COMPUTER NETWORKS

INTERNET

Definition

• CN is a collection of two or more computers, which are connected to share information and resources.

• Computers in a network are interconnected by telephone lines, coaxial cables, satellite links, radio or some other communication technique.

• The computers can be geographically located anywhere.

Applications of Networks

Resource SharingHardware (computing resources, disks, printers)Software (application software)

Information SharingEasy accessibility from anywhere (files, databases)Search Capability (WWW)

CommunicationEmailMessage broadcast -prerecorded message to people

Remote computing

- Access thru network

Distributed processing (GRID Computing)

-Combination of several computers to process large amount of data.

Types / categories:1. Local Area Network (LAN) -Network in

small geographical Area (Room, Building or a Campus) is called LAN (Local Area Network)

2. Metropolitan Area Network (MAN) -Network in a City is call MAN (Metropolitan Area Network)

3. Wide Area Network (WAN) -Network spread geographically (Country or across Globe) is called WAN (Wide Area Network)

Network Topology

The network topology defines the way in which

computers, printers, and other devices are

connected. A network topology describes the

layout of the wire and devices as well as the

paths used by data transmissions.

Bus Topology

Commonly referred toas a linear bus, all thedevices on a bustopology are connectedby one single cable.

Star & Tree Topology

Star topology resembles spokes in a bicycle wheel.

Larger networks use theextended star topology alsocalled tree topology. When usedwith network devices that filterframes or packets, like bridges,switches, and routers, thistopology significantly reducesthe traffic on the wires bysending packets only to thewires of the destination host.

Ring Topology

A frame travels around the ring,stopping at each node. If a nodewants to transmit data, it adds thedata as well as the destinationaddress to the frame.

The frame then continues aroundthe ring until it finds thedestination node, which takes thedata out of the frame.

Single ring – All the devices on thenetwork share a single cable

Dual ring – The dual ring topologyallows data to be sent in bothdirections.

Mesh Topology

The mesh topologyconnects all devices(nodes) to each otherfor redundancy andfault tolerance.

It is used in WANs tointerconnect LANs andfor mission criticalnetworks like thoseused by banks andfinancial institutions.

Implementing the meshtopology is expensiveand difficult.

OSI Model Overview

The Open Systems Interconnection (OSI) reference model is an industry standard framework that is used to divide the functions of networking into seven distinct layers.

Each layer provides specific services to the layers above and below it in order for the network to work effectively.

Functions of the Layers1. Physical

• transmit a bit stream over a physical medium.

2. Data-Link• Organizes bits into logical units called frames.

• Node-to-node delivery

3. Network• Source-to-destination delivery of a packet.

4. Transport• Source-to-destination delivery of the entire message.

5. Session• Establishes, maintains, and synchronizes

the dialog between communicating systems.

6. Presentation• Deals with the fact that different systems use

different coding methods.

7. Application• Enables the user to access the network.

Physical Layer It defines the physical and electrical characteristics

of the N/W.

This layer also defines what kind of network interface card must be installed in each computer and what kinds of hubs to be used.

In other words the physical layer is a conduit (connection) between the computers networking hardware and its networking software.

This layer communicates with the data link layer and regulates the transmission of a stream of bits over a physical medium.

This layer also defines which transmission technique is used to send data over the cable.

Data Link Layer The function of data link layer is to transform the

data into a line that is free of transmission errors and is responsible for node-to-node delivery

On the sender side the data link layer divides the layer divides the stream of bits fro the N/W layer into a manageable form known as frames.

These data frames are then transmitted sequentially to the receiver.

On the receiver end the data link layer detects and corrects any errors in the transmitted data which it gets from the physical layer.

Network Layer The network layer provides the physical routing of

the data that is it determines the path between the sender and receiver.

The outbound data is passed down from the transport layer is encapsulated in the networks layers protocol and then sent to the data link layer for segmentation and transmission.

