Data Communication and Computer Networks

Windhya RankothgeGCE (A/L) ICT Training for Teachers

People CommunicationPeople use voice as the main communication method to exchange information. If two people are close each other, the voice transmission is done through air. (air is the transmission media)

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If the two people are far away they cannot talk each other in natural voice through air. The sound signal should be converted to an electric signal and send through a transmission media. (Normally the transmission media is two copper wires)

Computer CommunicationComputer can communicate each other by using an electrical signals only. Normally this is called data.In order to send the data, a transmission media is needed. GCE (A/L) ICT Training for Teachers

Types of SignalsThe signals can be mainly categorized as,Analog Signals Digital Signals When a natural signal converts to an electrical signal it does not have a definite pattern. Such signals are called analog signals. (e.g. voice to electrical). The computers are communicating by using digital signals where it has a definite number of voltage or current levels.Analog signals are measured with frequency and frequency bandwidth and digital signals are measured with bits and bit rates. GCE (A/L) ICT Training for Teachers

Modern Communication MethodsTelephone Radio / TVSatellitePSTN (Dial UP)ISDNADSLCDMAGPRSGSMGCE (A/L) ICT Training for Teachers

011-2583868

Requires a modem and a phone line to dial into a service providers node, in order to get the connectionPSTN Dial Up ConnectionsRequires a modem and a phone line to dial into a service providers node, in order to get the connection

Leased LineA permanent telephone connection between two devicesA permanent telephone connection between two devices

Integrated Services Digital Network (ISDN)

ISDN cont.

Asymmetric Digital Subscriber Line (ADSL)

ADSL cont.

Communication Model

DTE / DCE

Asynchronous TransmissionData is not transmitted continuously. A character can be represented by a group of bits. (E.g. 8 bits) Each set is sent with a start bit and a stop bit.

Transmission Modes

Synchronous Transmission

Transmission Modes cont.

Simplex

Transmission Techniques

Half Duplex

Transmission Techniques cont.

Full Duplex

Transmission Techniques cont.

Base BandBandwidth of the cable is occupied by one signal

Bandwidth of the cable is occupied by more than one signal

Broad Band

Convert an analog signal to a digital signalThe most commonly used method is the Pulse Code Modulation (PCM). Normally all voice telephone channels use this method.Voice telephone channel frequency band is 0.3 kHz to 3.4 kHz.

The process can be described as follows.(i) Sampling(ii) Quantizing(iii) Encoding

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Pulse Code Modulation (PCM)The samples of an analog signal are taken. The sampled signal is called a pulse amplitude modulated signal.It can be shown that the original signal can be constructed at the receive end using these samples.

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Sampling Theorem ( Nyquists Theorem)In order to completely reconstruct the original signal from the samples, the sample rate should be at least twice its highest frequency.i.e. sampling rate 2 X highest frequencyThe highest frequency of telephone voice channel is 3.4 kHz.Hence sampling rate 2 X 3.4 6.8 kHzHence a sample rate of 8 kHz is selected.An analog signal is sampled at a rate of 8000 samples per second.GCE (A/L) ICT Training for Teachers

Quantizing The samples are divided into many discrete levels. Then each sample is numbered according to their corresponding level.There is no exact level for the above sample. The approximate level of the above sample is 50. Therefore the level of the sample is considered as 50GCE (A/L) ICT Training for Teachers

525150

3210

EncodingAfter quantizing the corresponding level it is to be represented in some manner.E.g. If the level is 50, it can be represented as,Decimal - 50Hexa - 32Octal - 62Binary - 110010

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1 1 0 0 1 0

ModulationModulation is a technique used to send information by modifying the characteristics of a basic electromagnetic signal. The basic signal is called the carrier signal.The characteristics of a signal are amplitude, frequency and phase.

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Modulation Cont.Modulation can be used to convert a low frequency analog signal to a high frequency analog signal

Or a digital signal to an analog signal.Ex : MODEMGCE (A/L) ICT Training for Teachers

DigitalModem

AnalogModem

Modulation ProcessGCE (A/L) ICT Training for Teachers

Modulation Process Cont.Modulating SignalThis is the useful signal. This can be an analog signal or a digital signal. If the modulating signal is analog it is called analog modulation. If the modulating signal is digital, it is called digital modulation.Carrier SignalThis is a high frequency analog signal.Modulated SignalThe three characteristics of any signal are amplitude, frequency and phase. One of these characteristics is changed according to the shape of the input analog signal or the bit pattern of the input digital signal.

