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Data Communications
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Frequency, Spectrum and Bandwidth
Time domain conceptsContinuous signal
Various in a smooth way over time
Discrete signalMaintains a constant level then changes to another constant level
Periodic signalPattern repeated over time
Aperiodic signalPattern not repeated over time
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Continuous & Discrete Signals
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Periodic Signals
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Wavelength
Distance occupied by one cycleDistance between two points of corresponding phase in two consecutive cyclesλAssuming signal velocity v
λ = vTλf = vc = 3*108 ms-1 (speed of light in free space)
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Frequency Domain Concepts
Signal usually made up of many frequenciesComponents are sine wavesCan be shown (Fourier analysis) that any signal is made up of component sine wavesCan plot frequency domain functions
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Spectrum & BandwidthSpectrum
range of frequencies contained in signalAbsolute bandwidth
width of spectrumEffective bandwidth
Often just bandwidthNarrow band of frequencies containing most of the energy
DC ComponentComponent of zero frequency
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Data Rate and Bandwidth
Any transmission system has a limited band of frequenciesThis limits the data rate that can be carried
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Analog and Digital Data Transmission
Data Entities that convey meaning
SignalsElectric or electromagnetic representations of data
TransmissionCommunication of data by propagation and processing of signals
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Data
AnalogContinuous values within some intervale.g. sound, video
DigitalDiscrete valuese.g. text, integers
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Acoustic Spectrum (Analog)
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Analog Transmission
Analog signal transmitted without regard to contentMay be analog or digital dataAttenuated over distance Use amplifiers to boost signalAlso amplifies noise
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Digital Transmission
Concerned with contentIntegrity endangered by noise, attenuation etc.Repeaters usedRepeater receives signalExtracts bit patternRetransmitsAttenuation is overcomeNoise is not amplified
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Advantages of Digital Transmission
Digital technologyLow cost LSI/VLSI technology
Data integrityLonger distances over lower quality lines
Capacity utilizationHigh bandwidth links economicalHigh degree of multiplexing easier with digital techniques
Security & PrivacyEncryption
IntegrationCan treat analog and digital data similarly
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Encoding Techniques
Digital data, digital signalAnalog data, digital signalDigital data, analog signalAnalog data, analog signal
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Digital Data, Digital Signal
Digital signalDiscrete, discontinuous voltage pulsesEach pulse is a signal elementBinary data encoded into signal elements
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Terms (1)Unipolar
All signal elements have same sign
PolarOne logic state represented by positive voltage the other by negative voltage
Data rateRate of data transmission in bits per second
Duration or length of a bitTime taken for transmitter to emit the bit
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Terms (2)
Modulation rateRate at which the signal level changesMeasured in baud = signal elements per second
Mark and SpaceBinary 1 and Binary 0 respectively
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Interpreting Signals
Need to knowTiming of bits - when they start and endSignal levels
Factors affecting successful interpreting of signals
Signal to noise ratioData rateBandwidth
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Digital Data, Analog Signal
Public telephone system300Hz to 3400HzUse modem (modulator-demodulator)
Amplitude shift keying (ASK)Frequency shift keying (FSK)Phase shift keying (PK)
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Modulation Techniques
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Amplitude Shift Keying
Values represented by different amplitudes of carrierUsually, one amplitude is zero
i.e. presence and absence of carrier is used
Susceptible to sudden gain changesInefficientUp to 1200bps on voice grade linesUsed over optical fiber
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Frequency Shift Keying
Values represented by different frequencies (near carrier)Less susceptible to error than ASKUp to 1200bps on voice grade linesHigh frequency radioEven higher frequency on LANs using co-ax
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FSK on Voice Grade Line
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Phase Shift Keying
Phase of carrier signal is shifted to represent dataDifferential PSK
Phase shifted relative to previous transmission rather than some reference signal
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Quadrature PSK
More efficient use by each signal element representing more than one bit
e.g. shifts of π/2 (90o)Each element represents two bitsCan use 8 phase angles and have more than one amplitude9600bps modem use 12 angles , four of which have two amplitudes
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Performance of Digital to Analog Modulation Schemes
BandwidthASK and PSK bandwidth directly related to bit rateFSK bandwidth related to data rate for lower frequencies, but to offset of modulated frequency from carrier at high frequencies
