Page | 1 Chapter 1 : Introduction to Computer Networks Topic List: A.Need to Share ResourcesB.Layering Concept and ExampleC.TCP/IP and OSI Layering ModelsD.Connection Oriented vs. Connectionless TransferE.Top-Down and Bottom Up Approaches to study layer.F.What is the need of resource sharing?G.Give functions of Each Layer in OSI/TCP Model.H.Distributed Systems and Networks___________________________________________________________________________ A.What is the need of Networks?For resource sharing.Major requirement of user community.We need to generate and disseminate information.Also information should be accessible by each one in user group. B.Layering Concept & Example:Based on Principle: Divide and Conquer.Each big problem divided into smaller pieces.Example:ABCisamiddlelevelcompany,locatedatA,selling computers, having: o Managero Secretaryo Route Operator o Warehouse Keeper Cum Security Officer oTransporterXYZ is also a company, located at B, ordered ABC for 5000 units of computers. How to deliver? Steps: 1. Manager passes the order to deliver 5000 computers to XYZ company and keep track of the delivery.2. Secretary communicates through letter with secretary of XYZ that consignment 5000 units of computer is sent, once delivered, give us acknowledgement.3. Secretary tells route operator to deliver the consignment to XYZ at B. He has map, using multiple transportations, he sends box.4. ABC has transport office at R1,R2,R3.5. At each destination, warehouse keeper takes care of delivery and security of goods at the time of unloading and loading. Advantage: 1 Clear Job Distribution: Work of each employee is clearly defined.Wecaneasilyreplaceanyoneofthemwithnewemployeewithoutaffecting efficiency of overall process.If new employee has different style of functioning, it doesnt affect us.When job is clearly divided, complex job becomes manageable.2. Every employee taking or giving services. Also called interface in computer networks.Page | 2 Chapter 1 : Introduction to Computer Networks Advantages of Layers Reducing the complexity Division of Work Standard Interfacing between Components Replacing a component is easy Independence in Protocol design Disadvantage of Layers Reduced Speed and Performance Increased Memory usage Sensor Networks Node Page | 3 C. Differentiate between OSI and TCP/IP layer: OSITCP/IP OSI (Open System Interconnection) Designed The ARPANET, a research network group connected by ISO (International Standards Organization) hundreds of universities and govn installations, using in 1983. leased telephone lines which later became TCP/IP. Layers 7Layers 5 Did not clearly distinguish between service, Distinction between these three concepts are explicit. interface and protocol. Protocols in the OSI model are better hidden Protocols in TCP/IP model are not hidden and tough than in the TCP/IP model and can be replacedto replace if technology changes. relatively easily as the technology changes. The protocols came first, and the model was The model was not biased toward one particular set of really just a description of the existingprotocols, a fact that made it quite general. protocols. Designers did not have much experience with Designers have much experience with the subject and the subject and did not have a good idea of have clear idea of which functionality to put in which which functionality to put in which layer.layer. Network Layer supports only connectionless Network Layer, supports both, connection oriented communication.and connectionless communication. More General.More precise. Describes the difference between protocol and Doesnt give emphasis on clear interface between interface. layers. Protocol works specially for transport and network layer. Page | 4 Chapter 1 : Introduction to Computer Networks D. Connection Oriented v/s Connectionless Transmission Connection Oriented TransmissionConnectionless Transmission Connection establishmentNo Connection establishment. Complete line occupiedComplete line not occupied Packet can be not be compressed (PktSize: 64 kb =Packet can be compressed (Pkt Size: 8kb) 8x8) Multiplexing not there, costly if transmitting smallMultiplexing multiple packets in one large pkt. packets. Robustness of the connection is not very good.Robustness of the connection is very high. Single Single link down results to connection break.link down results to alternate path. Cost of the connection highCost of the connection less Quality of service : Reliable, Delay if greedy userQuality of service: Unreliable, Minor delay if occupy channel with large flow.compressed. Order of delivery Important.Order of delivery not Important. Eg. ISD Telephone CallEg: Internet Voice Call E.Top-Down and Bottom-Up Approaches to study Layers:If we follow top-down approach, we need to discuss the lower layers to show how a specific service is implemented which is required at upper layers.If we follow bottom-up approach, we need to discuss the services that are to be provided for functionality that we are describing.It should be clearly described which layers renders which service to top and bottom.Similarly, description should also clearly describe which services are taken from peer layers by the layer described. Page | 5 Chapter 1 : Introduction to Computer Networks F. What is the need Share Resources? We need to generate information and disseminate information. Eg.Music files can be downloaded and uploaded on internet from LANEmailsSkype Voice Video CallsAccess information from blogsAccess Database at remote locationAudio and Video Conferencing G. Give functions of Each Layer in OSI/TCP Model. 