14. Computer Systems and Networks

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Indrajith Hapuarachchie

[email protected]

Computer Systems and Networks


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Computer Hardware

Types of Computers

Super Computers

Extremely powerful computers

With massive computing power

Large banks of processors Used for scientific purposes

Mainframe Computers

Very large and powerful

Hundreds of simultaneous users Kept in data centers

Temp. Humidity and dust is controlled

Specialized computer operators use them

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Computer Hardware

Types of Computers

Mini Computers

Much smaller

Fewer than 100 users

Can be used in office environment Staff can use it after training

Micro Computers ( PC / workstation)

On desktop

Generally one user Connected to a network (W/S) or can be standalone (PC)

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Computer Hardware Servers

Powerful micro computers (PC servers )

That server applications (web servers )

Routers Are computers or computer software that looks at the

destination of information and determines how best to get it

there Allows the connectivity of two different type of networks (LAN to

WAN )

Gateways / Communication servers Connect LAN to WAN

Connect different types of LANs

Repeaters Amplifies the signal

Bridges Connect two LANS of same type to each other

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Computer Hardware

Standards and Protocols

There should be a standard to allow

communication between computers and this is

called Protocols

Most are governed by OSI (Open System

Interconnection ) model developed by ISO

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Computer Hardware

OSI reference Model

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Computer Hardware Computer Addresses IP Address

Eg. 203.118.0.1 , 192.168.50.1, 172.16.5.

Address of each computer in the network

Internet routable addresses (Public IP) LAN only addresses ie. Private IP addresses

10.0.0.0 through 10.255.255.255

172.16.0.0 through 172.31.255.255

192.168.0.0 through 192.168.255.255

URL Uniform Resource Locator

Used instead of IP addresses

Central list is used to translate URLs in to IPs

Domain Name Servers (DNS)7


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Computer Hardware

Components of a URL Protocol

Host name

Port number (FTP 20,21,telnet 23, smtp 25, pop3

110, http 80, 53 Dns)

Directory/ path

File name

http://www.cbsl.gov.lk/htm/english/04_mp/mp.html

http://www.smib.lk:8080/user/Login.jsp

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http://www.cbsl.gov.lk/htm/english/04_mp/mp.htmlhttp://www.cbsl.gov.lk/htm/english/04_mp/mp.htmlhttp://203.94.66.193:8080/user/Login.jsphttp://203.94.66.193:8080/user/Login.jsphttp://www.cbsl.gov.lk/htm/english/04_mp/mp.htmlhttp://www.cbsl.gov.lk/htm/english/04_mp/mp.html


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Computer Hardware HTTP (Hyper Text Transfer Protocol ) Basic communication mechanisum in the web

Four stages of information retrieval Connect

Connection between the web client ( browser ) and webserver (site) is made

Request Request an object from the server

Respond Responds to the request

Disconnect

Separate connection is made for each object

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Computer Hardware

Cookies HTTP is a stateless protocol

No information is maintained across series of connections

between the server and the client

No way to know that we have already connected toa particular web site

This was over come by placing a small file in the

users computer with values

Privacy issues are associated with cookies

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LAN & WAN Local Area Network Components

File server SAN

architecture to attach remote computer storage devices (such as disk arrays,tape libraries, and optical jukeboxes) to servers in such a way that thedevices appear as locally attached to the operating system

NAS

uses file-based protocols such as NFS where it is clear that the storage is

remote, and computers request a portion of an abstract file rather than a diskblock

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http://en.wikipedia.org/wiki/Disk_arrayhttp://en.wikipedia.org/wiki/Tape_libraryhttp://en.wikipedia.org/wiki/Optical_jukeboxhttp://en.wikipedia.org/wiki/Server_(computing)http://en.wikipedia.org/wiki/Direct-attached_storagehttp://en.wikipedia.org/wiki/Operating_systemhttp://en.wikipedia.org/wiki/Network_File_System_(protocol)http://upload.wikimedia.org/wikipedia/commons/c/ce/Compingles.GIFhttp://en.wikipedia.org/wiki/Network_File_System_(protocol)http://en.wikipedia.org/wiki/Operating_systemhttp://en.wikipedia.org/wiki/Direct-attached_storagehttp://en.wikipedia.org/wiki/Server_(computing)http://en.wikipedia.org/wiki/Optical_jukeboxhttp://en.wikipedia.org/wiki/Tape_libraryhttp://en.wikipedia.org/wiki/Disk_array


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LAN & WAN

Local Area Network Components

Print server

Print spooling Microcomputers (PC or W/S)

