Chapter 3 – Data Transmission Chapter 3 – Data Transmission

8th and 9th editions

Data TransmissionData TransmissionQuality of transmitted signalTransmission Terminology.Frequency, Spectrum and Bandwidth.Analogue & Digital Signals.Frequency, Spectrum and BandwidthPeriodic and Aperiodic Signals.

The sine wave As the fundamental periodic signal. Sine wave representation.Wave lengthAmplitude Lambda ()

Frequency Domain Concepts

Data TransmissionData TransmissionAddition of Frequency Components.Frequency Domain Representations.Spectrum & Bandwidth.Data Rate and Bandwidth.Analog and Digital Data Transmission Terms.What is Decibels dB?Acoustic Spectrum (Analog).Advantages and Disadvantages o Digital

signalsTransmission Impairments

Noise and its typesChannel Capacity

Data TransmissionThe successful transmission of data depends principally on two factors: The quality of the signal being transmitted. The characteristics of the transmission medium

Transmission Terminologydata transmission occurs between a transmitter &

receiver via some mediumTransmission media may be classified as guided or unguided

guided medium eg. twisted pair, coaxial cable, optical fiber waves are guided along a physical path

unguided / wireless medium eg. air, water, vacuum Provide means for transmitting electromagnetic waves but do not guide

them;

In both cases, communication is in the form of electromagnetic waves

Transmission Terminologydirect link

propagate directly from transmitter to receiver with no intermediate devices (except for repeaters and amplifiers)

point-to-point (guided transmission medium)direct link only 2 devices share link

multi-pointmore than two devices share the link

Transmission Terminologysimplex

one direction (one station is transmitter and the other is receiver).Example: television

half duplex : both stations may transmit, but only one at a time. either direction

Example: police radiofull duplex

both directions at the same timeExample: telephone

Transmission modes

Frequency, Spectrum and Bandwidth

Viewed as a function of time, an electromagnetic signal can be either analog or digital

time domain conceptsanalog signal

various in a smooth way over timedigital signal

maintains a constant level then changes to another constant level

periodic signal (The simplest sort of signal) pattern repeated over time

aperiodic signal pattern not repeated over time

The signal is a function of time, but it can also be expressed as a function of frequency.. Will see soon

Analogue & Digital SignalsThe continuous signal might represent speech,

The discrete signal might represent binary 1s and 0s.

Periodic Signals The signal consists of components of different frequencies.

frequency domain view of a signal is more important to an understanding of data transmission than a time domain view.Frequency = Cycles per period of time

the signal consists of components of different frequencies

Sine Wave

peak amplitude (A)maximum strength of signal over timetypically measured volts

frequency (f)rate of change of signalHertz (Hz) or cycles per secondperiod = time for one repetition (T)T = 1/f

phase ()relative position in time

The sine wave is the fundamental periodic signal. A general sine wave can be represented by three parameters:

Varying Sine Wavesgeneral sine wave can be written as s(t) = A sin(2ft +) sinusoid function

Wavelength

There is a simple relationship between the two sine waves, one in time and one in space.

() is distance occupied by one cyclebetween two points of corresponding phase in

two consecutive cyclesassuming signal velocity v have = vT (distance =speed *time)T = 1/for equivalently f = v because ( = v * 1/f) especially when v = c

c = 3*108 ms-1 (speed of light in free space)

Frequency Domain Concepts

It turns out that the frequency domain view of a signal is more important to an understanding of data transmission than a time domain view.

signal are made up of many frequencies (cycles)

components are sine wavesFourier analysis discipline can show that any

signal is made up of component sine wavescan plot frequency domain functions

Addition of FrequencyComponents(T=1/f)

c is sum of f & 3f

By adding together enough sinusoidal signals, each with the appropriate amplitude, frequency, and phase, any electromagnetic signal can be constructed.

the components of this signal are just sine waves of frequencies f and 3f, as Shawn in parts (a) and (b).

Frequency Domain Representations

Similarly, there is a frequency domain function S(f) that specifies the peak amplitude of the constituent frequencies of the signal.

for each signal, there is a time domain function s(t) that specifies the amplitude of the signal at each instant in time

Spectrum & BandwidthSpectrum:

range of frequencies contained in signal (Fig 3.4c, it extends from f to 3f)

absolute bandwidthwidth of spectrum eg is 2f in Fig 3.4c (3f – f = 2f)Many Signals such as that of Figure 3.5b, have an infinite

bandwidth. effective bandwidth or just bandwidth

narrow band of frequencies containing most energyDC Component

component of zero frequency

Data Rate and BandwidthAny transmission system has a limited band

of frequenciesThis limits the data rate that can be carriedSquare wave have infinite components and

hence bandwidthBut most energy in first few componentslimited bandwidth increases distortion and the

greater the potential for error by the receiverThe greater the bandwidth transmitted, the greater the costThere is a direct relationship between data

rate & bandwidth:the higher the data rate of a signal, the greater is its required

effective bandwidth.

Analog and Digital Data Transmission TermsData

Entities that convey informationSignals

Electric or Electromagnetic representations of data

signalingPhysically propagates along a medium

TransmissionCommunication of data by propagation and

processing of signals

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The decibel (abbreviated dB)dB is the unit used to measure the intensity of a soundThe decibel scale is a little odd because the human ear

is incredibly sensitive. Your ears can hear everything from your fingertip

brushing lightly over your skin to a loud jet engine. In terms of power, the sound of the jet engine is about 1,000,000,000,000 times more powerful than the smallest audible sound. That's a big difference!

On the decibel scale, the smallest audible sound (near total silence) is 0 dB.

A sound 10 times more powerful is 10 dB. A sound 100 times more powerful than near total silence is 20 dB.

Sound sensibility is different

in humans than animals.