1. Physical Layer: Transmitting raw bits over a communication channel.Works as Transporter, to carry bits from one end to another.Converts the bits to voltage or light pulse and at the other end, converts incoming voltage or light pulse to bits.Checks whether message has to be broadcasted or unicasted. 2. Data Link Layer The No Monopoly idea and the framingFraming TechniquesError ControlFlow ControlClassification into 1) MAC 2)LCC 3. Medium Access Layer Receive/transmit normal framesHalf-duplex retransmission and backoff functionsAppend/check FCS (frame check sequence)Inter-frame gap enforcementDiscard malformed framesAppend(tx)/remove(rx) preamble, SFD(Start Frame Delimiter), and paddingHalf-duplex compatibility: append(tx)/remove(rx) MAC address Page | 6 Chapter 1 : Introduction to Computer Networks 4. Network Layer Handling accounting for usage of network resourcesDevise and implement mechanisms of identifying each machine uniquelyImplement connectionless or connection-oriented forwardingMultiplexing and de-multiplexing the transport layer and the data link layer jobs 5. Transport Layer RetransmissionAdapted Retransmission Time according to LoadOrdered DeliveryOptimum Utilization of BandwidthFlow ControlReal Time DeliveryQuality of Delivery 6. Session Layer Allow users on different machines to establish sessions between them.Sessions offer various services, includingDialog Control (keeping track of whose turn it is to transmit),Token Management (preventing two parties from attempting the same critical operation at the same time), andSynchronization (check pointing long transmissions to allow them to continue from where they were after a crash). 7. Presentation Layer The presentation layer is concerned with the syntax and semantics of the information transmitted.In order to make it possible for computers with different data representations to communicate, the data structures to be exchanged can be defined in an abstract way, along with a standard encoding to be used ''on the wire.''The presentation layer manages these abstract data structures and allows higher-level data structures (e.g., banking records), to be defined and exchanged. 8. Application Layer Application layer contains a variety of protocols that are commonly needed by users.Widely-used application protocol is HTTP (Hyper Text Transfer Protocol), which is the basis for the World Wide Web.When a browser wants a Web page, it sends the name of the page it wants to the server using HTTP.The server then sends the page back.Other application protocols are used for file transfer, electronic mail, and network news. Page | 7 Chapter 1 : Introduction to Computer Networks H. Distributed Systems and Networks: Node: The members of network are popularly known as nodes.Normal nodes and server nodes are connected by a network which acts as the medium to provide services.We need Username and Password for taking access of remote printer or remote file with additionally exact set of commands for every change we make in the file. It is not transparent to users.Internet that all of us are so familiar with, also contain quite a large number of servers known as web servers also known as www (world wide web).When we click on a link and transfer from one webpage to another webpage, it is not possible to shift from server from one country to server from another country. Problem? Yes.Solution? HTTP Protocol. It allows us to transfer from one link to another and similarly web server from one country to another without even knowing about the details of such movement to the users.This service that hides the underlying complexity from user is known as Distributed System.Distributed service is transparent to user where, no need to provide additional exact set of commands to make changes to files in remote servers.Network is a physical infrastructure on top of which the distributed systems are built. Page | 8 Chapter 2 : Network Fundamentals I.Home Network Applications:Refrigerator reminds about buying vegetablesStranger trying to break into your houseBabysitter is sleepingTV has a serious problem and it conveysElectric bulb tells us if it is going downLamp dims itself when you are asleepNight lamp automatically switches on and offSmart spectacles helps old peopleSmart KeysA phone receiver transfers the call automaticallyControl house via Internet or phone.The shower adjusts the temperatureSingle remote control for many home devicesIssues:RobustnessCostSecurity issuesRegular upgradesMaintenance and availabilityWired or WirelessHolding the vendor responsible J.Mobile Network Applications:MANETs has big set of applications.Exchanging notes with classmates with single click.Access to library from any location inside/outside college premises.VANETS, automatic intelligent solution for vehicles.Control home equipments using sms and android applications. Issue? Security and delay being mobile. C. Define Networks: The computer network can be defined as a collection of computing devices (nodes) interconnected by wires or other means and governed by a set of standards (protocols) in order to share the data and resources. D. Categorize networks based on multiple factors. 