Gateway to connect other LANs or WAN

Repeaters to extend the LAN

Bridges and routers to connect to other LANs

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BUS LAN

Most simplest network

Use of Carrier Sense Multiple Access with Collision Detect (CSMA/CD)

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STAR LAN

Use of Carrier Sense Multiple Access with Collision Detect (CSMA/CD)

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Ring LANMulti-station Access Unit

MAU

Use of Token to pass data15


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Wide Area Network - WAN

Main Frame

Modem Modem

Simple WAN

Modem Modem

Multiplexor

Multiplexor

Multiplexed WAN16


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WAN Technology

Multiplexing Frequency Division

Multiplexing (FDM) Transmit all of the signals along

the same high speed linksimultaneously

Each signal set at a differentfrequency.

Frequency overlap must beavoided

Operates similar way to radiobroadcasting

In order to prevent signaloverlap there are strips offrequency that separate thesignals called guard bands.17
http://upload.wikimedia.org/wikipedia/commons/6/6d/Sine_waves_different_frequencies.svg


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WAN Technology

Multiplexing

Time Division Multiplexing (TDM)

Multiplexor collect and store the incoming

transmissions and allocating a time slice on the fast

link to each in turn.

Messages are sent down the high speed link one

after the other.

Two ways of implementing TDM are:

Synchronous TDM

Asynchronous TDM

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WAN Technology

Multiplexing

Time Division Multiplexing (TDM)

Synchronous TDM

Gives exactly the same amount of time toeach device even if a device has nothing to

transmit

Asynchronous TDM

Length of time allocated is not fixed for each

device Time is given to devices that have data to

transmit.

Works by tagging each frame with an

identification number to note which device it

belongs to.

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WAN Technology

Packet Switched

Networks

Data is Broken in to

small parts called

packets Packets are given the

IP address that they

should reach

Then they are

dropped to thenetwork

They will use the best

path to reach the

destination

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Complex Networks

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Internet Technology

Hypertext Mark-up Language (HTML) Form of text

Information includes links

Used to connect to another page

Load more information like pictures, video etc.

XML (Extensible Markup Language)

Is a set of rules for encoding documents in machine-

readable form

It is a textual data format

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Internet Technology

Java & ActiveX Java

Programming language

Browsers have inbuilt java interpreters

Is a object-oriented programming language (OOP) Can be run on any platform

It runs on Java Run time environment

Features

Increased Interaction

Graphics

Reduced Server Load

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Internet Technology

Java Applets Is a Java Program specifically written for the

browsers

Strictly controlled access to host machine

Can not normally read or write files to the computer

Java Script

Programming script that can be embedded in HTML

Developed by Netscape ( this is different to java

programming )

Used to provide dynamic content in web pages

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Internet Technology

JavaBeans Reusable components of java programming

Created by JavaSoft (business unit of SUN)

Application Programming Interface (API)

This is where JavaBeans comes in

JavaBeans offers platform independent and

Architecture independent API for java software

ActiveX

Microsofts answer to Java

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Other Devices

Mobile Telephones

SMS

GSM

WAP ( Wirelessapplication protocol)

Strips complex graphics

from websites

Slow

3G (3rd Generationtelephony)

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Other Devices

How WAP works

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Other Devices

Use of 3G in Enterprises

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Other Devices

Bluetooth is an openwireless technology standard

for exchanging data over short distances (using

short length radio waves)

from fixed and mobile devices,

creating personal area networks (PANs)

with high levels of security

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http://en.wikipedia.org/wiki/Open_standardhttp://en.wikipedia.org/wiki/Wirelesshttp://en.wikipedia.org/wiki/Personal_area_networkhttp://en.wikipedia.org/wiki/File:Bluetooth.svghttp://en.wikipedia.org/wiki/Personal_area_networkhttp://en.wikipedia.org/wiki/Wirelesshttp://en.wikipedia.org/wiki/Open_standard


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Other Devices

Bluetooth Network

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From Viruses to Cybercrime

In the past, PCs were mainly under threat fromviruses and worms. The main purpose of theseprograms was to spread; however, some programswere also designed to cause damage to files andPCs. Such malicious software, or malware, could bedescribed as cyber vandalism. In the majority ofcases, the goal of viruses and worms was to spreadas much as possible, with a high infection rate leadingto fame for that program.

But in recent years, the situation has changeddrastically.

Today, the biggest threat faced by computers iscrimeware. This malicious software is written bycybercriminals with the purpose of making money

illegally Crimeware may take the form of viruses

Computer Threats

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14. Computer Systems and Networks

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