/2 Hz


Hence B (bandwidth) = n/2

Using more harmonics


However, as said earlier,
one harmonic does not approximate the

digital signal nicely and more harmonics are required to approximate

the digital signal.


As shown earlier, such a signal consists of odd harmonics


When we add 3
rd
harmonic to the worst case, we need B = n/2 + 3n/2

= 4n/2


When we add 5
th
harmonic to the worst case, we need B = n/2 + 3n/2

+ 5n/2= 9n/2 and so on.


In other words, B >= n/2 or n <= 2B

JPR COMPUTER NETWORKS

PHYSICAL LAYER


Hence we conclude that bit rate and the bandwidth of a channel are

proportional

to each other.


Low
-
pass channel : has a bandwidth with frequencies between 0 and f (f

could be anything including infinity).


Band
-
pass channel : has a bandwidth with frequencies between f1 (>=0)

and f2


A band
-
pass channel is more easily available th
an a low
-
pass channel.


Data rate depends on 3 factors:


The bandwidth available


Number of levels of signals


Quality of the channel (noise level)

JPR COMPUTER NETWORKS

PHYSICAL LAYER

Noiseless Channel: Nyquist Bit rate


b = 2 B log L (log is to base

2)

b : bit rate

B : Bandwidth

L : number of levels

Noisy channel : Shannon Capacity


C = B log (1 + SNR)

C = capacity of the channel in bps

B = Bandwidth

SNR = signal to noise ratio

JPR COMPUTER NETWORKS

20

PHYSICAL LAYER


We can calculate the theoretic
al highest bit rate of a regular telephone

line. A telephone line normally has a bandwidth of 3000 Hz (300 Hz to

3300 Hz). The signal
-
to
-
noise ratio is usually 3162. For this channel

the capacity is calculated as


C = B log
2
(1 + SNR) = 3000 log
2
(1 +
3162) = 3000 log
2
(3163)


C = 3000
11.62 = 34,860 bps


We have a channel with a 1 MHz bandwidth. The SNR for this channel

is 63; what is the appropriate bit rate and signal level?


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First, we use the Shannon formula to find our upper limit.

C = B log
2
(1

+ SNR) = 10
6
log
2
(1 + 63) = 10
6
log
2
(64) = 6 Mbps

Then we use the Nyquist formula to find the number of signal levels.

6 Mbps = 2
1 MHz
log
2
L
L = 4

JPR COMPUTER NETWORKS

PHYSICAL LAYER

A binary signal and its root
-
mean
-
square amplitudes

JPR
COMPUTER NETWORKS

PHYSICAL LAYER

Categories of Media


Transmission media divided into following sub
-
categories.


Note: Each of these forms a portion of the electromagnetic spectrum

JPR COMPUTER NETWORKS

Coaxi al

Cabl e

Fi ber
-
Optic

Cabl e Free Space

Tr ansmi s
sion

medi a

Gui ded

(Wi r e)

Un
-
Gui ded

(Wi r eless)

Twi sted
-
pair

cabl e

PHYSICAL LAYER

Following diagram shows the range of the spectrum.


Guided Media


media with a physical boundary


Twisted pair, coaxial, and fiber
-
optic


Unguided Media


no physical
boundaries


Radio waves, infrared light, visible light, and X, gamma, and

cosmic rays


Sent by microwave, satellite, and cellular transmission

JPR COMPUTER NETWORKS

21

PHYSICAL LAYER

Guided Media


Signal is directed and contained by physical limits of
medium


Twisted
-
pair and coaxial use copper conductors to accept and transport

signals in form of electrical current


Optical fiber is glass or plastic cable that accepts and transports signals

in form of light.

Twisted
-
Pair Cable


Two conductors surrou
nded by insulating material.


One wire used to carry signals, other used as a ground reference.


Twisting wires reduces the effect of noise interference or crosstalk

since both wires will likely be equally affected

JPR COMPUTER NETWORKS

PHYSICAL LAYER


More twists for better quality.


Twisted pairs are of two types


UTP


(un shielded twisted pair)


STP
-

(shielded twisted pair)

Unshielded Twisted Pair (UTP)


It is the most commonly used transmission medium.


It is suitable for both voice and data
transmission.


Frequency range is from 100 Hz to 5 MHz


In this cable there are various categories based on quality.

JPR COMPUTER NETWORKS

PHYSICAL LAYER



Cat 3 commonly used for telephone systems (up to 10 Mbps)



Cat 5 usually used for data networks
(up to 100 Mbps)


Performance is measured by attenuation versus frequency and

distance.


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UTP Cable

Its adv are


Cost is low.


flexible, easy to install.