1. Division based on Scope2. Division based on Connection3. Division based on Communication4. Division based on Usage 1. Network Division Based on Scope: a) PANb) LANc) MANd) WAN Page | 9 Chapter 2 : Network Fundamentals a. Personal Area Network +Network meant for one person. +Eg: Connecting Wireless mouse, keyboard, printer to computer. b. Local Area Network: +Privately owned networks +Size: few kilo meters +Widely used in? +To connect PC to warehouse in company and factories to share resources. +Restricted in Size. +Transmission Technology generally used is? +Cables. +Traditional LANs: oSpeed, 10 Mbps to 100 Mbps.pLow delay (micro second or nano second)qMake few errors.+Newer LANs: oSpeed Upto 40 Gbps. c. Metropolitan Area Networks A metropolitan area network based on cable TV. Covers a City Eg: Cable Television Network. Page | 10 Chapter 2 : Network Fundamentals Wireless Internet Connectivity. _Wired n/w got converted to Wireless n/w technology. _ Centralized Head End. d. Wide Area Networks I_Relation between hosts on LANs and the subnet. _Spans large geographical area, country or continent. _Purpose: Send message from Host to Host. _ Who fulfill this purpose? Subnet. _ Subnet Components: _ 1. Transmission Lines _Eg: optical fiber, co-axial cable __2. Switched Elements ___Eg: Router, Switch, Hub. __Two important characteristics of Subnet: _I_1. Store & Forward Subnet. __2. Packet Switched Subnet. __1. Store & Forward Subnet: What if congestion exist for line where packet is to be sent? Router stores that packet until congestion is resolved and once resolved, it forwards packets to that line. This is called Store & Forward Subnet. __2. Packet Switched Subnet: A stream of packets from sender to receiver. Packets follow route ACE, rather than ABDE or ACDE. Some n/w predetermine route that is to be followed. First decision is taken by A with the help of some routing algorithm and then by C. All WANs are not packet switched. See next figure. Page | 11 Chapter 2 : Network Fundamentals 2. Division Based on Connection: a) Wiredb) Wirelessi) Sensor Networksii) MANETs a) Wired Networks: Need some sort of cables for connectivity.Most popular are UTP (Unshielded Twisted Pair) and Fiber Optics.Fast Ethernet provides 100MB/s, Gigabit Ethernet provides 1Gbps/s, 10G provides 10Gbps/s and 40/100G would come to Indian market soon.Fiber Optics provides nearly 5000 Times more bandwidth of conventional telephone cables. b) Wireless Networks: First wireless network was ALOHA.Ethernet is successor of ALOHA.Other networks like Wi-Fi (802.11), Wireless Fidelity providing LAN Interpretability is widely accepted wireless networks.WiMax (802.16) wireless microwave access is covering large area of entire city. Page | 12 Chapter 2 : Network Fundamentals i) MANets: Mobile Ad Hoc NetworksNodes can come and join at their own will and leave on their own will.Services Provided are as follows:oOne can access internet from hotel by joining hotel network oOne can join network of railway station with session of 15 minutesStudent can access network of college from parking area or lobbyoWe can use internet during flight n share the files with co-passengers.oStudents can exchange class notes without writing it manually.Issues in MANets:oTopology is dynamicPower Consumption is important concern for mobile nodes. When A is communicating with C through intermediate node B, if the battery of B is low, it would not forward packets of A to C.Security must be more stringent with such networks as we expect guest to join the network any time.oWar Driving: It is a common Mechanism deployed by hackers to join wireless networks and extract information that is vital. This process is described as War Driving. ii) Sensor Networks: Sensor Networks are special type of Ad Hoc Networks, where the members of this network are tiny sensors rather than mobile computers or large devices.Sensors are sometimes mobile and sometimes stationary.Eg. Temperature Sensors detect temperature and sends it to some other nodes.Sensors are designed with much less memory and processing power is also too small. 