Transmission Basics

ICSA 733: Week 3Transmission BasicsElizabeth Lane Lawley, Instructor

ICSA733: Fundamentals of Telecommunication (Lawley)

Data Communication BasicsAnalog or DigitalThree ComponentsDataSignalTransmission


Electromagnetic SignalsFunction of timeAnalog (varies smoothly over time)Digital (constant level over time, followed by a change to another level)Function of frequencySpectrum (range of frequencies)Bandwidth (width of the spectrum)


Periodic Signal CharacteristicsAmplitude (A): signal value, measured in voltsFrequency (f): repetition rate, cycles per second or HertzPeriod (T): amount of time it takes for one repetition, T=1/fPhase (): relative position in time, measured in degrees


Analog Signalingrepresented by sine waves time(sec)amplitude (volts)1 cyclefrequency (hertz)= cycles per second phase difference


Digital Signalingrepresented by square waves or pulsestime(sec)amplitude (volts)1 cyclefrequency (hertz)= cycles per second


Digital Text SignalingTransmission of electronic pulses representing the binary digits 1 and 0How do we represent letters, numbers, characters in binary form?Earliest example: Morse code (dots and dashes)Most common current form: ASCII


ASCII Character CodesUse 7 bits of data (1 byte) to transmit one character7 binary bits has 128 possible outcomes (0 to 127)Represents alphanumeric characters, as well as special charactersEighth bit in a byte can be used for formatting; also known as high-order bit


Digital Image Signaling Pixelization and binary representationCode: 00000000 00111100 01110110 01111110 01111000 01111110 00111100 00000000


Why Study Analog?Telephone system is primarily analog rather than digital (designed to carry voice signals)Low-cost, ubiquitous transmission mediumIf we can convert digital information (1s and 0s) to analog form (audible tone), it can be transmitted inexpensively


Voice SignalsEasily converted from sound frequencies (measured in loudness/db) to electromagnetic frequencies, measured in voltageHuman voice has frequency components ranging from 20Hz to 20kHzFor practical purposes, the telephone system has a narrower bandwidth than human voice, from 300 to 3400Hz


BandwidthWidth of the spectrum of frequencies that can be transmittedif spectrum=300 to 3400Hz, bandwidth=3100HzGreater bandwidth leads to greater costsLimited bandwidth leads to distortionAnalog measured in Hertz, digital measured in baud


BPS vs. BaudBPS=bits per secondBaud=# of signal changes per secondEach signal change can represent more than one bit, through variations on amplitude, frequency, and/or phase


Analog Data Choices


Digital Data Choices


Transmission ChoicesAnalog transmissiononly transmits analog signals, without regard for data contentattenuation overcome with amplifiersDigital transmissiontransmits analog or digital signalsuses repeaters rather than amplifiers


Data, Signals, and TransmissionDataSignalTransmissionSystemADDDAA


Advantages of Digital TransmissionThe signal is exactSignals can be checked for errorsNoise/interference are easily filtered outA variety of services can be offered over one lineHigher bandwidth is possible with data compression


Analog Encoding of Digital Datadata encoding and decoding technique to represent data using the properties of analog wavesmodulation: the conversion of digital signals to analog formdemodulation: the conversion of analog data signals back to digital form


Modeman acronym for modulator-demodulatoruses a constant-frequency signal known as a carrier signalconverts a series of binary voltage pulses into an analog signal by modulating an audible carrier signalthe receiving modem translates the analog signal back into digital data


Methods of Modulationamplitude modulation (AM) or amplitude shift keying (ASK)frequency modulation (FM) or frequency shift keying (FSK)phase modulation or phase shift keying (PSK)


Amplitude Shift Keying (ASK)In radio transmission, known as amplitude modulation (AM)the amplitude (or height) of the sine wave varies to transmit the ones and zerosmajor disadvantagetelephone lines are very susceptible to variations in transmission quality that affect amplitude


ASK Illustration1001


Frequency Shift Keying (FSK)in radio transmission, known as frequency modulation (FM)the frequency of the carrier wave varies in accordance with the signal to be sentsignal is transmitted at constant amplitudemore immune to noise than ASKrequires more analog bandwidth than ASK