JPR COMPUTER NETWORKS

PHYSICAL LAYER

Its disadv


Signal is attenuated even for small noises.


It
supports less data rate.

UTP connectors


These are used to attach wires.


most commonly used is RJ45.


It is equipped with 8 conductors.

Shielded Twisted Pair (STP)


A metal foil or braided
-
mesh covering encases each pair of insulated

conductors

to prevent electromagnetic noise called crosstalk.


Crosstalk occurs when one line picks up some of the signals traveling

over another line.

JPR COMPUTER NETWORKS

22

PHYSICAL LAYER


More expensive but less susceptible to noise.


Use same connecters to
join the cables.

STP example

Adv


It supports more data rate than UTP.


Signal can transmit longer distance with out amplification.


Signal can be less attenuated.

DisAdv: Its cost is high


It supports less data rate than other cables(CC & FOC).

JPR
COMPUTER NETWORKS

PHYSICAL LAYER

Coaxial Cable


It equipped with central core conductor enclosed in an insulating

sheath, encased in an outer conductor of metal foil


Frequency range is between 100 KHz and 500 MHz


RG numbers denote physical specs such
as wire gauge, thickness and

type of insulator, construction of shield and size/type of outer casing


RG
-
8, RG
-
9, and RG
-
11 used in thick Ethernet, RG
-
58 used in thin

Ethernet, RG
-
59 used for TV.

JPR COMPUTER NETWORKS

PHYSICAL LAYER

Coaxial Cable
Connectors


Most common is barrel connector (BNC)


T
-
connectors are used to branch off to secondary cables


Terminators are required for bus topologies to prevent echoing of

signals

JPR COMPUTER NETWORKS

PHYSICAL LAYER

Coaxial Performance


Higher bandw
idth than twisted
-
pair


However, attenuation is higher and requires frequent use of repeaters

Coaxial Applications


Analog and digital phone networks


Cable TV networks


Traditional Ethernet LANs

JPR COMPUTER NETWORKS

23

PHYSICAL LAYER

Fiber
-
Optic
Cable


It is made of glass or plastic.


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
Signals are transmitted as light pulses from LED or laser


Light is also a form of electromagnetic energy


Speed depends on density of medium it is traveling through, fastest

when

in a vacuum, 186,000 miles/second.


When the data transmit in the form of light, it uses properties of light.


Basic properties of light are Refraction and Reflection


Refraction
often occurs when light bends as it passes from higher/less

dense medium
or less/high dense medium


Reflection
occurs and the light no longer passes into the less dense

medium

JPR COMPUTER NETWORKS

PHYSICAL LAYER

Refraction

Critical Angle

JPR COMPUTER NETWORKS

This image cannot curr ently be displayed.

PHYSICAL LAYER

Reflection


Optical fibers use reflection to guide light through a channel


Information is encoded onto a beam of light as a series of on
-
off pulses

representing 1s and 0s

JPR COMPUTER NETWORKS

PHYSICAL LAYER

Propagation Modes


Method for transmitting optical
signals


There are two methods multimode & single mode.


Multimode are of two types step index & graded index.

Single Mode


Only one beam from a light source is transmitted using a smaller range

of angles


Smaller diameter and lower density


Makes pro
pagation of beams almost horizontal, delays are negligible


All beams arrive together and very difficult recombined the signals.

Multimode


Multiple beams from light source move through core at different paths

JPR COMPUTER NETWORKS

24

PHYSICAL LAYER


Multimode step
-
index fiber


Density remains constant from center to edges


Light moves in a straight line until it reaches the cladding


Some beams penetrate the cladding and are lost, while others are

reflected down the channel to the destination.


Due t
o sudden change at the cladding, possibility of change the

signal/beam.


Multimode Graded
-
index


The word index here refers to the index of refraction.


The index of refraction is related to density.


Graded
-
index refers to varying densities of the
fiber, highest at center

and decreases at edge.

JPR COMPUTER NETWORKS

PHYSICAL LAYER


Since the core density decreases with distance from the center, the

light beams refract into a curve.


Eliminate problems with some of the signals penetrating at the

cl
adding and lost.

JPR COMPUTER NETWORKS

PHYSICAL LAYER

Fiber Construction

JPR COMPUTER NETWORKS

PHYSICAL LAYER

Applications of Fiber Optics


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
Backbone networks due to wide bandwidth and cost effectiveness


Up to 1600 Gbps


Cable TV


LANS like 100Base
-
FX
(Fast Ethernet) & 1000Base
-
X

Advantages of Fiber Optics


Higher bandwidth than twisted
-
pair and coaxial cable


Noise resistance & Less signal attenuation


Lightweight & Greater security

Disadvantages of Fiber Optics


Installation/maintenance


Unidirec
tional


Cost

JPR COMPUTER NETWORKS

25

PHYSICAL LAYER

UnguidedMedia: Wireless


Wireless communication, transporting electromagnetic waves without

a physical conductor.