In the presence of noise, bit error rate of PSK and QPSK are about 3dB superior to ASK and FSK
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Analog Data, Digital Signal
DigitizationConversion of analog data into digital dataDigital data can then be transmitted using NRZ-LDigital data can then be transmitted using code other than NRZ-LDigital data can then be converted to analog signalAnalog to digital conversion done using a codecPulse code modulationDelta modulation
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Pulse Code Modulation(PCM) (1)
If a signal is sampled at regular intervals at a rate higher than twice the highest signal frequency, the samples contain all the information of the original signal
Voice data limited to below 4000HzRequire 8000 sample per secondAnalog samples (Pulse Amplitude Modulation, PAM)Each sample assigned digital value
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Pulse Code Modulation(PCM) (2)
4 bit system gives 16 levelsQuantized
Quantizing error or noiseApproximations mean it is impossible to recover original exactly
8 bit sample gives 256 levelsQuality comparable with analog transmission8000 samples per second of 8 bits each gives 64kbps
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Nonlinear Encoding
Quantization levels not evenly spacedReduces overall signal distortionCan also be done by companding
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Delta Modulation
Analog input is approximated by a staircase functionMove up or down one level (δ) at each sample intervalBinary behavior
Function moves up or down at each sample interval
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Delta Modulation - example
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Delta Modulation -Performance
Good voice reproduction PCM - 128 levels (7 bit)Voice bandwidth 4khzShould be 8000 x 7 = 56kbps for PCM
Data compression can improve on thise.g. Interframe coding techniques for video
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Analog Data, Analog Signals
Why modulate analog signals?Higher frequency can give more efficient transmissionPermits frequency division multiplexing Types of modulationAmplitudeFrequencyPhase
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Analog Modulation
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Transmission Impairments
Signal received may differ from signal transmittedAnalog - degradation of signal qualityDigital - bit errorsCaused by
Attenuation and attenuation distortionDelay distortionNoise
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AttenuationSignal strength falls off with distanceDepends on mediumReceived signal strength:
must be enough to be detectedmust be sufficiently higher than noise to be received without error
Attenuation is an increasing function of frequency
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Delay Distortion
Only in guided mediaPropagation velocity varies with frequency
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Noise (1)
Additional signals inserted between transmitter and receiverThermal
Due to thermal agitation of electronsUniformly distributedWhite noise
IntermodulationSignals that are the sum and difference of original frequencies sharing a medium
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Noise (2)
CrosstalkA signal from one line is picked up by another
ImpulseIrregular pulses or spikese.g. External electromagnetic interferenceShort durationHigh amplitude
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Channel Capacity
Data rateIn bits per secondRate at which data can be communicated
BandwidthIn cycles per second of HertzConstrained by transmitter and medium
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Encoding Schemes
Nonreturn to Zero-Level (NRZ-L)Nonreturn to Zero Inverted (NRZI)Bipolar -AMIPseudoternaryManchesterDifferential ManchesterB8ZSHDB3
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Nonreturn to Zero-Level (NRZ-L)
Two different voltages for 0 and 1 bitsVoltage constant during bit interval
no transition I.e. no return to zero voltage
e.g. Absence of voltage for zero, constant positive voltage for oneMore often, negative voltage for one value and positive for the otherThis is NRZ-L
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Nonreturn to Zero Inverted
Nonreturn to zero inverted on onesConstant voltage pulse for duration of bitData encoded as presence or absence of signal transition at beginning of bit timeTransition (low to high or high to low) denotes a binary 1No transition denotes binary 0An example of differential encoding
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NRZ
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Differential Encoding
Data represented by changes rather than levelsMore reliable detection of transition rather than levelIn complex transmission layouts it is easy to lose sense of polarity
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NRZ pros and cons
ProsEasy to engineerMake good use of bandwidth
Consdc componentLack of synchronization capability
Used for magnetic recordingNot often used for signal transmission
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Multilevel BinaryUse more than two levelsBipolar-AMI
zero represented by no line signalone represented by positive or negative pulseone pulses alternate in polarityNo loss of sync if a long string of ones (zeros still a problem)No net dc componentLower bandwidthEasy error detection
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Pseudoternary
One represented by absence of line signalZero represented by alternating positive and negativeNo advantage or disadvantage over bipolar-AMI