3) Division Based on Communication Type: a) Point to Point b ) Multicast c) Broadcast a)Point to Point: P2P requires to know where the recipient isSender must know where each router isSimilarly each router must be aware of where other routers are locatedFor inherently broadcast network P2P communication is not possible.Little advantage for high bandwidth case.See next figure. Page | 13 Chapter 2 : Network Fundamentals b) Broadcast Networks: Better when a very low probability of more than one user transmitting simultaneouslyIt is useful when the network is inherently a broadcast network.Better when routing through neighbours or addressing each node of the network is not requiredBroadcasting helps when the topology is not fixed.See the next two given figures. Page | 14 Chapter 2 : Network Fundamentals Page | 15 Chapter 2 : Network Fundamentals 4. Division based on Usage: a) Home Networksb) Sensor Networksc) Other Networks a) Home Networks Refrigerator reminds about buying vegetablesStranger trying to break into your houseBabysitter is sleepingTV has a serious problem and it conveysElectric bulb tells us if it is going downLamp dims itself when you are asleepNight lamp automatically switches on and offSmart spectacles helps old peopleSmart KeysA phone receiver transfers the call automaticallyControl house via Internet or phone.The shower adjusts the temperatureSingle remote control for many home devices Home Networking issues: RobustnessCostSecurity issuesRegular upgradesMaintenance and availabilityWired or WirelessHolding the vendor responsible b) Sensor Networks: Used for specific purpose like monitoring a process or recording the outcomes of continuous experiment.What is the need of sensor networks?oFire erupts in a warehouse in an industry can call fire brigade by using sensor networks. oIf sudden rainfall in Rajkot, can cause adverse effect to the farm, to what extent of humidity is there in the farm, can be determined by sensor networksoAfter the earthquake, building with sensors can monitor record vibration data to confirm that structure is safe to enter.Sensor networks have large number of nodes.Sensor networks have no fixed topology.Sensor nodes have less memory and processing power. Page | 16 Chapter 2 : Network Fundamentals c) Other Networks: Pervasive computing and research on wearable computers are helping.Pervasive networking means, if you are out on vacation, all the emails are copied to yr outlook and they are msged that u r on vacation.Wearable computers are those which can be worn on the body like a dress.Microprocessors are also the example for it.Passive Environment becomes Active: eg. U enter the room and light automatically switches on and light music starts playing.In todays scenario, toll booth have long queues. But if we have sensors, RFID can just pass by, all the details are recorded, and toll is deducted from the RFID card.Phones are used to check emails and computers answer the phone calls. E. Differentiate between MANETS and SENSORS: Points of DifferenceMANETsSENSORS Number of nodes?Small (100s)Large (1000s) Mobility?Low (Fixed)High (Mobile) Processing Power?LowHigh Memory Embedded inside the nodes?LowHigh Addressability to the nodes are clear or confusing?ConfusingClear Power Consumption by nodes?LowHigh Routing Empowerment? Is complex routing algo can beNoYes implemented? Requirement of security to nodes? Are the nodes subjects to beNeed MoreNo. Chances are captured? Do we need to them to be configured under strictSecurityless. vigilance? Size of the nodes?Small (inLarge (in centimeters)meters) F. Network Components: a) NICb) Cables for Wired Communicationc) Frequency Band for Wireless Communicationd) Servers and Nodese) Interconnecting Devicesi) Hubii) Switchiii) Repeateriv) Bridgev) Router a) NIC: The first requirement of connectivity at nodes end is Network Interface Card NIC or LAN Card.Eg. Ethernet Card (Wired ) and Centrino Card (Wireless). Page | 17 Chapter 2 : Network Fundamentals b) Cables for Wired Communication: There is a LAN cable slot in motherboard on which network cable fits. Cables can be for different types. Eg. UTP (Cat 3,5,6,7), Copper Cable (Thin and Thick) and Fiber Optics. c) Frequency Band for Wireless Communication: More the frequency more is the data rate. Rights has to be taken from Government of India, body DoT (Department of Telecommunication) to transmit data using frequency. d) Servers and Nodes: Networking machine in networking parlance is known as a node or host. We have LAN card and wire to connect. Nodes giving service is server and nodes demanding service is client. e) Interconnecting Devices: Its not possible to get connected to every client directly. So we need some interconnecting device that connects each of us. i) Hub: A Hub is an intermediate device that connects to server with its clients through wires. It has multiple ports. It is broadcast by nature. Impose network load as do not store addresses of communicating clients to its memory. ii) Switch: Switch is an interconnecting device that supports P2P communication in a local network. Switch stores the information of its connected nodes in its buffer so that it can utilize it for point to point communication. It reduces