In this communication signals are normally broadcast through free

space

and thus are available to anyone who has a device capable of

receiving them.


The following diagram shows spectrum for wireless communication.

JPR COMPUTER NETWORKS

PHYSICAL LAYER

PropagationMethods


Unguided signals can travel from source to destinatio
n in several

ways.

1. Ground Propagation: radio waves travel through lowest portion of

atmosphere, hugging the Earth.


The lowest frequency signals emanate from the antenna and follows

curvature of the earth.

2. Sky Propagation: higher
-
frequency radio wav
es radiate upward into

ionosphere and then reflect back to Earth.

3. Line
-
of
-
sight Propagation: high
-
frequency signals transmitted in

straight lines directly from antenna to antenna

JPR COMPUTER NETWORKS

PHYSICAL LAYER

The following diagram shows the propa
gation modes

JPR COMPUTER NETWORKS

PHYSICAL LAYER

The electromagnetic spectrum is divided into 8 bands and are regulated

by government authority.

JPR COMPUTER NETWORKS

Band Range Propagation Application

VLF 3

30 KHz Ground Long
-
range radio navigation

LF
30

300 KHz Ground

Radio beacons and

navigational locators

MF 300 KHz

3 MHz Sky AM radio

HF 3

30 MHz Sky

Citizens band (CB),

ship/aircraft communication

VHF 30

300 MHz

Sky and

line
-
of
-
sight

VHF TV,

FM radio

UHF 300 MHz

3 GHz Line
-
of
-
sight

UHF TV, cellular p
hones,

paging, satellite

SHF 3

30 GHz Line
-
of
-
sight Satellite communication

EHF 30

300 GHz Line
-
of
-
sight Long
-
range radio navigation

26

PHYSICAL LAYER

Wireless Communication is broadly divided into three groups

1. RadioWaves

2. MicroWaves


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3. InfraredWaves

Radio waves


Its frequency range is 3 KHz to 1 GHz.


Radio waves are Omni directional .


In this communication sending & receiving antenna need not face

with each other.


It major disadvantage is Susceptible to interference by other antennas

using same

frequency or band.


These are suitable for long
-
distance broadcasting and also penetrate

walls.

JPR COMPUTER NETWORKS

PHYSICAL LAYER

Its Applications are


AM and FM radio and TV etc.

MicroWaves


Its frequency range in between 1 and 300 GHz


Its
signals are Unidirectional.


Micro wave propagation is line
-
of
-
sight.


Very high frequency waves can’t penetrate walls.


It uses unidirectional antennas and there are two types of antennas 1.

Parabolic Dish 2. Horn.

JPR COMPUTER NETWORKS

PHYSICAL LAYER

The following diagrams shows the two antennas


Its Applications are

Unicasting


one
-
to
-
one communication between sender and receiver



Cellular phones



Satellite networks



Wireless LANs

JPR COMPUTER NETWORKS

PHYSICAL LAYER

Infrared


Frequencies
between 300 GHz and 400 THz


Short
-
range communication


High frequencies cannot penetrate walls


Requires line
-
of
-
sight propagation


Adv: prevents interference between systems in adjacent rooms


Disadv: cannot use for long
-
range communication or outsi
de a

building due to sun’s rays

Infrared Applications


Wide bandwidth available for data transmission


Communication between keyboards, mice, PCs, and printers

JPR COMPUTER NETWORKS

27

PHYSICAL LAYER

Satellite Communication


A satellite network is a com
bination of nodes , which are satellites,

earth station and end
-
user terminal.


A satellite the provides communication from one point on the earth to

another.


It can provide transmission capability to and from any location on

earth, no matter how remote
.


The natural satellite moon can be used as a relaying node in the

network, the use of artificial satellite is preferred.


It allows install electric equipment / transponders, it listen signal,

amplify the incoming signal and rebroadcast it at another f
requency to

avoid interference with incoming signal.

JPR COMPUTER NETWORKS

PHYSICAL LAYER


Another restriction on using natural satellite is their distance from the

earth, which creates long delay in the communication.


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
An artificial satellite needs to
have an orbit, the path in which travel an

earth.


The orbit can be equatorial, inclined or polar.


Important points to artificial satellites

1. A satellite period

2. Position of the satellite (where to place)

3. VanAllen Belts.


Van Allen Belts, layers

of highly charged particles, any satellite flying

within them destroyed very quickly.