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Modulation MethodsIf the modulating signal is an analog signal, the three modulation methods are called,Amplitude Modulation (AM)Frequency Modulation (FM)Phase Modulation (PM)

If the modulating signal is a digital signal, the three modulation methods are called,Amplitude Shift Keying (ASK)Frequency Shift Keying (FSK)Phase Shift Keying (PSK)

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Amplitude Modulation (AM)Amplitude of carrier signal varies according to the amplitude of modulating signal. Please note that the frequency or phase of the carrier signal is not changed.

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Frequency Modulation (FM)The carrier signal frequency changes according to the amplitude of the modulating signal. GCE (A/L) ICT Training for Teachers

Amplitude Shift Keying (ASK)The two states are,0 amplitude 1 (a1)1 amplitude 2 (a2)

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Frequency Shift Keying (FSK)The two states are,0 - frequency 1 (f1)1 - frequency 2 (f2)

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Phase Shift Keying (PSK)The carrier signal phase is shifted according to the input digital signal.

PSK versionsBPSKQPSK 8PSK16PSKGCE (A/L) ICT Training for Teachers

Bipolar Phase Shift Keying (BPSK)GCE (A/L) ICT Training for Teachers

MultiplexingSuppose we need to transmit 4 64 kb/s signals from A to B.For this purpose, it is required to have four channels. Each channel needs at least 2 wires. If the length from A to B is 100m, we need 4 X 2 X 100 = 800m Copper Cable. If we can combine all four channels together without any mixing, a single pair of cable is sufficient. This type of combination (packing) of signal is called Multiplexing.There are mainly two types of Multiplexing. Frequency Division Multiplexing (for Analog Signals) Time Division Multiplexing (for Digital Signals)

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Frequency Division Multiplexing (FDM)GCE (A/L) ICT Training for Teachers

Time Division Multiplexing (TDM)GCE (A/L) ICT Training for Teachers

What is a computer network?A collection of computers and other associated devices (printers, fax machines, or any devices capable of sending/receiving data) interconnected by intermediate networking devices (hubs, routers and switches) using some transmission media (copper, fiber, free space)

Network Advantages

Resource sharing - printers, databases, applications, files

Remote information access- web and ftp

User communications (email)

Direct ConnectivityPoint-to-Point (p-to-p) Provides a dedicated link between two devices

Multipoint There is a single link shared among several devices

Classification of NetworksBased on the covered geographical distance (size or scale)

Local-Area Networks (LANs)Connects a set of devices in a relatively small geographical area to exchange information and share resources

Metropolitan-Area Network (MANs)Extend over an entire city

Wide-Area Networks (WANs)WANs provide connectivity for LANs that are dispersed over a large geographic area

LANsFeaturesSmaller Scope (Building or Small Campus)Usually owned by same organizationData rates are much higherUsually broadcast systemsEasy to manageTechnologiesEthernet, Fast Ethernet, Gigabit Ethernet, Token RingLAN devicesRepeaters, hubs, bridges, LAN switches

Network TopologyThe way in which a network is physically laid out (physical structure)

Mesh TopologyEvery device has a dedicated point-to-point link to every other device

Star Topology

Bus TopologyShare a common media

Ring TopologyShare a common media

Tree TopologyA combination of star and bus topologiesGCE (A/L) ICT Training for Teachers

Hybrid TopologyA combination of different topologies such as star, Ring, Mesh, Bus etc.GCE (A/L) ICT Training for Teachers

ISO OSI Seven Layers

Why ISO-OSI Seven Layers ?Inter operatabilityMarket for different manufacturersReduce costHigh maintainability

Physical LayerDefines the DTE/DCE interface standardsMechanical , Electrical ,Functional CharacteristicsInterface with the communications hardware and transmission medium. Transmission of an unstructured stream of data bits.

Data Link LayerTransmission of frames containing data and/or control information. Provides error control and flow control over the data link.