network load. Page | 18 Chapter 2 : Network Fundamentals iii) Repeater: Signals strength reduces gradually by covering the distance. The device that reshapes the digital signal is called a repeater. For this, Amplifiers are needed by analog signals. Repeaters also remove noise from the digital signals which is not done by amplifiers for analog signals. iv) Bridge: Special device called bridge is used to connect more networks. A bridge is similar to switch but is designed to provide additional services like broadcasting in a particular LAN segment, finding out the shortest path between different LAN segments. A bridge keeps broadcast in network limited to that network only. It keeps, logically separate segments physically separate. Bridge operates upon Data Link Layer. v) Router: A router is a device or, in some cases, software in a computer, that determines the next network point to which a packet should be forwarded toward its destination. A router may create or maintain a table of the available routes and their conditions and use this information along with distance and cost algorithms to determine the best route for a given packet. vi) Gateway: Gateways are conventionally used to connect the networks. It operates on higher layers like transport layer and application layer. It provide an application seamless access to remote networks. Eg. Proxy Server. Two types of gateways are there. 1) Transport gateway: connect two computers using different connection-oriented transport protocols (TCP, ATM). 2) Application gateway: translate format and contents of data. E.g., email gateway: translate Internet message into SMS for mobile phones. Page | 19 Chapter 3: Data Communication Fundamentals Topic List: Topic List, IntroductionBandwidth & Data RateAnalog & Digital SignallingAnalog & Digital TransmissionCoding MechanismModulationModulation in Practice A. Bandwidth & Data Rate:(a) Bandwidth: Represents the range of frequencies that can pass thru a medium.More the bandwidth larger the data it can transmit.FixedEg. Wider the road, larger #Vehicles accommodated.(b) Data Rate: Data rate is the actual traffic passing at given time.Purely depends on bandwidth.Variable.Eg: Driver asking for infinite speed (bandwidth) for transmission.Data Rate depends on:oBandwidth utilization o If Analog, #constellation points o If Digital, Number of levels oReceivers sensitivity Media resistance and temperature B. Frequency & Band:Signal travels thru a medium in range of frequencies.This range of frequencies is called frequency band.If we try to accommodate more radio stations in a band, they become too noisy.Similarly for LANs, limits the #users. C. Media Effect on BandwidthDistortion: Due to elimination of higher frequency components from signal results to distortion.Attenuation: Adding more number of different frequency causes attenuation.Swimmers lane: Middle lanes provide less resistance than outer lanes. Larger the frequency, more the noise, smaller the frequency, larger the data transmission rate. D. Harmonics and composite wave:Fourier Component: Components of a composite signal are known as Fourier Component.Composite Signals: When a signal contains multiple frequencies, it is called composite signals.Fundamental Frequency: Frequency of first harmonic (Fourier Component) is knows as Fundamental Frequency.If first component is f, 2nd is 3f and 3rd is 5f.Frequency is inversely proportional to amplitude.Large amplitude signals cover smaller distance. Page | 20 Chapter 3: Data Communication Fundamentals c)Properties of Channel:i)More the bandwidth of the media, more the number of harmonics that can pass through the media.ii)Higher the data rate, less will be the number of harmonics that can pass through the media.iii) Baud Rate: #signals that passes thru media in unit time, usually in seconds. As bandwidth is fixed, Baud Rate is also fixed.iv) Bit Rate: #bits that passes thru a media in unit time, usually in seconds. If 1 signal carry n bits then Baud Rate is 1 and Bit Rate is n.v)Signal: Combination of zeros and ones.vi) Maximum Baud Rate: No of harmonics that can be accommodated in a given bandwidth.vii) We can obtain Maximum Data Rate (MDR) if we know bandwidth and sensitivity of receiver (#harmonics recognized correctly).viii)Data rate is doubled when baud rate is quadrupled, tripled when baud rate is 8 times, and so on. d) Baud Rate & Bit Rate Relation:Nyquist: MDR of a channel = 2 BandwidthMDR of a channel = 2 Bandwidth log2 (signal levels) Claude Shannon:MDR of a channel = Bandwidth log2 (1 + S/N)Signal to noise ratio (decibel): When signal to noice ratio is equal:S/N =10 log (Ps/Pn)Where, Ps Signal Power, Pn Noise PowerWhen Ps/Pn becomes 1MDR = Bandwidth log2 (1+10 log e 1)Bandwidth log 2 (1+0)Bandwidth log 2 1Page | 21 Chapter 3: Data Communication Fundamentals Filtering a specific frequency:Filter is a device used for restricting bandwidth of a medium to specified limit.Used with telephone lines,Actual bandwidth = 1.1 MHzUsing filter reduced to = 4 kHzRemaining