JPR COMPUTER NETWORKS

PHYSICAL LAYER


According to Kepler’s law, the orbital period of the satellite varies as

the radius of the orbit to the 3/2 power.


Period = C X
distance
3/2


Based on the location of the orbit, satellites can be divided into three

categories

1. Geostationary Earth Orbit (GEO)

2. Low Earth Orbit

3. Middle Earth Orbit (MEO)

Frequency Bands for Satellite Communication


The frequencies are reserved
for satellite microwave communication

are in the GHz.


Each satellite send and receive over two bands


uplink/downlink

JPR COMPUTER NETWORKS

PHYSICAL LAYER

GEO Satellite


It uses line of sight propagation that the sending and receiving

antennas

be locked onto each other’s location all the time.


A satellite that moves slow or faster than the earth rotation,

communication is only short period.


To ensure constant communication, the satellite must move at the same

speed as the earth.


So that
it seems to remain fixed above certain spot, such satellites are

called
geostationary
.


This orbit occurs at the equatorial plane and is approx 22,000 mi from

the surface of the earth.


But one GEO satellite cannot cover the whole earth.

JPR COMPUTER NET
WORKS

28

PHYSICAL LAYER


It takes minimum of 3 satellites equi
-
distance from each other to prove

full global transmission.

Applications


INSAT
or the
Indian National Satellite System
is a series of

multipurpose Geo
-
stationary satellites launched by ISRO
to satisfy the

telecommunication broadcasting etc.


The
Geostationary Satellite
System
(
GOES
), operated by the United

States National Environmental Satellite, Data, and Information

Service (NESDIS), supports weather forecasting, severe storm

tracking etc.

JPR COMPUTER NETWORKS

PHYSICAL LAYER

MEO Satellites


These are positioned in between the two VanAllen Belts.


A satellite at the orbit takes approximately 6
-
8 hrs to circle the earth.

Applications


An Application/Example of MEO Satellite is the Global
Positioning

System (GPS).


Constructed & Operated by the US department of defense , orbiting at

an altitude about 18,000 km above the earth.


It consists of 24 satellites and is used for land, sea and air navigation to


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provide time & locations for vehicl
es & ships.


GPS is based on a principle called
trilateration.

JPR COMPUTER NETWORKS

PHYSICAL LAYER

LEO Satellites


Its having polar orbits.


The altitude is between 500 & 2000 km, with rotation period of 90 to

120 min.


The satellite has a speed of 20
,000 to 25,000 km/h.


The round
-
trip propagation delay is normally less than 20ms, which is

acceptable for audio communication.


LEO System is made of collection of satellites that works together as a

network, each acts a switch.

Iridium System


The
iridium system has 66 satellites in six LEO orbits, each at an

altitude 750 km.

JPR COMPUTER NETWORKS

PHYSICAL LAYER


It provide direct worldwide communication using handled terminals.


This system can provide connectivity between users at different

locations where other types of communication is not possible.


It supports voice, data, paging etc.

Globalstar


The system uses 48 satellites in the six polar orbits with each orbit

hosting 8 satellites.


The orbits are located at an altitude of above 1
400km.


More than 3,15,000 subscribers, Globalstar is the world’s largest

provider of mobile satellite voice and data services.


Globalstar offers these services to commercial and recreational users

in more than 120 countries around the world.

JPR COMPUT
ER NETWORKS

29

PHYSICAL LAYER

Teledesic


It is a system of satellites that provides fiber
-
optic like communication.


Its main is to provide broadband internet access for users all over the

world.


It is sometimes called
“internet in the sky”


It is hav
ing 288 satellites in 12 LEO orbits each at an altitude of 1350

km.


The project was started in 1990 by craig & Bill Gates.


It provides end
-
end internet communication.

JPR COMPUTER NETWORKS

PHYSICAL LAYER

Communication Satellites


Communication
satellites and some of their properties, including

altitude above the earth, round
-
trip delay time and number of satellites

needed for global coverage.

JPR COMPUTER NETWORKS

PHYSICAL LAYER

The public switched telephone network


When two computers owned by

the same company and located each

other, simply connected by LAN.


When distance is large, cable pass through a road, running of private

cables through public road is prohibited.


Consequently, the network designers are depend on the existing

telecommun
ication facilities.


Alexander Graham Bell, designed many years ago, transmitting human

voice from one to another.

Structure of the telephone system


If telephone owner wanted to talk to n other telephone owners,


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separate wire looped to all
n
houses.

JPR

COMPUTER NETWORKS

PHYSICAL LAYER


Bell formed, Bell Telephony company, which opened first switching

office in 1878.


The company ran a wire to each customer’s house or office.


To make call,


Customer first communicate to the switching office.