Network LayerTransfer data from a source to a destination via one or more networksPerforms routing functions

Transport LayerEstablish connection between processesSegmentation and reassembly of data Flow control & Error control

Session LayerManages the session

Dialog Control- Allows communication between two processes to be half duplex / full duplex

Synchronization- Adds checkpoints into a stream of dataEx : Total Number of pages 2000Insert check points after every 100 pages

Presentation LayerTranslationAt Tx : Changes information from sender-dependent format into common formatAt Rx : Changes information from common format into receiver-dependent format EncryptionSender transforms original information to another formatProvides securityCompressionReduce number of bits contain informationImportant for transmission of multimedia

Application LayerProvides user interfacesSupports for services- e mail- Remote file access and share- Shared database management

AddressesPort Address Port numberIdentifies the process (Transport Layer)Logical Address IP addressIdentifies the computer (Network Layer)Physical Address MAC addressIdentifies computers at two ends of a link (Data Link Layer)

Networking DevicesRepeaterHubBridgeL2 SwitchL3 SwitchRouters

RepeaterOperates in Physical Layer

HubOperates in physical layer

BridgeConnects two or more similar types of LANsMore performances and securityOperates at data link layer (intelligent device)

SwitchAn intelligent deviceOperates in layer 2 layer2 switchOperates in layer 3 layer3 switchOperates in half duplex / full duplex modeSupports 10Mb/s and 100 Mb/s

RouterRouters can be used to connect dissimilar types of LANsOperates in Layer 3

GatewayCan operates in higher layers (Transport / Application layer)

ISO/OSI Model and TCP/IP ModelGCE (A/L) ICT Training for Teachers

ProtocolA set of rules agreed by both transmit and receive endsDefines- What are the frames- Frame structure- Order of the frames

ProtocolsGCE (A/L) ICT Training for Teachers

The station senses the link before start to transmit the signal ( Carrier Sense CS )So, multiple computers access media at different times ( Multiple Access MA )Once the station starts to transmit, it monitors the medium to see if transmission is successful( Collision Detection CD )If collision detected, the station stops the transmission immediatelyIt will retransmit the signal, when medium is free

IEEE 802.3 - Ethernet

IEEE 802.5 - Token Ring

Transport layer protocol - TCPTransport Control ProtocolConnection Oriented : Connection is establish between client and serverThen only data is transferredAn acknowledgement is received for each data segmentError control and flow control can be doneAfter data transfer, the connection is terminated Data transfer is reliable

Transport layer protocol - UDPUser Datagram ProtocolConnectionless : No connection is established prior to sending dataData will go through the network and reach serverServer does not send any acknowledgementData transfer is unreliable

Internet ProtocolIP protocol is unreliableconnectionlessNo error control and flowTCP and IP together - reliable UDP and IP together - not reliable

Network layer protocol - IP

Address Resolution Protocol (ARP)IP address is known, Wants to find the MAC address

Application Layer ProtocolsDynamic Host Configuration Protocol (DHCP)Domain Name System (DNS)TELNETFile Transfer Protocol (FTP)Trivial File Transfer Protocol (TFTP)Simple Network Management Protocol (SNMP)Simple Mail Transfer Protocol (SMTP)SMTP is used to send e-mails.Hyper Text Transfer Protocol (HTTP)

Dynamic Host Configuration Protocol (DHCP)

Domain Name System (DNS)192.168.10.1

TELNETUsed to access a server located in a remote location in the networkNetworkTelnet Server

File Transfer Protocol (FTP)

Trivial File Transfer Protocol (TFTP)

Simple Network Management Protocol (SNMP)

Simple Mail Transfer Protocol (SMTP)- used to send e-mails

Hyper Text Transfer Protocol (HTTP)- used for web applications

Addressing with TCP/IPMAC AddressIP AddressPort AddressIdentify a ComputerIdentify a Process

Logical Address Address scheme depends on the protocol usedWidely used protocol is TCP/IP and the logical address is called IP AddressLogical address operates in the Network Layer (Layer 3)

IP AddressIP Version 4 (IPV4)32 bit scheme

IP Version 4 (IPV4)The 32 bits are represented in following manner. Byte 1. Byte 2. Byte 3. Byte 4(one byte is 8 bits)

IP Version 4 (IPV4) cont.