This layer organizes frames from data link layer into packets and is passed to the transport layer

Network layer provides uniform addressing mechanism so that more than one networks can be interconnected.

Transport Layer The basic function of the transport layer is to

handle error recognition and recovery of the data packets.

The T L establishes, maintains, and terminates communication between the sender and the receiver.

At the receiving end transport layer rebuilds packets into the original message, and to ensure that the packets arrived correctly, the receiving transport layer sends receipt acknowledgments.

Session Layer The session layer organizes and synchronizes the

exchange of data between the sending and receiving applications.

The session layer lets each application at one end know the status of the other at the other end.

An error in the sending application is handled by the session layer in such a manner so that the receiving application may know that the error has occurred.

The SL can resynchronize applications that are currently connected to each other.

This may be necessary when communications are temporarily interrupted or when an error has occurred that results in loss of data.

Presentation Layer The basic function of the presentation layer is to

ensure that information sent from the application layer of one system would be readable by the application layer of another system.

This is where application data is packed or unpacked ready for use by the running application.

This layer also manages security issues by providing services such as data encryption and compresses data so that fewer bits need to be transferred on the N/W .

Application Layer The A L is the entrance point that programs use to

access OSI model and utilize network resources.

This layer represents the services that directly support applications.

This OSI layer is closest to the end user.

Application layer includes network software that directly serves the user , providing such things as the user interface and application featurs such as electronic Mail.

NETWORK STRUCTURE

The subnet interconnects hosts.

Subnet

◦ Carries messages from host to host. It is made

up of telecommunication lines (i.e. circuits,

channels, trunks) and switching elements (i.e.

Interface Message Processor’s, routers).

Hosts

◦ End user machines or computers.

Services

Connection-Oriented and Connectionless

◦ Connection-Oriented – before data is sent, the

service from the sending computer must

establish a connection with the receiving

computer.

◦ Connectionless – data can be sent at any time by

the service from the sending computer.

Service Primitives

Request – entity wants the service to do some work

Indicate – entity is to be informed about an event

Response – entity responds to an event

Confirm – entity is to be informed about its request

Sending Computer Receiving Computer

3 Network

1. request

3 Network

2. indicate 3. response4. confirm

4 Transport 4 Transport

Read/writeRead/writeRead/writeRead/writeRead/writeRead only

1 2 3 4

Networking Devices

Introduction to Computer Networks

HUB, Switches, Routers,Wireless Access Points,Modems etc.

Network Devices• These devices interconnect individual computers

and ensure that they communicate efficiently.

• Some network devices are as below

1. Network Interface Card

2. Hub

3. Repeater

4. Switch

5. Bridge

6. Router

7. Gateway

Network DevicesNetwork Interface Card

• Interface between the machine and the N/W.

• It connects the clients, servers and peripherals to the network via a port.

• Most network interfaces come as small circuit board that can be inserted onto one of the computers motherboard slots.

• Each network interface is associated with a unique address called its media access control(MAC) address.

• The MAC address helps in sending information to its intended destination.

Network DevicesHub

• Is a small box that connects individual devices on a network so that they can communicate with one another.

• The hub operates by gathering the signals from individual network devices, optionally amplifying the signals and then sending them onto all other connected devices.

• Amplification of the signal ensures that devices on the network receive reliable information.

• Also known as concentrator, a hub works on physical layer of the OSI model.

Network DevicesRepeater

• Is an electronic device that operates on the physical layer of the OSI model.

• Signals that carry information within a network can travel a fixed distance.

• A repeater installed on the link receives signal , regenerates it and sends the refreshed copy back to the link.

• Nowadays the terms repeater and hub are used synonymously, but they are actually not the same

• Although at its very basic level, a hub can be thought of as a multiport repeater.

Network DevicesSwitch

• Like a hub a switch too connects individual devices on a network so that they can communicate with one another.

• Switches work on the data link layer of OSI model.

