03-signals.ppt

7/27/2019 03-Signals.ppt

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Physical Layer:

Data and Signals

01204325: Data Communication

and Computer Networks

Asst. Prof. Chaiporn Jaikaeo, [email protected]

http://www.cpe.ku.ac.th/~cpj

Computer Engineering DepartmentKasetsart University, Bangkok, Thailand
mailto:[email protected]:[email protected]


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Outline

Analog and digital data/signals Time and frequency domain views of signals

Bandwidth and bit rate

Transmitting digital signals as analog

Theoretical data rate

Signal impairment


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Analog vs. Digital Data

Analog data Data take on continuous values

E.g., human voice, temperature reading

Digital data Data take on discrete values

E.g., text, integers


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Analog vs. Digital Signals

Analog signals

have an infinite number ofvalues in a range

Digital signals Have a limited number of

values

value

time

value

time

To be transmi tted, data must betransformed to electromagnetic signals


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Data and Signals

Telephone

Analog Data Analog Signal

Modem

Digital Data Analog Signal

Codec

Analog Data Digital Signal

Digitaltransmitter

Digital Data Digital Signal


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Simplest form of periodic signal

General form:x(t) =Asin(2ft+ )

periodT= 1/f

peakamplitude

time

signal strength

Sine Waves

phase / phase shift


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-3

-2

-1

0

1

2

3

0 0.5 1 1.5 2 2.5 3

A = 1,f= 1, = 0 -3

-2

-1

0

1

2

3

0 0.5 1 1.5 2 2.5 3

A = 2,f= 1, = 0

-3

-2

-1

0

1

2

3

0 0.5 1 1.5 2 2.5 3

A = 1,f= 2, = 0 -3

-2

-1

0

1

2

3

0 0.5 1 1.5 2 2.5 3

A = 1,f= 1, = /4

Varying Sine Waves


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Bandwidth of a Medium

Transmission medium

1

(low-pass channel)gain

freq

t t

0 f0 3f0 5f0 f0 f0 3f0 5f0 7f0...

9f0 f


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Digital Signals

Properties: Bit rate number of bits per second

Bit interval duration of 1 bit

1 0 0 1 1 10 0

...

time

amplitude

bit interval


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Two digi tal signals: one with two signal levels and the other

with four signal levels


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Baseband transmission

Baseband transmission Sending a digital signal over a channelwithout changing it to an analog signal

Baseband transmission requires a low-pass channel


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A digital signal is a composite analogsignal with an infinite bandwidth.

Note


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Baseband transmission using a dedicated medium


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Digital vs. Analog

Using one harmonic

1 1 1 1 1 1

1 sec

Bit rate = 6

Digital

1 0 1 0 1 0

Bit rate = 6

Digital

1 0 1 0 1 0

f = 3

Analog

1 1 1 1 1 1

f = 0

Analog


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Digital vs. Analog

Using more harmonicsAdding 3rd harmonic to improve quality

1 0 1 0 1 0

f0 = 3, fmax= 9

Analog

1 0 1 0 1 0

Bit rate = 6

Digital


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Digital vs. Analog Bandwidth

Digital bandwidth Expressed in bits per second (bps)

Analog bandwidth

Expressed in Hertz (Hz)

Bit rate and bandwidth are proportional to each other


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Low-Pass and Band-Pass Channels

Low-pass channel

Band-pass channel

frequencyf1 f2

gain

frequencyf1

gain


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Modulation of a digital signal for transmission on a bandpass channel


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Transmission Impairment

Attenuation Distortion

Noise


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Signal Attenuation

Attenuation

Loss of energy

Signal strength falls off with distance

Attenuation depends on medium

Attenuation is an increasing function offrequency

Transmission medium


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Relative Signal Strength

Measured in Decibel (dB)

P1 andP2 are signal powers at points 1 and 2,

respectively

Positive dB signal is amplified (gains strength)

Negative dB signal is attenuated (loses strength)

dB = 10 log10 (P2/P1)

Point 1 Point 2


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Signal Distortion

Distortion Change in signal shape

Only happens in guided media

Propagation velocity varies with frequency


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Noise

Noise

Undesirable signals addedbetween the transmitter and the receiver

Types of noise Thermal

Due to random motion of electrons in a wire


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Noise

Types of noise (contd) Crosstalk

Signal from one line picked up by another

Impulse Irregular pulses or spikes E.g., lightning

Short duration

High amplitude

Wire 1

Wire 2


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Data Rate: Noiseless Channels

Nyquist Theorem

Bit rate in bps

Bandwidth in Hz

L number of signal levels

Bit Rate = 2 Bandwidth log2LHarry Nyquist(1889-1976)


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Data Rate: Noisy Channels

Shannon Capacity

Capacity (maximum bit rate) in bps Bandwidth in Hz

SNR Signal-to-Noise Ratio

Capacity = Bandwidth log2(1+SNR)

Claude Elwood Shannon(1916-2001)


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The Shannon capacity gives us the

upper limit; the Nyquist formula tells ushow many signal levels we need.

Note


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Network Performance

Bandwidth Hertz

Bits per second (bps)

ThroughputActual data rate

Latency (delay)

Time it takes for an entire message tocompletely arrive at the destination


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Latency

Composed of Propagation time

Transmission time

Queuing time

Processing time

Entiremessage

transmission

time

propagationtime


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Data bits

Latency

Time Time

First bit leaves

Last bit leaves

First bit arrives

Last bit arrives

Sender Receiver

Propagation time

Transmission time


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Bandwidth-Delay Product

The link is seen as a pipe Cross section = bandwidth

Length = delay

Bandwidth-delay product defines thenumber of bits that can fill the link


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Figure Fill ing the link with bits for case 1


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Summary Data need to take form of signal to be

transmitted

Frequency domain representation of signalallows easier analysis Fourier analysis

Medium's bandwidth limits certainfrequencies to pass

Bit rate is proportional to bandwidth Signals get impaired by attenuation,

distortion, and noise

03-signals.ppt

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