IP Version 4 (IPV4) cont.The minimum value of a byte00000000=0The maximum value of a byte 11111111=255The minimum IP Address0.0.0.0The maximum IP Address 255.255.255.255

Network ID and Host IDTelephone number - Area Code + Telephone NumberIP Addresses - Network ID + Host ID

Part of the IP Address is allocated to Network ID Remaining part is allocated to Host ID (Computer ID)

Class ANetwork IDHost ID

Subnet MaskNet ID part : All 1sHost ID part : All 0s

Transmission MediaGCE (A/L) ICT Training for Teachers

Guided MediaIt is a point-to-point communication.The signal can be transmitted without changing the frequencies. These signals normally cannot be interfered with other signals.

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Unguided MediaThe signal is transmitted into free space.Therefore, each signal should operate with a unique frequency. If two signals have the same frequency, then those two signals can interfere. GCE (A/L) ICT Training for Teachers

Copper CablesCopper cables are used for different purposes.

For voice communication in telecommunication systems. (Exchange to Distribution Point (DP) and DP to home). The DP to home copper cables is called Aerial Cable.For multichannel (high bandwidth) signal transmission. These are called, Coaxial Cables. For data transmission, Unshielded Twisted Pair (UTP) or Shielded Twisted Pair (STP) is used.

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Types of Copper CablesTwisted Pair CableA twisted pair consists of two insulated copper wires. These wires are twisted together in a helical form

Coaxial CableIn between the cables, there is an insulator. Since both cables have the same axis, this is called a Coaxial Cable.

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Twisted PairGCE (A/L) ICT Training for Teachers

Twisted Pair cont.Twisted-pair cable is a type of cabling that is used for telephone communications and most modern Ethernet networks.A pair of wires forms a circuit that can transmit data.The pairs are twisted to provide protection against crosstalk, the noise generated by adjacent pairs.TPs small size can be advantageous during installation. TP cable is more prone to electrical noise and interference than other types of networking mediaDistance between signal boosts is shorter for TP GCE (A/L) ICT Training for Teachers

Coaxial CablesThere are two types of Coaxial cables.Thin Coaxial CableThick Coaxial Cable

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Coaxial Cables cont.Coaxial cable is still the most common means of data transmission over short distancesCoaxial cables were designed to transmit of high-frequency radio signals efficiently to any location possible

Limitations :- Along the length of the coaxial cable, part of the transmitted signal will be lost or attenuated- Coaxial cable efficiency partly depends on keeping the physical dimensions of the cable uniform

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Fiber Optics Fiber Optics are cables made of optical fibers and can transmit large amounts of information at the speed of light

Fiber Optics cont.

Fiber optics cont.AdvantagesLess expensiveThinnerHigher carrying capacityLess signal degradation No interferenceLow powerDigital signalsNon-flammable Lightweight Flexible

Fiber optics cont.DisadvantagesInstallation and Maintenance CostUnidirectional light propagation

ApplicationsBackbone networksCable TV networks100Base-FX , 1000Base-FX LANs

Radio TransmissionA radio signal is an electromagnetic wave, which travels through free space (unguided media).

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Problems in transmission mediaNoiseNoise is an unwanted electrical signal (voltage or current). This mainly occurs due to random movement of electrons

AttenuationSignal strength reduces when it ravel through the media

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Problems in transmission media cont.DistortionThe velocity of an electromagnetic signal travelling through a transmission media depends on the frequency of the signal. Any signal is a combination of many sinusoidal waves, which have different frequencies. Therefore, the signal wave components travel with different velocities and reach the destination at different times. GCE (A/L) ICT Training for Teachers

The InternetThe Internet is the global network connecting thousands of network spanned across the globe. It is a Network of NetworksInternet is opened to anyone who has a need to connect to itInternet is regulated by the Internet Society

Information SecurityAny information system is secure if it confirms three things:AvailabilityIntegrityConfidentialityThis is often called the CIA triad

Information SecurityAvailabilityLegitimate users should have timely and efficient access to information and systemsIntegrityAny unauthorized modifications should be preventedConfidentialityThere should be a necessary level of secrecy for information and unauthorized disclosure of information should be provided

In the early days.Mainframe systems contained dumb terminals with limited functionality and processingFew individuals had knowledge and access to computer systemsCompanies did not depend heavily on computer systems for their business functions