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Bipolar-AMI and Pseudoternary
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Trade Off for Multilevel Binary
Not as efficient as NRZEach signal element only represents one bitIn a 3 level system could represent log23 = 1.58 bitsReceiver must distinguish between three levels (+A, -A, 0)Requires approx. 3dB more signal power for same probability of bit error
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BiphaseManchester
Transition in middle of each bit periodTransition serves as clock and dataLow to high represents oneHigh to low represents zeroUsed by IEEE 802.3
Differential ManchesterMidbit transition is clocking onlyTransition at start of a bit period represents zeroNo transition at start of a bit period represents oneNote: this is a differential encoding schemeUsed by IEEE 802.5
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Biphase Pros and Cons
ConAt least one transition per bit time and possibly twoMaximum modulation rate is twice NRZRequires more bandwidth
ProsSynchronization on mid bit transition (self clocking)No dc componentError detection
Absence of expected transition
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Modulation Rate
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Scrambling
Use scrambling to replace sequences that would produce constant voltageFilling sequence
Must produce enough transitions to syncMust be recognized by receiver and replace with originalSame length as original
No dc componentNo long sequences of zero level line signalNo reduction in data rateError detection capability
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B8ZS
Bipolar With 8 Zeros SubstitutionBased on bipolar-AMIIf octet of all zeros and last voltage pulse preceding was positive encode as 000+-0-+If octet of all zeros and last voltage pulse preceding was negative encode as 000-+0+-Causes two violations of AMI codeUnlikely to occur as a result of noiseReceiver detects and interprets as octet of all zeros
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HDB3
High Density Bipolar 3 ZerosBased on bipolar-AMIString of four zeros replaced with one or two pulses
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B8ZS and HDB3
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Guided Transmission Media
Twisted PairCoaxial cableOptical fiber
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Twisted Pair
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Twisted Pair - Applications
Most common mediumTelephone network
Between house and local exchange (subscriber loop)
Within buildingsTo private branch exchange (PBX)
For local area networks (LAN)10Mbps or 100Mbps
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Twisted Pair - Pros and Cons
CheapEasy to work withLow data rateShort range
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Twisted Pair - Transmission Characteristics
Analog Amplifiers every 5km to 6km
DigitalUse either analog or digital signalsrepeater every 2km or 3km
Limited distanceLimited bandwidth (1MHz)Limited data rate (100MHz)Susceptible to interference and noise
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Unshielded and Shielded TPUnshielded Twisted Pair (UTP)
Ordinary telephone wireCheapestEasiest to installSuffers from external EM interference
Shielded Twisted Pair (STP)Metal braid or sheathing that reduces interferenceMore expensiveHarder to handle (thick, heavy)
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UTP CategoriesCat 3
up to 16MHzVoice grade found in most officesTwist length of 7.5 cm to 10 cm
Cat 4up to 20 MHz
Cat 5up to 100MHzCommonly pre-installed in new office buildingsTwist length 0.6 cm to 0.85 cm
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Near End Crosstalk
Coupling of signal from one pair to anotherCoupling takes place when transmit signal entering the link couples back to receiving pairi.e. near transmitted signal is picked up by near receiving pair
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Coaxial Cable
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Coaxial Cable ApplicationsMost versatile mediumTelevision distribution
Ariel to TVCable TV
Long distance telephone transmissionCan carry 10,000 voice calls simultaneouslyBeing replaced by fiber optic
Short distance computer systems linksLocal area networks
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Coaxial Cable - Transmission Characteristics
AnalogAmplifiers every few kmCloser if higher frequencyUp to 500MHz
DigitalRepeater every 1kmCloser for higher data rates
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Optical Fiber
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Optical Fiber - Benefits
Greater capacityData rates of hundreds of Gbps
Smaller size & weightLower attenuationElectromagnetic isolationGreater repeater spacing
10s of km at least
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Optical Fiber - Applications
Long-haul trunksMetropolitan trunksRural exchange trunksSubscriber loopsLANs
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Optical Fiber - Transmission Characteristics
Act as wave guide for 1014 to 1015 Hz Portions of infrared and visible spectrum
Light Emitting Diode (LED)CheaperWider operating temp rangeLast longer
Injection Laser Diode (ILD)More efficientGreater data rate
Wavelength Division Multiplexing
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Optical Fiber Transmission Modes
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Wireless Transmission
Unguided mediaTransmission and reception via antennaDirectional
Focused beamCareful alignment required
OmnidirectionalSignal spreads in all directionsCan be received by many antennae
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Frequencies2GHz to 40GHz
MicrowaveHighly directionalPoint to pointSatellite