The o
perator manually connect the caller to the calle.


To make long distance calls between cities, bell system began connect

switching offices.


The wire between switching office to every other switching office is

quickly unmanageable.


So, Second level swi
tching offices are invented, this hierarchy

gradually grew up to five levels.

JPR COMPUTER NETWORKS

30

PHYSICAL LAYER

Structure of the Telephone System

JPR COMPUTER NETWORKS

(a)
Fully
-
interconnected network.

(b)
Centralized switch.

(c)
Two
-
level hierarchy.

PHYSICAL LAYER


The modern PSTN made with three components


Local loop


Trunks


Switching offices

Local Loop:
it is physical link/circuit established in between customer’s

office or house to nearest telephone network end
-
office.


Twisted pair & coaxi
al cables are popular local loops.

Trunk:
A trunk is a line or link designed to handle many signals

simultaneously, and that connects major switching centers or nodes in a

communications system.


The transmitted data can be voice, text, images, video etc.

JPR COMPUTER NETWORKS

PHYSICAL LAYER

Trunks are usually coaxial cable, fiber optic cable etc.

Switching office


where calls moved from one trunk to another


PSTN having several levels of switching offices

1. End offices

2. Tandem offices

3. Regional
offices

End
-
Office


A local office (or end office) was a switching center that connected

directly to the customers’ telephone instruments.


This distance typically 1 to 10km.

JPR COMPUTER NETWORKS

PHYSICAL LAYER

Tandam Offices/ Toll offices


A telephone

switching center (central office) that does not connect

directly to the customer.


It connects offices in the same network or between networks, but

always deals with trunks rather than customer lines.

Regional Offices / Intertoll offices


If the caller
and callee do not have a toll office in common, the path

will have to be established somewhere higher up in the hierarchy.


Primary, sectional, and regional offices form a network by which toll

offices are connected through regional offices.

JPR COMPUTER
NETWORKS

31


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PHYSICAL LAYER

The Local Loop


Most of the people are familiar with: the two wire local loop coming

from a telephone company end
-
office into houses & small businesses.


The local is also referred to as the
“last mile”
.


When home computer
whishes to communicate over existing PSTN/

analog dial
-
up line, the digital data must first be converted to analog

signal from for transmission over the local loop.


This conversion is done by a device called a
modem
.


The term modem is a composite word,

that refers two function, a

signal modulator and a signal demodulator.


Amodulator creates analog signal from binary data


at the sender site.


A demodulator recollect binary data from analog signal


at the

receiver site.

JPR COMPUTER NETWORKS

PHYSICA
L LAYER


Following diagram shows structure of modern PSTN.

JPR COMPUTER NETWORKS

PHYSICAL LAYER


Analog signaling consists of infinite values wr to information stream.


If transmission medium is perfect, every thing goes correct.


But, every transmissi
on medium suffer from three major problems

1. Attenuation

2. Delay Distortion

3. Noise


Due to the above problems, the square wave used in the digital signal

have a wide frequency and also strong attenuation & delay distortion.


Its amplitude, frequency
or phase can be modulated to transmit

information

JPR COMPUTER NETWORKS

PHYSICAL LAYER


Amplitude is changed

amplitude modulation (AM)


Frequency is changed

frequency shift keying (FSK)


Phase is changed

phase shift keying (PSK)


Quadrature Phase

Shift Keying (QPSK)


Quadrature Amplitude Modulation(QAM) 4QAM, 16QAM, 32QAM

etc.


To reduce the chance of an error, high speed modems do error

correction by adding extra bits to each sample, this scheme is know as

TCM (Trellis Coded Modulation).


For
EX. V.32 modem uses 32 constellation points to transmit 4 bits &

one parity at data rate of 96000bps.


V.32bis

140000 bps, this speed is achieved by transmitting 6 data

bits & one parity bit.

JPR COMPUTER NETWORKS

32

PHYSICAL LAYER

Digital Subscriber Li
nes (DSL)


It support high
-
speed digital communication over the existing local

loops.


DSL is a set of technologies, each of differ in the first letter


ADSL, VDSL, HDSL, SDSL, the set simply referred as xDSL.

ADSL


Short for
a
symmetric/
a
synchronous
d
igital
s
ubscriber
l
ine
, ADSL is a

type of DSL broadband communications technology used for

connecting to the Internet.


ADSL allows more data to be sent over existing copper telephone lines

(PSTN), when compared to traditional modem lines.


A special fil
ter, called a
microfilter
, is installed on a subscriber's

telephone line to allow both ADSL and regular voice (telephone)


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services to be used at the same time.

