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Chapter 4-6

Signals, Media, And

Data Transmission

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Transmission of Information

Transmission of Information

Well-understood basicsFrom physics

EnergyElectromagnetic wave propagation

From mathematicsCoding theory

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Transmission MediaTransmission MediaCopper wire

Need two wiresPossibilities

Twisted pairCoaxial cable

Optical fiberFlexibleLight “stays in”

Air / spaceUsed for electromagnetic transmission

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Forms of Energy Used To Transmit Data

Forms of Energy Used To Transmit Data

Electric currentAudible soundsOmni-directional electromagnetic waves

Radio Frequency (RF)Infrared

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Forms of Energy Used to Transmit Data (continued)Forms of Energy Used to Transmit Data (continued)

Directional electromagnetic wavesPoint-to-point satellite channelLimited broadcast (spot beam)MicrowaveLaser beam

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Types of SatellitesTypes of Satellites

Geosynchronous Earth Orbit (GEO)

Low Earth Orbit (LEO)

Array needed

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Two Important Physical Limits

Of a Transmission System

Two Important Physical Limits

Of a Transmission SystemPropagation delay

Time required for signal to travel across mediaExample: electromagnetic radiation travels

through space at the speed of light (c = 3*108 meters per second)

BandwidthMaximum times per second the signal can

change

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Transmission of DataTransmission of Data

Network hardware encodes information for transmission

Two types of encodingAnalog (amount of energy proportional to value

of item)Digital (two forms of energy to encode 0 and 1

Computer networks use the latter

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Example Digital EncodingExample Digital Encoding

MediumCopper wire

Energy formElectric current

EncodingNegative voltage encodes 1Positive voltage encodes 0

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Illustration Of Digital Encoding

Illustration Of Digital Encoding

Known as waveform diagramX-axis corresponds to timeY-axis corresponds to voltage

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Encoding DetailsEncoding Details

Several organizations produce networking standards

IEEEITUEIA

Hardware that adheres to standard interoperable

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The RS-232C StandardThe RS-232C Standard

Example useConnection to keyboard / mouseSerial port on PC

Specified by EIAVoltage is +15 or –15Cable limited to ~50 feetLatest EIA standard is RS-422 (ITU standard is V.24)Uses asynchronous communication

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Asynchronous CommunicationAsynchronous

CommunicationSender and receiver must agree on

Number of bits per characterDuration of each bit

ReceiverDoes not know when a character will arriveMay wait forever

To ensure meaningful exchange sendStart bit before characterOne or more stop bits after character

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Illustration of RS-232Illustration of RS-232

Start bitSame as 0Not part of data

Stop bit Same as 1Follows data

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Duration of a Bit in RS-232C

Duration of a Bit in RS-232C

Determined by baud rateTypical baud rates: 9.6 Kbaud, 14.4 Kbaud, 28.8

KbaudDuration of bit is 1 / baud rateSender and receiver must agree a prioriReceiver samples signalDisagreement results in framing error

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Two-Way CommunicationTwo-Way Communication

Desirable in practiceRequires each side to have transmitter and

receiverCalled full duplex

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Illustration Of Full-Duplex

Communication

Illustration Of Full-Duplex

Communication

Transmitter on one side connected to receiver on otherSeparate wires needed to carry current in each directionCommon ground wireDB-25 connector used

Pin 2 is transmitPin 3 is receive

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Electrical Transmission(The Bad News)

Electrical Transmission(The Bad News)

It’s an ugly worldElectrical energy dissipates as it travels alongWires have resistance, capacitance, and

inductance which distort signalsMagnetic or electrical interference distorts signalsDistortion can result in loss or misinterpretation

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Illustration of DistortedSignal for a Single Bit

Illustration of DistortedSignal for a Single Bit

In practiceDistortion can be much worse than illustrated

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ConsequencesConsequences

RS-232 hardware must handle minor distortionsTake multiple samples per bitTolerate less than full voltage

Cannot use electrical current for long-distance transmission

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Long-Distance CommunicationLong-Distance

CommunicationImportant fact: an oscillating signal travels

farther than direct currentFor long-distance communication

Send a sine wave (called a carrier wave)Change (modulate) the carrier to encode date

Note: modulated carrier technique used for radio and television

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Illustration of a CarrierIllustration of a Carrier

CarrierUsually a sine waveOscillates continuouslyFrequency of carrier fixed

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Types of ModulationTypes of Modulation

Amplitude modulation (used in AM radio)Frequency modulation (used in FM radio)Phase shift modulation (used for data)

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Illustration ofAmplitude Modulation

Illustration ofAmplitude Modulation

Strength of signal encodes 0 or 1One cycle of wave needed for each bitData rate limited by carrier bandwidth

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Illustration ofPhase-Shift Modulation

Illustration ofPhase-Shift Modulation

Change in phase encodes K bitsData rate higher than carrier bandwidth

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Phase-Shift ExamplePhase-Shift Example

Section of wave is omitted at phase shiftData bits determine size of omitted section

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ModemModem

Hardware deviceUsed for long-distance communicationContains separate circuitry for

Modulation of outgoing signalDemodulation of incoming signal

Name abbreviates modulator / demodulator

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Illustration of ModemsUsed Over a Long

Distance

Illustration of ModemsUsed Over a Long

Distance

One modem at each endSeparate wires carry signals in each directionModulator on one modem connects to

demodulator on other

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Types of ModemsTypes of Modems