30MHz to 1GHzOmnidirectionalBroadcast radio
3 x 1011 to 2 x 1014
InfraredLocal
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Terrestrial Microwave
Parabolic dishFocused beamLine of sightLong haul telecommunicationsHigher frequencies give higher data rates
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Satellite MicrowaveSatellite is relay stationSatellite receives on one frequency, amplifies or repeats signal and transmits on another frequencyRequires geo-stationary orbit
Height of 35,784km
TelevisionLong distance telephonePrivate business networks
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Broadcast Radio
OmnidirectionalFM radioUHF and VHF televisionLine of sightSuffers from multipath interference
Reflections
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Infrared
Modulate noncoherent infrared lightLine of sight (or reflection)Blocked by wallse.g. TV remote control, IRD port
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StandardsRequired to allow for interoperability between equipmentAdvantages
Ensures a large market for equipment and softwareAllows products from different vendors to communicate
DisadvantagesFreeze technologyMay be multiple standards for the same thing
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Standards Organizations
Internet SocietyISOITU-T (formally CCITT)ATM forum
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OSI - The Model
A layer modelEach layer performs a subset of the required communication functionsEach layer relies on the next lower layer to perform more primitive functionsEach layer provides services to the next higher layerChanges in one layer should not require changes in other layers
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The OSI Environment
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OSI as Framework for Standardization
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Layer Specific Standards
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Elements of Standardization
Protocol specificationOperates between the same layer on two systemsMay involve different operating systemProtocol specification must be precise
Format of data unitsSemantics of all fieldsallowable sequence of PCUs
Service definitionFunctional description of what is provided
AddressingReferenced by SAPs
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OSI Layers (1)
PhysicalPhysical interface between devices
MechanicalElectricalFunctionalProcedural
Data LinkMeans of activating, maintaining and deactivating a reliable linkError detection and controlHigher layers may assume error free transmission
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OSI Layers (2)Network
Transport of informationHigher layers do not need to know about underlying technologyNot needed on direct links
TransportExchange of data between end systemsError freeIn sequenceNo lossesNo duplicatesQuality of service
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OSI Layers (3)Session
Control of dialogues between applicationsDialogue disciplineGroupingRecovery
PresentationData formats and codingData compressionEncryption
ApplicationMeans for applications to access OSI environment
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Terminology (1)
TransmitterReceiverMedium
Guided mediume.g. twisted pair, optical fiber
Unguided mediume.g. air, water, vacuum
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Terminology (2)
Direct linkNo intermediate devices
Point-to-pointDirect link Only 2 devices share link
Multi-pointMore than two devices share the link
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Terminology (3)
SimplexOne direction
e.g. Television
Half duplexEither direction, but only one way at a time
e.g. police radio
Full duplexBoth directions at the same time
e.g. telephone
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A Communications ModelSource
generates data to be transmittedTransmitter
Converts data into transmittable signalsTransmission System
Carries dataReceiver
Converts received signal into dataDestination
Takes incoming data
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Simplified Communications Model - Diagram
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Key Communications TasksTransmission System UtilizationInterfacingSignal GenerationSynchronizationExchange ManagementError detection and correctionAddressing and routingRecoveryMessage formattingSecurityNetwork Management
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Simplified Data Communications Model
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Networking
Point to point communication not usually practical
Devices are too far apartLarge set of devices would need impractical number of connections
Solution is a communications network
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Simplified Network Model
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Local Area Networks
Smaller scopeBuilding or small campus
Usually owned by same organization as attached devicesData rates much higherUsually broadcast systemsNow some switched systems and ATM are being introduced
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LAN Applications (1)
Personal computer LANsLow costLimited data rate
Back end networks and storage area networks
Interconnecting large systems (mainframes and large storage devices)
High data rateHigh speed interfaceDistributed accessLimited distanceLimited number of devices
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LAN Applications (2)
High speed office networksDesktop image processingHigh capacity local storage
Backbone LANsInterconnect low speed local LANsReliabilityCapacityCost
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LAN Topologies