JPR COMPUTER NETWORKS

PHYSICAL LAYER


ADSL requires a special ADSL modem and subscribers must b
e in

close geographical locations to the provider's central office to receive

ADSL service.


Typically this distance is within a radius of 2 to 2.5 miles.


ADSL supports data rates up to 9 Mbps downstream and 8 Mbps

upstream.

JPR COMPUTER NETWORKS

PHYSICAL LAYER

A typical ADSL arrangement as shown in the following figure.

JPR COMPUTER NETWORKS

PHYSICAL LAYER


In this architecture, a telephone company technician must install a NID

(network interface device) on the customer’s premises.


This is a
small plastic box, end of the telephone company property and

start of the customer’s property.


In the customers property, the first one and closer to NID is
splitter
.


It separates/splits voice band (0
-
4000 Hz) from data, the voice band

routed to the te
lephone/fax and data signal routed to the ADSL

modem.


ADSL modem is a digital signal processor, the computer must be

connected to it at high speed.


This done by putting Ethernet (NIC) card in the computer.


On the other end, corresponding splitter is
installed, voice portion of

the signal is filtered out and send to the normal voice switch.

JPR COMPUTER NETWORKS

33

PHYSICAL LAYER

Wireless Local Loop (WLL)/ LMDS (Local multipoint Distribution

Service)


WLL (Wireless in Local Loop) is a communication sy
stem that connects

subscribers to the public switched telephone network (PSTN) using

radio frequency signals as a substitute for conventional wires for all or

part of the connection between the subscriber and the telephone

exchange.

OR


WLL is a system
that connects subscribers to the local telephone station

wirelessly.


Subscribers to a WLL system are linked via radio to a network of radio

base stations, which, in turn, are tied, by a backhaul network to the

PSTN.

JPR COMPUTER NETWORKS

PHYSICAL LAYER



Each base station supports a cell or several sectors of coverage,

servicing within coverage area.


Extent of coverage area determined by transmit power and frequency

of service.

JPR COMPUTER NETWORKS

PHYSICAL LAYER

Air Interface(WLL) Technologies :


DECT


Digitally Enhanced Cordless Technology


PHS


Personal Handy phone System


CDMA


Code Division Multiple Access

CDMA


Direct Sequence Spread Spectrum(DS
-
SS) technique


Each user convey baseband information at 9.6kbps


RF Bandwidth 1.25MHz (Appr
ox.)

JPR COMPUTER NETWORKS

PHYSICAL LAYER

Technical requirements of WLL systems:


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
Communication Quality


Low cost


Absence of interference with other wireless systems

Benefits of WLL:


Rapid deployment capability.


Acceptable investment per line.


Low Initial Capital and Maintenance Cost.


Flexibility and Robust installation characteristics.


Ideal for teleworking or Remote Access to Corporate LANs.

JPR COMPUTER NETWORKS

34

PHYSICAL LAYER

Connection Setup

JPR COMPUTER NETWORKS

PSTN Switch

function

WLL

Controller

AM

HLR

Transceiver
WASU

Air

Interface

U
WLL

T
WLL

Wireless Access Network Unit(WANU)



Interface between underlying telephone network and wireless link



consists of

• Base Station Transceivers (BTS)

• Radio Controller(RPCU)

• Access Manager(
AM)

• Home Location Register(HLR)

WANU

Wireless Access Subscriber Unit(WASU)



located at the subscriber



translates wireless link into a

traditional telephone connection

PHYSICAL LAYER

Factors should be considered while designing the transmission mediom


Factors considering the design of data transmission systems, a key

concern, generally, is data rate and distance: the greater the data rate

and distance, the better.


A number of design factors relating to the transmission medium and to

the signal deter
mine the data rate and distance:


1. Bandwidth:
All other factors remaining constant, the greater the

bandwidth of a signal, the higher the data rate that can be achieved.


2. Transmission impairments:
Impairments, such as attenuation,

limit the distance
.


For guided media, twisted pair generally suffers more impairment than

coaxial cable, which in turn suffers more than optical fiber.

JPR COMPUTER NETWORKS

PHYSICAL LAYER


3. Interference:
Interference from competing signals in overlapping

frequency ban
ds 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 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.


4. 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 distance and/or data rate.

JPR COMPUTER NETWORKS

PHYSICAL LAYER


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Trunks & Multiplexing


A

trunk
is a transmission medium between two points, each point

being either the switching center or the node.


A
trunk
is a line or link designed to handle many signals

simultaneously, and that connects major switching centers or nodes in

a communications

system.


The transmitted data can be voice (as in the conventional telephone

system) data, computer programs, images, video or control signals.


Trunking
also refers to the connection of switches and circuits within

a telephone exchange.


Cost is very
high to replace low bandwidth trunks with high bandwidth

trunks between switching offices.