NowadaysWith the introduction of PC computer systems became popular and majority started using themCompanies depend heavily on data processing and storage capability of computersMany individuals have knowledge on computer system operations and how they can be used in destructive ways (Specially with the popularity of the Internet)

The Closed Network

The Network Today

Who are your Enemies?HackersAccess systems in an unauthorized manner. However, hackers have no malicious intent (i.e., they do not intend to cause harm). They are only motivated by curiosity, personal satisfaction, or gaining reputation etcCrackersIndividuals who cause damages to information systems with a malicious intent often for financial gains

Malicious SoftwareThese are software that can cause harm to your data and information systems (e.g., Viruses, Worms, Trojan Horses, Spyware etc)

Who are your Enemies? Cont.

EncryptionEncryption makes your data unreadable to othersEncryption takes your normal messages (called clear text) and changes it to an unreadable format called cipher textExample: Take the word Hello and replace each letter by three letters ahead in the alphabet. You end up with Khoor which is unreadable

Strong PasswordsPasswords are not stored in clear-text (i.e., readable) format in your computerHowever, it is possible for someone to find out your password either by guessing it or by carrying out a password attackStrong passwords cannot be easily guessed by others or cracked by password cracking programsStrong passwords are essential to protect your information

Attacks on PasswordsBrute force attacksHere the attacker tries all possible combinations for a password until he gets the correct oneThere are programs written to do this task Dictionary attacksThe attacker tries all the words in a dictionary with the hope of discovering the password (including names, places, etc..)There are dictionaries of frequentlyused passwords that can be used for this purpose

Keystroke MonitoringAttacker tries to obtain a password by looking at your key strokes while you enter your passwordDumpster divingAttacker searches through trash bins with the hope of finding written down passwords or other confidential informationAttacks on Passwords cont.

Key loggersThese are programs planted in your Computer by an attacker to record every keystroke you type in to a file. The attacker can later analyze the file to obtain your password Social EngineeringAttacker tries to trick you into revealing your password.Attacks on Passwords cont.

Password Best PracticesAlways use a password of a minimum of eight charactersDo not use your name, birthday, name of a close relative as your password since these can be easily guessedUse non-dictionary words for your passwordAlways use a combination of uppercase/lowercase characters, numbers.Use at least one special character in your password (e.g., !,#,$,@)

Change your password at least twice every monthNever write down your password in books, pieces of paper, diary etc.Never send your password via email or disclose it to someone even if you trust that personPassword Best Practices cont.

Malicious SoftwareVirusesA Virus is a piece of malicious code which attaches to another program to spread and cause harm to your informationWormsA worm is a self-contained program which can spread and cause damage on its own without attaching to another program

Trojan HorsesA Trojan Horse is a harmful program which disguises it self as some otheruseful program SpywareThese are malicious software installed in your computer without you Knowing about them. They may provide others access to your system,display unwanted banner ads, or steal your confidential informationMalicious Software cont.

Always use a virus scanner and keep it up to date with latest updatesEnable auto-protect features of your virus scannerUse a spyware scanner to scan and remove spyware and update it regularly Never download content from unknown web sitesNever open email attachments coming from unknown sourcessources

Preventing Malicious Software cont.

Use a personal firewallKeep your operating system updated with latest updates and patchesNever click yes in unknown popup ads that appear.Always close them using x on the upper right hand corner

Preventing Malicious Software cont.

Identity TheftIn the Internet sometimes you have to disclose your personal information such as name, telephone numbers and email addressesTo make online purchases you need a to give your credit card numberHowever, you got to be careful when you disclose your personal information over the Internet

Identity theft is the act of using someones Identity and good reputation by another individual for financial gainsOne of the fastest growing crimes in United StatesA popular way to obtain private information is by using phishing scamsIn phishing scams attacker sends an email to the victim which looks like a legitimate request for victims personal informationIdentity Theft cont.

Preventing Identity Theft Do not disclose your private information over emailsAlways check whether a website is a trusted one before you enter any sensitive informationAlways check whether the website supports secure transactions (others cannot see the information you send to secure sites)Always read privacy policies given on websites

Identifying Secure WebsitesSecure websites have a URL starting with https://

There is a closed padlock icon at the bottom of the browser status bar