FSK Illustration1101


Phase Shift Keying (PSK)also known as phase modulation (PM)frequency and amplitude of the carrier signal are kept constantthe carrier is shifted in phase according to the input data streameach phase can have a constant value, or value can be based on whether or not phase changes (differential keying)


PSK Illustration0011


Differential Phase Shift Keying (DPSK)0110


Complex ModulationsCombining modulation techniques allows us to transmit multiple bit values per signal change (baud)Increases information-carrying capacity of a channel without increasing bandwidthIncreased combinations also leads to increased likelihood of errorsTypically, amplitude and phase modulation are combined


Quadrature Amplitude Modulation (QAM)the most common method for quadbit transfercombination of 8 different angles in phase modulation and two amplitudes of signalprovides 16 different signals, each of which can represent 4 bits


Quadrature Amplitude Modulation Illustration90450135180225270315amplitude 1amplitude 2


Quadrature Amplitude Modulation UsesCCITT V.22 bis modemthe "bis" qualifier is a French term for "duo" or "twice" supports transmission of full-duplex 2400 bps synchronous or asynchronous data over a switched, 2-Wire, voice circuitthe modulation rate is 600 baud, with each baud representing four data bits


Trellis Coded Modulation (TCM)sophisticated mathematics are used to predict the best fit between the incoming signal and a large set of possible combinations of amplitude and phase changesa Forward Error Correcting (FEC)used in the V.32 modem (9600 bps) and all the higher speed modems


Digital Encodingof Digital DataMost common, easiest method is different voltage levels for the two binary digitsTypically, negative=1 and positive=0Known as NRZ-L, or nonreturn-to-zero level, because signal never returns to zero, and the voltage during a bit transmission is level


Differential NRZDifferential version is NRZI (NRZ, invert on ones)Change=1, no change=0Advantage of differential encoding is that it is more reliable to detect a change in polarity than it is to accurately detect a specific level


Problems With NRZDifficult to determine where one bit ends and the next beginsIn NRZ-L, long strings of ones and zeroes would appear as constant voltage pulsesTiming is critical, because any drift results in lack of synchronization and incorrect bit values being transmitted


Biphase Alternatives to NRZRequire at least one transition per bit time, and may even have twoModulation rate is greater, so bandwidth requirements are higherAdvantagesSynchronization due to predictable transitionsError detection based on absence of a transition


Manchester CodeTransition in the middle of each bit periodTransition provides clocking and dataLow-to-high=1 , high-to-low=0Used in Ethernet


Differential ManchesterMidbit transition is only for clockingTransition at beginning of bit period=0Transition absent at beginning=1Has added advantage of differential encodingUsed in token-ring


Digital Encoding Schemes


Transmitting Digital DataCodes determine what needs to be transmitted, not how to transmitTwo primary transmission methodsSerialParallel


Parallel Transmissionsending a character at a timethe components of each character are transmitted in parallelcommon transmission method between a personal computer and a printermultiple wires are required for transmission


Parallel Illustration


Serial Transmissionsending bits one after another rather than several at the same timerequires only one wire to transmit dataslower than parallel transmissionused when transmitting data over a telephone line as there is only one set of wires


Serial Illustration


Asynchronous & Synchronous TransmissionConcerned with timing issues in serial communicationHow does the receiver know when the bit period begins and ends?Small timing difference become more significant over time if no synchronization takes place between sender and receiver


Timing of Serial Dataasynchronous transmissionsynchronous transmission


Asynchronous TransmissionData transmitted 1 character at a timeCharacter format is 1 start & 1+ stop bit, plus data of 5-8 bitsCharacter may include parity bitResynchronization each start bitUses simple, cheap technologyWastes 20-30% of bandwidthExample: VT100 terminal


Synchronous TransmissionLarge blocks of bits transmitted without start/stop codesSynchronized by clock signal or clocking data, usually sent over a separate wire or channelData framed by preamble and postamble bit patternsMore efficient than asynchronousOverhead typically below 5% Used at higher speeds than asynchronousRequires error checkingExample: IBM3270 terminal


Communication PathsSimplexHalf-DuplexFull-Duplex


Simplex Transmissiononly transmit in one directionrarely used in data communicationse.g., receiving signals from the radio station or CATVthe sending station has only one transmitter the receiving station has only one receiver