JPR COMPUTER NETWORKS

35

PHYSICAL LAYER


Telephone companies have developed many technologies/schemes for

multiplexing many conversations over a single physical lin
k/trunk.


Multiplexing
is the set of technologies that allows the simultaneous

transmission of multiple signals across a single link/trunk.


Under the simplest conditions, a medium can carry only one signal at

any moment in time.


For multiple signals
to share one medium, the medium must somehow

be divided, giving each signal a portion of the total bandwidth.


Multiplexing schemes can be divided into two basic categories

1. Frequency Division Multiplexing(FDM)

2. Time Division Multiplexing(TDM)

JPR COM
PUTER NETWORKS

PHYSICAL LAYER

Frequency Division Multiplexing(FDM)


Frequency
-
division multiplexing (FDM) is a scheme in which

numerous signals are combined for transmission on a single

communications line or channel. Each signal is assigned a different

f
requency (subchannel) within the main channel.


Multiplexing performed at the source , Demultiplexing performed at

destination.


At source, N inputs onto 1 link, at destination 1 link into N outputs.

JPR COMPUTER NETWORKS

PHYSICAL LAYER


Assignment of
non
-
overlapping frequency ranges to each “user” or

signal on a medium. Thus, all signals are transmitted at the same time,

each using different frequencies.


Amultiplexor accepts inputs and assigns frequencies to each device.


The multiplexor is attached

to a high
-
speed communications line.


A corresponding multiplexor, or demultiplexor, is on the end of the

high
-
speed line and separates the multiplexed signals.

JPR COMPUTER NETWORKS

PHYSICAL LAYER


Analog signaling is used to transmits the signals.


Broadcast radio, television, and the AMPS cellular phone systems use

frequency division multiplexing.


This technique is the oldest multiplexing technique.


Since it involves analog signaling, it is more susceptible to noise.

Wavelength Division Multiple
xing (WDM)


For fiber optic channels, a variation of FDM is used, it is calledWDM.


Wave Division Mutiplexing (WDM) multiplexes multiple optical

carrier signals on a single optical fiber by using different wavelengths

(colors) of laser light to carry dif
ferent signals.


At the far end, the beam split up over as many fibers/signals as there

on the input side.

JPR COMPUTER NETWORKS

36


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PHYSICAL LAYER


The system often referred to as DWDM.

Advantages


The DWDM point
-
to
-
point architecture is simple to build

and

troubleshoot .


It enables protocol transparency, incremental growth, and capacity

expansion over time, while dramatically reducing start
-
up costs.


Point
-
to
-
point solutions are also extremely efficient.


No amplifiers or additional equipment requi
red.

JPR COMPUTER NETWORKS

PHYSICAL LAYER

Time Division Multiplexing


Sharing of the signal is accomplished by dividing available

transmission time on a medium among users.


Digital signaling is used exclusively.


Time division multiplexing comes in two

basic forms:


1. Synchronous time division multiplexing, and


2. Statistical, or asynchronous time division multiplexing.

Synchronous Time Division Multiplexing


The multiplexor accepts input from attached devices in a round
-
robin

fashion and transmit
the data in a never ending pattern.


T
-
1 and ISDN telephone lines are common examples of synchronous

time division multiplexing.

JPR COMPUTER NETWORKS

PHYSICAL LAYER

Ex of basic TDM


If one device generates data at a faster rate than other devices, then
the

multiplexor must either sample the incoming data stream from that

device more often than it samples the other devices, or buffer the faster

incoming stream.

JPR COMPUTER NETWORKS

PHYSICAL LAYER


If a device has nothing to transmit, the multiplexor mus
t still insert a

piece of data from that device into the multiplexed stream.

JPR COMPUTER NETWORKS

37

PHYSICAL LAYER

Statistical Time Division Multiplexing


A statistical multiplexor transmits only the data from active

workstations (
or why work when you
don’t have to
).


If a workstation is not active, no space is wasted on the multiplexed

stream.


A statistical multiplexor accepts the incoming data streams and creates

a frame containing only the data to be transmitted.


To identify each piece of data,
an address is included.


Good for low bandwidth lines (used for LANs)

JPR COMPUTER NETWORKS

PHYSICAL LAYER

For Ex:

JPR COMPUTER NETWORKS

PHYSICAL LAYER

Switching


A telephone engineer point of view, the phone system is divided into

two parts.

1. Outside
plant

local loop, trunks.

2. Inside plant (switches)

which are inside the switching offices.


Switches are devices capable of creating temporary connections

between two or more devices linked to the switch.


In a switched network some of the end
devices and other are used only

for routing.