channel can be used for other telephone connections data transmission at a time.Remaining channel can also be used for broadband.Thats y we feel voice interference in laneline phone when internet is on.DSL (Digital Subscribers Line) removes the filter from line to use entire bandwidth for internet access. Filter Types:1. Low Pass Filter: Such filters keep only lower part of frequency out of given range.2. Band Pass Filter: keeps middle part of frequency.3. High Pass Filter: eliminates lower parts to keep only higher part of frequency.Widely used is low pass filter. H. Analog Signals:Used to transmit video and audio signals.Higher error rate.Continuous by nature.Uses curved wave forms.Used when we dont have large bandwidth.High error rate due to sine property.Amplifiers are used which gives strength to signal but cant correct the signals.Three different properties:Amplitude: signals max voltage value.Frequency: #oscillations per second.Phase: direction of a signalCan be used to transmit digital data also.Also supports modulation techniques.Technique to represent zeros and ones using one or more properties of wave is called modulation. Page | 22 Chapter 3: Data Communication Fundamentals Digital Signals:oUsed when we have large bandwidth.pUsed to transfer (0,1) bits generally for file transfer.qLow error rate.rDiscrete by nature.sSquare wave forms.tRepeaters are used to give strength to signal which can also correct signals.uHow to transfer digital data over wire?Using electromagnetic wavesUsing lightvMP3 (Music Player), MPEG (Moving Picture Expert Group) are the types where audio video signals are converted from analog to digital.wThese signals can be of two types: Periodic and A periodic.xPeriodic Signals: Continuously repeats its shape after a specified interval. Used for data transmission. yA-Periodic Signals: does not follow pattern. Digital signaling and bit rate: Baud Rate: #voltage signals we send per second is known as baud rate.Bit Rate: #bits sent per second are known as bit rate.If we use 2n different voltage signals, thenbit rate = baud rate n If we use n voltage signals, thenbit rate = baud rate log2 n n is number of levels.Suppose we have 4 different voltage levels, say, 1,2,-1,-2.1v = 10, 2v=11, -1v=01, -2v=00.This means, 4(voltage) levels can represent 8(combination of 2) bits of data.Suppose it takes 0.1 msec then voltage transmitted are 10 Mbps, but total bits transmitted is 20 Mbps.Characteristics: Square waveforms Limited discrete values of voltage levels Page | 23 Chapter 3: Data Communication Fundamentals iiNeed more bandwidthiiiNoise can be completely removed Repeater reshapes(Digital), amplifier amplifies(Analog): Amplification: Noise addition to waves Analog Signal TransmissionDigital Signal Transmission Used to transmit video and audio signals.Used to transfer (0,1) bits generally for file transfer. Used when we dont have large bandwidth.Used when we have large bandwidth. Higher error rate due to sine property.Low error rate. Continuous by nature.Discrete by nature. Uses curved wave forms.Square wave forms. Can travel short distance.Can travel long distance. Amplifiers are used which gives strength to signalRepeaters are used to give strength to signal which but cant correct the signals.can also correct signals. Analog signals can be used for digitalDigital signals can be used for analog transmission. E.g.; Modem sends digital data overtransmission. E.g. You tube, Skype transmitting analog telephone lines.audio video signals using digital signals. J. Modulation: Amplitude modulationFrequency modulationPhase ModulationModulation in PracticeModulation in cable TVModulation in ADSL Page | 24 Chapter 3: Data Communication Fundamentals Amplitude modulation: 5 represent 0 and 10 represents 1.Easy to implement.Problems:o Noisy channel, difficult to correct errors. o Impossible to detect errors also. oEffects of temperature, rain, etc. Distortion in digital signals: Digital error occurs.Atmospheric disturbance, electromagnetic disturbance (mobile beside tv),Faulty circuitry, etc results to problems like crosstalk and noise.It is possible to reshape the signals after some distance using repeaters. Frequency modulation Two different frequency representing 0 and 1.f0 has more oscillations than 1. Page | 25 Chapter 3: Data Communication Fundamentals Phase Modulation Out of phase situation for all three signals. Done by shifting the phase at regular interval. See next figure. Justify the statement: Phase modulation is better than other modulation techniques: Unlike frequency modulation, it doesn't require two different frequencies. (So better than frequency modulation)If error occurs and amplitude changes, will not have any adverse effect. (Better than amplitude modulation).In Amplitude modulation, gap between two amplitudes must be high, to transfer that signal, more power is consumed.Phase modulation, less power consumption.Thus, Phase modulation is better than other modulation techniques. Modulation in Practice (a) Modulation