Simplex Illustration


Half Duplex Communicationdata may travel in both directions, but only in one direction at a timeprovides nonsimultaneous two-way communicationcomputers use special control signals to negotiate which system will send data and which will receive datathe amount of time it takes computers to switch between sending and receiving is called turnaround time


Half Duplex Illustration


Full Duplex Communicationcomplete two-way simultaneous transmissionfaster than half-duplex communication because no turnaround time is neededrequires higher bandwidth


Full Duplex Illustration


Digital InterfacesThe point at which one device connects to anotherStandards define what signals are sent, and howSome standards also define physical connector to be used


RS-232C (EIA 232C)Uses NRZ-L encodingDefines two types of interfaceDTE: Data Terminal EquipmentDCE: Data Circuit-Terminating EquipmentWe often define entire devices based on their interface (e.g terminal=DTE, or modem=DCE)


DTE and DCE


RS-232C DB-25 ConnectorsFor pin assignments, see page 58 of the textbook


RS-232C ExamplesOdd ParityEven ParityNo Parity


Initial HandshakingDTE raises DTR (data terminal ready) signal to DCEDCE raises DSR (data set ready) signalDTE raises RTS (request to send) signalDCE raises CTS (clear to send) signalDCE sends a carrier signalRemote DCE detects carrier and raises DCD (data carrier detect) signal to DCEDTE sends data on TD (transmit data)

Completion HandshakingDCE modulates data onto the carrier waveRemote DCE demodulates data onto RD (receive data)DTE lowers RTS signalDCE drops CTS and carrier waveRemote DCE drops DCDTransmission is complete

Null Modem Cablespecial wiring of an RS-232-C cable to enable computers to talk to one another without a modem

Null Modem Cable

EIA-232-Dnew version of RS-232-C adopted in 1987improvements in grounding shield, test and loop-back signalsthe prevalence of RS-232-C in use made it difficult for EIA-232-D to enter into the marketplace


RS-449an EIA standard that improves on the capabilities of RS-232-Cprovides for a 37-pin connection, cable lengths up to 200 feet, and data transmission rates up to 2 million bpsequates with the functional and procedural portions of R-232-Cthe electrical and mechanical specifications are covered by RS-422 and RS-423


Baseband vs. BroadbandBaseband transmissionA single data signal transmitted directly on a wire (as in RS-232)Commonly used for LANsBroadband transmissionData is sent using a carrier signalDifferent frequencies allow multiple simultaneous signalsCable TV is a good example


Transmission Mediathe physical path between transmitter and receiverdesign factorsbandwidthattenuation: weakening of signal over distancesinterference: number of receivers


Impairments and CapacityImpairments exist in all forms of data transmissionAnalog signal impairments result in random modifications that impair signal qualityDigital signal impairments result in bit errors (1s and 0s transposed)


Transmission ImpairmentsAttenuationloss of signal strength over distanceAttenuation Distortiondifferent losses at different frequenciesDelay Distortiondifferent speeds for different frequenciesNoise


Types of NoiseThermal (aka white noise)Uniformly distributed, cannot be eliminatedIntermodulationwhen different frequenciesCrosstalkImpulse noiseLess predictable


Types of Transmission Mediaconducted or guided mediause a conductor such as a wire or a fiber optic cable to move the signal from sender to receiverwireless or unguided mediause radio waves of different frequencies and do not need a wire or cable conductor to transmit signals


Guided Transmission Mediathe transmission capacity depends on the distance and on whether the medium is point-to-point or multipointExamplestwisted pair wirescoaxial cablesoptical fiber


Twisted Pair Wiresconsists of two insulated copper wires arranged in a regular spiral pattern to minimize the electromagnetic interference between adjacent pairsoften used at customer facilities and also over distances to carry voice as well as data communicationslow frequency transmission medium


Twisted Pair Wirestwo varietiesSTP (shielded twisted pair)the pair is wrapped with metallic foil or braid to insulate the pair from electromagnetic interferenceUTP (unshielded twisted pair)each wire is insulated with plastic wrap, but the pair is encased in an outer covering


Twisted Pair WiresCategory 3 UTPdata rates of up to 16mbps are achievableCategory 5 UTPdata rates of up to 100mbps are achievablemore tightly twisted than Category 3 cablesmore expensive, but better performanceSTPMore expensive, harder to work with