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Bus and TreeMultipoint mediumTransmission propagates throughout medium Heard by all stations
Need to identify target stationEach station has unique address
Full duplex connection between station and tapAllows for transmission and reception
Need to regulate transmissionTo avoid collisionsTo avoid hogging
Data in small blocks - framesTerminator absorbs frames at end of medium
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Frame Transmission - Bus LAN
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Ring TopologyRepeaters joined by point to point links in closed loop
Receive data on one link and retransmit on anotherLinks unidirectionalStations attach to repeaters
Data in framesCirculate past all stationsDestination recognizes address and copies frameFrame circulates back to source where it is removed
Media access control determines when station can insert frame
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Frame Transmission Ring LAN
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Star Topology
Each station connected directly to central node
Usually via two point to point links
Central node can broadcastPhysical star, logical busOnly one station can transmit at a time
Central node can act as frame switch
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Wide Area Networks
Large geographical areaCrossing public rights of wayRely in part on common carrier circuitsAlternative technologies
Circuit switchingPacket switchingFrame relayAsynchronous Transfer Mode (ATM)
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Circuit Switching
Dedicated communications path established for the duration of the conversatione.g. telephone network
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Packet Switching
Data sent out of sequenceSmall chunks (packets) of data at a timePackets passed from node to node between source and destinationUsed for terminal to computer and computer to computer communications
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Frame Relay
Packet switching systems have large overheads to compensate for errorsModern systems are more reliableErrors can be caught in end systemMost overhead for error control is stripped out
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Asynchronous Transfer Mode
ATMEvolution of frame relayLittle overhead for error controlFixed packet (called cell) lengthAnything from 10Mbps to GbpsConstant data rate using packet switching technique
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Protocols
Used for communications between entities in a systemMust speak the same languageEntities
User applicationse-mail facilitiesterminals
SystemsComputerTerminalRemote sensor
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Key Elements of a ProtocolSyntax
Data formatsSignal levels
SemanticsControl informationError handling
TimingSpeed matchingSequencing
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Characteristics
Direct or indirectMonolithic or structuredSymmetric or asymmetricStandard or nonstandard
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Direct or Indirect
DirectSystems share a point to point link orSystems share a multi-point linkData can pass without intervening active agent
IndirectSwitched networks orInternetworks or internetsData transfer depend on other entities
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Symmetric or Asymmetric
SymmetricCommunication between peer entities
AsymmetricClient/server
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Standard or Nonstandard
Nonstandard protocols built for specific computers and tasksK sources and L receivers leads to K*L protocols and 2*K*L implementationsIf common protocol used, K + L implementations needed
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Use of Standard Protocols
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Monolithic or Structured
Communications is a complex taskTo complex for single unitStructured design breaks down problem into smaller unitsLayered structure
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FunctionsEncapsulationSegmentation and reassmeblyConnection controlOrdered deliveryFlow controlError controlAddressingMultiplexingTransmission services
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Encapsulation
Addition of control information to dataAddress informationError-detecting codeProtocol control
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Segmentation (Fragmentation)Data blocks are of bounded sizeApplication layer messages may be largeNetwork packets may be smallerSplitting larger blocks into smaller ones is segmentation (or fragmentation in TCP/IP)
ATM blocks (cells) are 53 octets longEthernet blocks (frames) are up to 1526 octets long
Checkpoints and restart/recovery
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Why Fragment?
AdvantagesMore efficient error controlMore equitable access to network facilitiesShorter delaysSmaller buffers needed
DisadvantagesOverheadsIncreased interrupts at receiverMore processing time
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Connection Control
Connection EstablishmentData transferConnection terminationMay be connection interruption and recoverySequence numbers used for
Ordered deliveryFlow controlError control
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TCP/IP Protocol Architecture
Developed by the US Defense Advanced Research Project Agency (DARPA) for its packet switched network (ARPANET)Used by the global InternetNo official model but a working one.
Application layerHost to host or transport layerInternet layerNetwork access layerPhysical layer
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TCP/IP Protocol Architecture Model
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