There are three methods of switching


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1. Circuit
-
Switching

2. Message switching

3. Packet switching

JPR COMPUTER NETWORKS

PHYSICAL LAYER

Circuit Switching


A circuit switched network consists of a set of switc
hes connected by

physical links.


A connection between two stations is a dedicated path made of one or

more links.


Each link is divided into n channels either by using FDM or TDM.


Circuit switching takes place at the physical layer.


Before stating
communication, connection must be established.


Data transferred between the stations is continues flow.


There is no addressing involved during data transfer.


For communication in a circuit switched network requires three pahes

1. Setup Phase

2. Data
Transfer Phase

3. Teardown Phase

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PHYSICAL LAYER

For Ex:


Advantages:


The communication channel (once established) is dedicated.


Disadvantages:


Possible long wait to establish a connection


More expensive than any other
switching techniques, because a

dedicated path is required for each connection.


Inefficient use of the communication channel, because the channel

is not used when the connected systems are not using it.

JPR COMPUTER NETWORKS

PHYSICAL LAYER

Message Switch
ing


With message switching there is no need to establish a dedicated path

between two stations.


When a station sends a message, the destination address is appended to

the message.


The message is then transmitted through the network, in its entirety,

from node to node.


Each node receives the entire message, stores it in its entirety on disk,

and then transmits the message to the next node.


This type of network is called a
store
-
and
-
forward network.

JPR COMPUTER NETWORKS

PHYSICAL LAYER


Amessage
-
sw
itching node is typically a general
-
purpose computer.


The device needs sufficient secondary
-
storage capacity to store the

incoming messages.


A time delay is introduced using this type of scheme due to store
-

andforward

time, plus the time required to f
ind the next node in the

transmission path.

JPR COMPUTER NETWORKS

PHYSICAL LAYER

Advantages:


Channel efficiency can be greater compared to circuit
-
switched

systems, because more devices are sharing the channel.


Traffic congestion can be reduced, becaus
e messages may be

temporarily stored in route.


Message priorities can be established due to store
-
and
-
forward

technique.


Message broadcasting can be achieved with the use of broadcast


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address appended in the message.

Disadvantages


Message switching
is not compatible with interactive

applications.


Store
-
and
-
forward devices are expensive, because they must have

large disks to hold potentially long messages.

JPR COMPUTER NETWORKS

39

PHYSICAL LAYER

Packet
-
Switching


Packet switching
can be seen as a s
olution that tries to combine the

advantages of message and circuit switching and to minimize the

disadvantages of both.


There are two methods of packet switching: Datagram and virtual

circuit.

JPR COMPUTER NETWORKS

PHYSICAL LAYER


In both packet switch
ing methods, a message is broken into small

parts, called packets.


Each packet is tagged with appropriate source and destination

addresses.


Each packets have a strictly defined maximum length, they can be

stored in main memory instead of disk, therefor
e access delay and cost

are minimized.


Also the transmission speeds, between nodes, are optimized.


With current technology, packets are generally accepted onto the

network on a first
-
come, first
-
served basis.


If the network becomes overloaded,
packets are delayed or discarded.

JPR COMPUTER NETWORKS

PHYSICAL LAYER


Datagram packet switching is similar to message switching in that

each packet is a self
-
contained unit with complete addressing

information attached.


This fact allows packets to
take a variety of possible paths through the

network.


In the virtual circuit approach, a preplanned route is established before

any data packets are sent.

A logical connection is established when


A sender send a "call request packet" to the receiver


And the receiver send back an acknowledge packet "call accepted

packet" to the sender.


if the receiver agrees on conversational parameters, communication is

going to takes place.

JPR COMPUTER NETWORKS

PHYSICAL LAYER

Advantages:


Packet switching is cost

effective, because switching devices do not

need massive amount of secondary storage.


Packet switching offers improved delay characteristics, because there

are no long messages in the queue (maximum packet size is fixed).


Packet can be rerouted if the
re is any problem, such as, busy or

disabled links.


The advantage of packet switching is that many network users can

share the same channel at the same time.


Packet switching can maximize link efficiency by making optimal use

of link bandwidth.

JPR COM
PUTER NETWORKS

40

PHYSICAL LAYER

Disadvantages:


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
Protocols for packet switching are typically more complex.


It can add some initial costs in implementation.


If packet is lost, sender needs to retransmit the data.


Another disadvantage is that
packet
-
switched systems still can’t

deliver the same quality as dedicated circuits in applications requiring

very little delay
-

like voice conversations or moving images.

JPR COMPUTER NETWORKS

PHYSICAL LAYER

A comparison of circuit switched and packet
-
swi
tched networks.

JPR COMPUTER NETWORKS