• Allows reducing overall network traffic

Network DevicesBridge

• A bridge filters data traffic at a network boundry.

• It reduces the amount of traffic on a LAN by dividing it into two segments.

• Bridges operate at the data link layer of the OSI model.

• It inspects each incoming traffic and decides whether to forward or discard it .

• When a frame enters a bridge the bridge not only regenerates the signals but also checks the address of the destination and forwards the new copy only to the segment to which the address belongs

Network DevicesRouter• A router is an essential network device for

interconnecting two or more networks.• Routers aim is to trace the best route for information to

travel .• As network traffic changes during the day routers can

redirect information to take less congested routes.• A router creates or maintains a table called a routing

table that stores the best route to certain destinations.• They can permit or deny network communications with a

particular network.

Network DevicesGateway• Is internetworking device which joins two different

network protocols together.• It works on all seven layers of the OSI model• A gateway accepts the packet formatted for one protocol

and converts the formatted packet into another protocol.

Networking Media

Networking media can bedefined simply as themeans by which signals(data) are sent from onecomputer to another(either by cable orwireless means).

Introduction to Computer Networks

TRANSMISSION MEDIA

1. Guided

Data is sent via a wire or optical cable.

Twisted Pair

Two copper wires are twisted together to reduce the effect of

crosstalk noise.

Baseband Coaxial Cable

A 50-ohm cable used for digital transmission.

Broadband Coaxial Cable

A 75-ohm cable used for analog transmission such as Cable

TV.

TRANSMISSION MEDIA

Fiber Optic Cables

Two general types are multimode and single mode.

In multimode, light is reflected internally. Light source is an

LED.

In single mode, the light propagates in a straight line. Light

source come from expensive laser diodes. Faster and longer

distances as compared to multimode.

* Fiber optic cables are difficult to tap (higher security)

and are normally used for backbone cabling.

Twisted Pair

Two insulated copper wires in a spiral

Number of pairs are bundled together

Twisting decreases crosstalk

Most common form for analog and digital

Used in telephone system

Subscriber loops

From a person’s home to the local office of the phone

company

LANS

10Mbps with newer at 100Mbps

Long Distance

4 Mbps

ISDN –Integrated Services Digital Network

Digital

Repeaters required every 2 –3 kilometers

Analog

Amplifiers required every 5-6 kilometers

Bandwidth of 250KHz, carry a few voice channels

Susceptible to noise, shielded and unshielded

Compared to optical and coax twisted pair is

limited in bandwidth, distance, and data rate

Coaxial Cable

• Hollow outer cylindrical conductor

surrounding a single view

• Most versatile of mediums, used for TV,

long distance telephone, and LAN’S

Optical fiber• Thin, flexible light passing material made from glass or

plastic

• Grouped into cables

• Better than coaxial cable or twisted pair

• Data rates of 2Gbps over 10’s of Km

• Light weight –good for buildings

• Lower attenuation than coax or twisted

TRANSMISSION MEDIA

2. Unguided

Data is sent through the air.

Line-of-sight

Transmitter and receiver must “see” each other, such as a

terrestrial microwave system.

Communication Satellites

A big microwave repeater in the sky. Data is broadcasted, and

can be “pirated.”

Radio

Term used to include all frequency bands, such as FM, UHF,

and VHF television.