in cable TV(b) Modulation in ADSL Constellation pattern for QPSK: QPSK: Quadrature Phase Shift Keying.Total #bits sent are twice the total #signals. (i.e. while sending x signals, 2x bits sent).Only mechanism where, amplitude is not affected.Four phase values: 45*, 135*, 225* and 315*.Distance from origin indicates amplitude (constant). Page | 26 Chapter 3: Data Communication Fundamentals QAM-16 constellation pattern QAM-16 Quadrature Amplification Modulation.Combination of phase and amplitude.16 combinations represented by different combination of phase and amplitude.Arrow represents phase.Curves represent amplitude.3 amplitude and 3 phase values represent 4 constellation points in one quadrant.Such 4 quadrants (4x4) makes it 16.Similarly we can have QAM-64 using 16 bits in each quadrant and QAM-256 with 64 bits in each quadrant.Disadvantage: More the constellation points in constellation diagram, more vulnerable to noise. Vulnerability increases from QPSK, QAM-16, QAM-64, QAM-256...Thus, prefer QPSK for noisy channels and prefer QAM-256 for noiseless channel. (a) Modulation in Cable TV: Cable TV uses analog signals to transmit.Thick black TV cables having bandwidth 650 to 700 Mbps has much spare bandwidth other than utilized by TV channel signals that can be utilized for Internet traffic.This brought the era of Cable Internet.See the next figure.Up-stream uses QPSK, downstream transmission uses QAM-16 or QAM-256.Frequency bands used in cable TV transmission: Page | 27 Chapter 3: Data Communication Fundamentals (b) Modulation in ADSL: Asymmetric Digital Subscriber Line.Technique that uses telephone lines for voice communication as well as internet connectivity.Telephone lines use small fraction of bandwidth.i.e 4KHz out of 1.1 MHz.Filters are kept to use remaining bandwidth for data transmission over internet.If no telephone needed, filters are removed and entire bandwidth is used for internet.Even division of bandwidth used for internet traffic need to be distributed into Upstream and Downstream data transmission.When division is equal for Upstream and Downstream, SDSL Symmetric Digital Subscriber Line is used.Generally preferred is more downstream and less upstream bandwidth for which A-symmetric Digital Subscriber Line is used.See previous figure for distribution. Q. What is Multiplexing and Demultiplexing? The process of collecting individual connections at one end and distributing at the other end is known as multiplexing and demultiplexing.Examples:Proxy Server multiplexes users URL request and sends to next router in chainoWindows Server , Linux Server, client.oRemote telnet serveroFTP Client connecting to FTP ServeroHTTP Client talking to Web ServeroOracle or SQL Client connecting to Remote Database Server.oSingle TCP Process handling multiple ApplicationsoUDP application running VoIP and video conferencing application.oA switch relies on multiple desktops in LAN relies on multiplexing to combine all transmission.Two types:oTime Division MultiplexingoFrequency Division Multiplexing oTime Division Multiplexing: TDM divides available spectrum such that each user gets a specific time slot to transmit its data. Each sender gets its turn once in a cycle. When specific users turn comes, entire bandwidth is given to him. This multiplexing does not impose any kind of delay. TDM is useful when the data is transmitted using digital signaling. oFrequency Division Multiplexing: in FDM, each user is given an exclusive frequency band. Each channel is given a specific frequency. The entire spectrum available is multiplexed for all these Stations. While multiplexing, the channel frequencies are separated by guard bands to prevent interference from adjacent stations. FDM is used with analog signaling for data transmission.oWDM: A variant of FDM known as WDM (Wavelength Division Multiplexing ) is famous for fiber optics which provides 100 channels of 10 Gbps each. WDM also provides quality of service.oGuard Bands: While multiplexing, the channel frequencies are separated by guard bands to prevent interference from adjacent station. Page | 28 Chapter 3: Data Communication Fundamentals Q. Write a note on switching. Switching is an operation where the decision of where to send the incoming data is instantaneous and predetermined. In routing, decision is taken dynamically and hence service is little delayed than switching.Switching is generally done at second layer.Layer 4 and Layer 5 are transport and application layer where data are populated.Layer 4 switch separate UDP and TCP data flows.Layer 5 switch send mail traffic in one direction and http traffic in another direction.Layer 3 switches are like routers.Telephone Lines use switches to route the traffic at physical layer.Though switching is faster, it is monotonous.Data does not flow continuously and so, it needs store and forwarding transmission facility.Types: 1) Circuit Switching 2) Packet Switching 3) Message Switching. Circuit Switching Packet Switching 1) Circuit Switching: In circuit