Hint, dogs are used in

rescue operations when

earthquakes

Decibels dBA sound 1,000 times more powerful than

near total silence is 30 dB. Here are some common sounds and their

decibel ratings:Near total silence - 0 dBA whisper - 15 dBNormal conversation - 60 dBA lawnmower - 90 dBA car horn - 110 dBA rock concert or a jet engine - 120 dBA gunshot or firecracker - 140 dB

Db and distance relationshipYou know from your own experience that distance

affects the intensity of sound -- if you are far away, the power is greatly diminished. All of the ratings above are taken while standing near the sound.

Any sound above 85 dB can cause hearing loss, and the loss is related both to the power of the sound as well as the length of exposure.

You know that you are listening to an 85-dB sound if you have to raise your voice to be heard by somebody else.

Eight hours of 90-dB sound can cause damage to your ears; any exposure to 140-dB sound causes immediate damage (and causes actual pain).

Acoustic Spectrum (Analog)Analog data take on continuous values in some interval, the most familiar example being audio, which, in the form of acoustic sound waves, can be perceived directly by human beings.

Figure 3.9 see slide 22 shows the acoustic spectrum for human speech and for music (note log scales). Frequency components of typical speech may be found between approximately 100 Hz and 7 kHz, and has a dynamic range of about 25 dB (a shout is approx 300 times louder than whisper).Another common example of analog data is video, as seen on a TV screen.

Acoustic Spectrum (Analog)

Audio SignalsThe most familiar example of analog information is

audio/acoustic sound wave information, eg. human speechfreq range 20Hz-20kHz (speech 100Hz-7kHz)easily converted into electromagnetic signalsvarying volume converted to varying voltagecan limit frequency range for voice channel to

300-3400Hz

narrower bandwidth will produce acceptable voice reproduction. The standard spectrum for a voice channel is 300 to 3400 Hz

Digital DataAs generated by computers (1s and 0s). then converted

into digital voltage pulses for transmission.Has two dc componentsBandwidth depends on data rate

The greater the bandwidth of the signal, the more faithfully it approximates a digital pulse stream.

Analog SignalsData are propagated from one point to another by means of electromagnetic signals. Both analog and digital signals may be transmitted on suitable transmission media

Digital Signals• A sequence of voltage pulses that may be transmitted over a wire

medium.• Digital signals can be used to transmit both analog signals and digital

data.• Analog data can converted to digital using a codec (coder-decoder),

A digital signal can be transmitted only within a limited distance before attenuation, noise, and other impairments

. A repeater receives the digital signal, recovers the pattern of 1s and 0s, and retransmits a new signal.

Thus the attenuation is overcome.

Advantages & Disadvantages of Digital Signals

CheaperLess susceptible to noiseBut greater attenuation than AnalogDigital signals are now the preferred choice

Because of the attenuation, or reduction, of signal strength at higher frequencies, the pulses become rounded and smaller.

Attenuatio

n:

Reduction in

strength

Analog OR Digital?Which is the preferred method of transmission?

The answer being supplied by the telecommunications industry and its customers is digital. Both long-haul telecommunications facilities and intra-building services have moved to digital transmission and, where possible, digital signaling techniques, for a range of reasons.

Transmission Impairments

Signal received may differ from signal transmitted causing:Analog - degradation of signal qualityDigital - bit errors

Most significant impairments areAttenuation and attenuation distortionDelay distortionNoise

AttenuationAttenuationWhere signal strength falls off with distanceDepends on medium For unguided media, attenuation is a more complex

function of distance and the makeup of the atmosphere.Attenuation introduces three considerations for the transmission

engineer Received signal strength must be strong enough to

be detectedSufficiently higher than noise to receive without

errorAttenuation varies with frequency causing distortion

Increase strength using amplifiers/repeaters (first and second problems)

Note: This also an increasing function of frequency.

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Delay DistortionOnly occurs in guided mediaPropagation velocity varies with frequencyVarious frequency components arrive at

different times resulting in phase shifts between the different frequencies.

The velocity tends to be highest near the center frequency and fall off toward the two edges of the band

Particularly critical for digital dataParts of one bit spill over into others causing

intersymbol interference

Techniques to equalizing attenuation

Using loading coils: changes the properties of the electrical signal on the line

Using amplifiers: Amplifies higher frequencies more than lower ones

These techniques result in smoothing the attunation effect on the transmitted signal

Noise. What is Nose?

Additional signals inserted between transmitter and receiver.

Types of Noise:Thermal (static Noise)

Due to thermal agitation of electrons Uniformly distributed ( often referred to as white noise)

Intermodulation Noise Signals that are the sum and difference of original

frequencies sharing a medium. Pproduced by nonlinearities in the transmitter, receiver, and/or

intervening transmission medium.

Types of Noise.. continueCrosstalk

A signal from one line is picked up by anotherOccurs by electrical coupling between nearby twisted pairs or, rarely, coax cable lines carrying

multiple signals. It can also occur when microwave antennas pick up unwanted signals;

Impulse Irregular pulses or spikes

eg. external electromagnetic interference short duration high amplitudeIt is generated from a variety of causes, including external electromagnetic disturbances, such as

lightning, and faults and flaws in the communications system.

A minor annoyance for analog signals A major source of error in digital data, a noise spike

could corrupt many bits

Channel CapacityMax possible data rate on communication

channel There are four concepts here that we are trying to relate to

one another:Data rate - in bits per second at which data can be

communicatedBandwidth – dictated by the transmitter and the

medium in cycles per second or HertzNoise – Average of noise on communications link

Error rate - of corrupted bits 0 to 1 or 1 to 0

There are limitations due to physical properties for all transmission channels

Summarylooked at data transmission issuesFrequency, spectrum & bandwidthAnalog vs Digital signalsTransmission impairments