ConventionalUse four wiresTransmit modulated electrical wave

OpticalUse glass fibersTransmit modulated light

WirelessUse air / spaceTransmit modulated RF wave

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Types of Modems(continued)

Types of Modems(continued)

DialupUse voice telephone systemTransmit modulated audio tone

Note: in practice, a dialup modem uses multiple tones simultaneously

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Illustration of Dialup Modem

Illustration of Dialup Modem

Modem canDialAnswer

Carrier is audio tone

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Modem TerminologyModem Terminology

Full-duplex modemProvides 2-way communicationAllows simultaneous transmissionUses four wires

Half-duplex modemDoes provide 2-way communicationTransmits in one direction at any timeUses two wires

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RecallRecall

Propagation delayDetermined by physicsTime required for signal to travel across medium

BandwidthElectrical property of physical transmission

systemMaximum times per second signal can change

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Fundamental Measures Of A

Digital Transmission System

Fundamental Measures Of A

Digital Transmission SystemDelay

The amount of time required for a bit of data to travel from one end to the other

Usually the same as the propagation delay in underlying hardware

ThroughputThe number of bits per second that can be

transmittedRelated to underlying hardware bandwidth

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Relationship Between Digital

Throughput and Bandwidth

Relationship Between Digital

Throughput and BandwidthGiven by Nyquist’s theorem:

D = 2 B log2 K

whereD is maximum data rateB is hardware bandwidthK is number of values used to encode data

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Applications of Nyquist’s Theorem

Applications of Nyquist’s Theorem

For RS-232K is 2 because RS-232 only uses two values,

+15 or –15 volts, to encode data bitsD is 2 B log2 2 = 2 B

For phase-shift encodingSuppose K is 8 (possible shifts)D is 2 B log2 8 = 2 B * 3 = 6 B

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More Bad NewsMore Bad News

Physics tells us that real systems emit and absorb energy (e.g., thermal)

Engineers call unwanted energy noiseNyquist’s theorem

Assumes a noise-free systemOnly works in theory

Shannon’s theorem corrects for noise

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Shannon’s TheoremShannon’s Theorem

Gives capacity in presence of noise:

C = B log2 (1 + S/N)where

C is the effective channel capacity in bits per secondB is hardware bandwidthS is the average power (signal)N is the noise

S/N is signal-to-noise ratio

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Application of Shannon’s Theorem

Application of Shannon’s Theorem

Conventional telephone systemEngineered for voiceBandwidth is 3000 HzSignal-to-noise ratio is approximately 1000Effective capacity is

3000 log2 (1 + 1000) = ~30000 bps Conclusion: dialup modems have little hope

of exceeding 28.8 Kbps

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The Bottom LineThe Bottom Line

Nyquist’s theorem means finding a way to encode more bits per cycle improves the data rate

Shannon’s theorem means that no amount of clever engineering can overcome the fundamental limits of a real transmission system

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MultiplexingMultiplexing

Fundamental to networkingGeneral conceptUsed in

Lowest level of transmission systemsHigher levels of network hardwareProtocol softwareApplications

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The General Concept of Multiplexing

The General Concept of Multiplexing

Separate pairs of communications travel across shared channel

Multiplexing prevents interferenceEach destination receives only data sent by

corresponding source

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Multiplexing TerminologyMultiplexing Terminology

MultiplexorDevice or mechanismAccepts data from multiple sourcesSends data across shared channel

DemultiplexorDevice or mechanismEstracts data from shared channelSends to correct destination

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Two Basic Types of Multiplexing

Two Basic Types of Multiplexing

Time Division Multiplexing (TDM)Only one item at a time on shared channelItem marked to identify sourceDemultiplexor uses identifying mark to know

where to deliverFrequency Division Multiplexing (FDM)

Multiple items transmitted simultaneouslyUses multiple “channels”

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Transmission SchemesTransmission Schemes

Baseband transmissionUses only low frequenciesEncodes data directly

Broadband transmissionUses multiple carriersCan use higher frequenciesAchieves higher throughputHardware more complex and expensive

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Scientific Principle BehindFrequency Division

Multiplexing

Scientific Principle BehindFrequency Division

Multiplexing

Note: this is the same principle that allows a cable TV company to send multiple television signals across a single cable

Two or more signals that use different carrier frequencies can be transmitted over a single medium simultaneously without interference

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Wave Division MultiplexingWave Division Multiplexing

FactsFDM can be used with any electromagnetic

radiationLight is electromagnetic radiation

When applied to light, FDM is called wave division multiplexing

Informally called color division multiplexing

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SummarySummary

Various transmission schemes and media availableElectric current over copperLight over glassElectromagnetic waves

Digital encoding used for dataAsynchronous communication

Used for keyboards and serial portsRS-232 is standardSender and receiver agree on baud rate

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Summary (continued)Summary (continued)

ModemsUsed for long-distance communicationAvailable for copper, optical fiber, dialupTransmit modulated carrier

Phase-shift modulation popularClassified as full- or half-duplex

Two measures of digital communication systemDelayThroughput

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Summary (continued)Summary (continued)

Nyquist’s theoremRelates throughput to bandwidthEncourages engineers to use complex encoding

Shannon’s theoremAdjusts for noiseSpecifies limits on real transmission systems

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Summary (continued)Summary (continued)

MultiplexingFundamental conceptUsed at many levelsApplied in both hardware and softwareTwo basic types

Time-division multiplexing (TDM)Frequency-division multiplexing (FDM)

When applied to light, FDM is called wave-division multiplexing