switching, there is a dedicated line that connects the sender and the receiver.Here, path is reserved for duration of connection. Page | 29 Chapter 3: Data Communication Fundamentals Eg. Telephonic Conversation.Advantages of Circuit Switching: o Guaranteed bandwidthoPredictable communication performanceo Not best-effort delivery with no real guarantees o Simple abstractiono Reliable communication channel between hosts o No worries about lost or out-of-order packets o Simple forwardingo Forwarding based on time slot or frequency o No need to inspect a packet headeroLow per-packet overheado Forwarding based on time slot or frequency o No IP (and TCP/UDP) header on each packetDisadvantages of Circuit Switching:oWasted bandwidth o Bursty traffic leads to idle connection during silent period o Unable to achieve gains from statistical multiplexing oBlocked connections o Connection refused when resources are not sufficient o Unable to offer okay service to everybody oConnection set-up delay oNo communication until the connection is set up o Unable to avoid extra latency for small data transfers o Network state o Network nodes must store per-connection information o Unable to avoid per-connection storage and state Page | 30 Chapter 3: Data Communication Fundamentals 2) Message Switching: It is possible to have packets that are large enough to hold entire message. Such a mechanism is not in practice is known as message switching.Not used nowadays.Communication is not continuous.Each packets travel independently.No physical path is established in advance between sender and receiver.Instead, when the sender has a block of data to be sent, it is stored in the first switching office (i.e., router) and then forwarded later, one hop at a time.Each block is received in its entirety, inspected for errors, and then retransmitted.A network using this technique is called a store-and-forward network.Used with telegrams. 3) Packet Switching: Doesnt reserve entire path in advance.Reserves only small portion of path.Data is segregated into pieces and transmitted as packets.In packet-based networks, the message gets broken into small data packets. These packets are sent out from the computer and they travel around the network seeking out the most efficient route to travel as circuits become available. This does not necessarily mean that they seek out the shortest route. Page | 31 Chapter 3: Data Communication Fundamentals Each packet may go a different route from the others.Each packet is sent with a header address. This tells it where its final destination is, so it knows where to go.The header address also describes the sequence for reassembly at the destination computer so that the packets are put back into the correct order.One packet also contains details of how many packets should be arriving so that the recipient computer knows if one packet has failed to turn up.If a packet fails to arrive, the recipient computer sends a message back to the computer which originally sent the data, asking for the missing packet to be resent.Advantages:oSecurity oBandwidth used to full potential oDevices of different speeds can communicateoNot affected by line failure (rediverts signal)oAvailability do not have to wait for a direct connection to become availableoDuring a crisis or disaster, when the public telephone network might stop working, e-mails and texts can still be sent via packet switchingDisadvantagesoUnder heavy use there can be a delayo Data packets can get lost or become corrupted o Protocols are needed for a reliable transfer oNot so good for some types data streams e.g real-time video streams can lose frames due to the way packets arrive out of sequence. Q. Differentiate between Circuit Switching and Packet Switching. Page | 32 Chapter 3: Data Communication Fundamentals Q. Define Attenuation, Distortion and Noise: 1) Attenuation: Change in shape of a signal due to loss of energy after travelling some distance. Different fouier components have different attenuation and thus the resultant signal not only gets diminished, but also acquired signal requires reshaping. This is known as attenuation. Attenuated signals require reshaping. Attenuation also depends on media. 2) Distortion: A signal is said to be distorted when different frequency components reach at the other end at different times. This effect is known as distortion. The speed mismatch between different components changes the shape of the signals. This is known as delayed distortion or distortion in short. 3) Noise: It is the error produced in a signal due to various reasons. Noise is the external interference which changes the shape of the signal.Thermal Noise: The disturbance caused due to electrons present in the media. This noise depends on the temperature. More the temperature, more is the thermal Noise.Non-Thermal Noise: It is caused due to other external reasons like cross-talking, malfunctioning of devices, etc.Noise is also used as a synonym of errors. Eg. An image with noise means an image with errors.