• Wireless

• Broadcast Radio

Radio -3kHz to 300Ghz

Broadcast radio –30MHz to 1GHz

• Satellite Microwave 1 –10 GHz

• Infrared

Features & Working of the Internet

• Connected by many small networks in the world

using TCP/IP protocol stack

• No specific path between any two hosts that are

communicating

• Path is determined dynamically by Router, that

relays the data from the source host to the

destination host

• Data may need to go thru many routers before they

reach the destination

• When data from different sources need to go thru a

particular router, it will become busy and may delay

or even lost the data

• Hence Internet is NOT a reliable network

Performance: Latency and

Bandwidth• Latency

–How long minimum communication takes in seconds (s)

–Round trip vs. single trip

–More difficult to overcome than bandwidth

• Bandwidth

–Number of bits per time unit usually seconds (bps)

Addressing in TCP/IP

• Each TCP/IP address includes:

– Internet Address

– Protocol (TCP)

– Port Number

What is an IP Address?• Internet identifier including information about how to reach

a network location (via the Internet routing system)

• IPv4: 32-bit* number. Written in Dotted Decimal Notation

205.150.58.7

• 4 billion different host addresses

• IPv6: 128-bit* number. Written in Hex Decimal Notation

2001:0503:0C27:0000:0000:0000:0000:0000

• 16 billion billion network addresses

• Each Network I.D. on the Internet needs to be registered to

the Internet Assigned number Authority (IANA)

Address

• Way to identify people / computers

• On the Internet, the term “address” is used loosely

• –Can mean many different things from an email address to

a URL (Uniform resource locator)

• More specifically, 2 types:

• –Internet protocol (IP) address(or network address): 4-part

numeric address

• e.g. 158.132.148.28

• –Domain name system (DNS) address

• e.g. LMS

The four formats of IP Addresses

0 NetID

10

110 NetID

1110 Multicast Address

HostID

NetID HostID

HostID

Class

A

B

C

D

8 bits 8 bits 8 bits8 bits

Hardware vs. Software Firewalls

• Hardware Firewalls

–Protect an entire network

–Implemented on the router level

–Usually more expensive, harder to configure

• Software Firewalls

–Protect a single computer

–Usually less expensive, easier to configure

Firewall Rules

• Allow –traffic that flows automatically

because it has been deemed as “safe” (Ex.

Meeting Maker, Eudora, etc.)

• Block –traffic that is blocked because it has

been deemed dangerous to your computer

• Ask –asks the user whether or not the traffic

is allowed to pass through

SUPERVISORY CONTROL AND DATA

ACQUISITION(SCADA)

SCADA AS A SYSTEM

• SCADA refers to a system that collects data from

various sensors at a factory, plant or in other

remote locations and then sends this data to a

central computer which then manages and controls

the data.

• Data acquisition is the process of retrieving

control information from the equipment which is

out of order or may lead to some problem or when

decisions are need to be taken according to the

situation in the equipment. So this acquisition is

done by continuous monitoring of the equipment

to which it is employed. The data accessed are

then forwarded onto a telemetry system ready for

transfer to the different sites.

• Components of SCADA System

1. Field Instrumentation 2. Remote Stations 3.

Communications Network 4. Central

Monitoring Station

BENEFITS OF SCADA

• Long distance monitoring

• Long distance training

• Protection against terrorism/vandalism-

alarm

• Data management (engineering and

operations)

• Automated operations with real time control

What is Internet & Web?

Internet is a world wide, noncommercial, freely

accessible Network of Computer Networks.

World Wide Web (WWW) is a vast, ever-

expanding collection of online documents and

information formatted in HTML and

distributed over the Internet.

Working of the Web• Web documents live on Computers that run

HTTP servers.

• HTTP servers can be hosted on UNIX , Macintosh or PC’s.

• Browser requests the Web page from the Web server.

• Server sends the HTML pages over the Internet to your computer.

• Browser interprets the data and displays it on your screen.

Working of the Web

Web Page & Home Page

• Web Page : Document written in Web

formatting language, HTML

• Home Page : This is the first page or the

opening page of any web document.

Elements of Web Page

• Text : This is simple 7-bit ASCII , which can be easily represented by using the keys available on a standard keyboard.

• Graphics : Graphics add life to the web pages and makes them interesting to browse.

• Links and URL’s : Links are transporters of the Web. Links contain addresses of the Web files that they are referencing.

Web address / site address is also known as URL (Uniform Resource Locator)