Twisted Pair Advantagesinexpensive and readily availableflexible and light weight easy to work with and install


Twisted Pair Disadvantagessusceptibility to interference and noiseattenuation problemFor analog, repeaters needed every 5-6kmFor digital, repeaters needed every 2-3kmrelatively low bandwidth (3000Hz)


Coaxial Cable (or Coax)bandwidth of up to 400 MHzhas an inner conductor surrounded by a braided meshboth conductors share a common center axial, hence the term co-axial


Coax Layerscopper or aluminum conductorinsulating materialshield (braided wire)outer jacket(polyethylene)


Coax Advantageshigher bandwidth400 to 600Mhzup to 10,800 voice conversationscan be tapped easily (pros and cons)much less susceptible to interference than twisted pair


Coax Disadvantageshigh attenuation rate makes it expensive over long distancebulky


Fiber Optic Cablerelatively new transmission medium used by telephone companies in place of long-distance trunk linesalso used by private companies in implementing local data communications networksrequire a light source with injection laser diode (ILD) or light-emitting diodes (LED)


Fiber Optic Layersconsists of three concentric sections


Fiber Optic Typesmultimode step-index fiberthe reflective walls of the fiber move the light pulses to the receivermultimode graded-index fiberacts to refract the light toward the center of the fiber by variations in the densitysingle mode fiberthe light is guided down the center of an extremely narrow core


Fiber Optic Signalsfiber optic multimodestep-indexfiber optic multimodegraded-indexfiber optic single mode


Fiber Optic Advantagesgreater capacity (bandwidth of up to 2 Gbps)smaller size and lighter weightlower attenuationimmunity to environmental interferencehighly secure due to tap difficulty and lack of signal radiation


Fiber Optic Disadvantagesexpensive over short distancerequires highly skilled installersadding additional nodes is difficult


Wireless (Unguided Media) Transmissiontransmission and reception are achieved by means of an antennadirectionaltransmitting antenna puts out focused beamtransmitter and receiver must be alignedomnidirectionalsignal spreads out in all directionscan be received by many antennas


Wireless Examplesterrestrial microwave transmissionsatellite transmissionbroadcast radioinfrared


Terrestrial Microwave Transmissionuses the radio frequency spectrum, commonly from 2 to 40 Ghztransmitter is a parabolic dish, mounted as high as possibleused by common carriers as well as by private networksrequires unobstructed line of sight between source and receivercurvature of the earth requires stations (called repeaters) to be ~30 miles apart


Microwave Transmission Applicationslong-haul telecommunications service for both voice and television transmissionshort point-to-point links between buildings for closed-circuit TV or a data link between LANsbypass application


Microwave Transmission Advantagesno cabling needed between siteswide bandwidth multichannel transmissions


Microwave Transmission Disadvantagesline of sight requirementexpensive towers and repeaterssubject to interference such as passing airplanes and rain


Satellite Microwave Transmissiona microwave relay station in spacecan relay signals over long distancesgeostationary satellites remain above the equator at a height of 22,300 miles (geosynchronous orbit)travel around the earth in exactly the time the earth takes to rotate


Satellite Transmission Linksearth stations communicate by sending signals to the satellite on an uplinkthe satellite then repeats those signals on a downlinkthe broadcast nature of the downlink makes it attractive for services such as the distribution of television programming


Satellite Transmission Processdishdishuplink stationdownlink stationsatellitetransponder22,300 miles


Satellite Transmission Applicationstelevision distributiona network provides programming from a central locationdirect broadcast satellite (DBS)long-distance telephone transmissionhigh-usage international trunksprivate business networks


Principal Satellite Transmission BandsC band: 4(downlink) - 6(uplink) GHzthe first to be designated Ku band: 12(downlink) -14(uplink) GHzrain interference is the major problemKa band: 19(downlink) - 29(uplink) GHzequipment needed to use the band is still very expensive


Satellite Advantagescan reach a large geographical areahigh bandwidthcheaper over long distances


Satellite Disadvantageshigh initial costsusceptible to noise and interferencepropagation delay


Analog data, analog signaleasy conversion processAnalog data, digital signalpermits use of modern digital switching and transmission equipment

Digital data, analog signalequipment is less expensive than digital to analog conversionDigital data, analog signalsome transmission media (e.g. optical fiber or unguided media) can only send analog signals

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