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Chapter 4-6

Signals, Media, And Data Transmission

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Transmission of Information

Well-understood basicsFrom physics

EnergyElectromagnetic wave propagation

From mathematicsCoding theory

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Transmission 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

Electric currentAudible soundsOmni-directional electromagnetic waves

Radio Frequency (RF)Infrared

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Forms of Energy Used to Transmit Data (continued)Directional electromagnetic waves

Point-to-point satellite channelLimited broadcast (spot beam)MicrowaveLaser beam

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Types of SatellitesGeosynchronou

s Earth Orbit (GEO)

Low Earth Orbit (LEO)

Array needed

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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 DataNetwork hardware encodes information for

transmission Two types of encoding

Analog (amount of energy proportional to value of item)

Digital (two forms of energy to encode 0 and 1Computer networks use the latter

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Example Digital EncodingMedium

Copper wireEnergy form

Electric currentEncoding

Negative voltage encodes 1Positive voltage encodes 0

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Illustration Of Digital Encoding

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

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Encoding DetailsSeveral organizations produce networking

standardsIEEEITUEIA

Hardware that adheres to standard interoperable

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The RS-232C StandardExample use

Connection 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 Communication

Sender and receiver must agree onNumber 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-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

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 CommunicationDesirable in practiceRequires each side to have transmitter and

receiverCalled full duplex

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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)

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

In practiceDistortion can be much worse than illustrated

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ConsequencesRS-232 hardware must handle minor distortions

Take multiple samples per bitTolerate less than full voltage

Cannot use electrical current for long-distance transmission

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Long-Distance Communication

Important fact: an oscillating signal travels farther than direct current

For long-distance communicationSend 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 Carrier

CarrierUsually a sine waveOscillates continuouslyFrequency of carrier fixed

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Types of ModulationAmplitude modulation (used in AM radio)Frequency modulation (used in FM radio)Phase shift modulation (used for data)

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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

Change in phase encodes K bitsData rate higher than carrier bandwidth

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Phase-Shift Example

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

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ModemHardware 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

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 ModemsConventional

Use four wiresTransmit modulated electrical wave

OpticalUse glass fibersTransmit modulated light

WirelessUse air / spaceTransmit modulated RF wave

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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

Modem canDialAnswer

Carrier is audio tone

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Modem TerminologyFull-duplex modem

Provides 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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RecallPropagation delay

Determined 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 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 BandwidthGiven by Nyquist’s theorem:

D = 2 B log2 Kwhere

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

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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 NewsPhysics 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 TheoremGives 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

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 LineNyquist’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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MultiplexingFundamental to networkingGeneral conceptUsed in

Lowest level of transmission systemsHigher levels of network hardwareProtocol softwareApplications

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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 TerminologyMultiplexor

Device 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

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 SchemesBaseband transmission

Uses 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

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 MultiplexingFacts

FDM can be used with any electromagnetic radiation

Light is electromagnetic radiationWhen applied to light, FDM is called wave

division multiplexingInformally called color division multiplexing

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SummaryVarious transmission schemes and media available

Electric 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)Modems

Used 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)Nyquist’s theorem

Relates throughput to bandwidthEncourages engineers to use complex encoding

Shannon’s theoremAdjusts for noiseSpecifies limits on real transmission systems

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Summary (continued)Multiplexing

Fundamental 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