I 3phy, Slides for Computer Networks. University of Liberia
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Austin.Doe31 August 2016

I 3phy, Slides for Computer Networks. University of Liberia

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Transmission System Utilization
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Data Transmission

3-1 ©2005 Raj JainCSE473sWashington University in St. Louis

Data Transmission

Raj Jain Professor of CSE

Washington University in Saint Louis Saint Louis, MO 63130

[email protected]

These slides are available on-line at: http://www.cse.wustl.edu/~jain/cse473-05/

3-2 ©2005 Raj JainCSE473sWashington University in St. Louis

Overview

 Time Domain and Frequency Domain

 Decibels

 Data vs Signal

 Attenuation, Delay Distortion, Noise, Capacity

3-3 ©2005 Raj JainCSE473sWashington University in St. Louis

Transmission Terminology  Guided Media: Wire, Fiber , coax

Unguided Media: Air, Vacuum, sea water

 Direct Link

 Point to Point vs Point to Multipoint

T R

T R T

R1 R2 R3

3-4 ©2005 Raj JainCSE473sWashington University in St. Louis

Line Duplexity

 Simplex: Transmit or receive, e.g., Television

 Half-Duplex: Transmit and receive alternately, e.g., Police Radio

 Full Duplex: Transmit and receive simultaneously, e.g., Telephone

T R

T/R T/R

3-5 ©2005 Raj JainCSE473sWashington University in St. Louis

Analog vs Digital Signals

 Analog:

 Digital:

3-6 ©2005 Raj JainCSE473sWashington University in St. Louis

Periodic vs Aperiodic

 Periodic: Signal pattern repeats over time

 Aperiodic: Not periodic

T

S(t+T) = s(t) for all t Period = T

3-7 ©2005 Raj JainCSE473sWashington University in St. Louis

Examples of Periodic Signals

 Sine Wave:

 Square Wave:

3-8 ©2005 Raj JainCSE473sWashington University in St. Louis

Frequency, Period, and Phase  A Sin(2πft + θ), Period T=1/f, Frequency in Hertz

3-9 ©2005 Raj JainCSE473sWashington University in St. Louis

Sine Wave

 Peak Amplitude (A): Maximum strength of signal in volts

 Frequency (f): Hertz (Hz) or cycles per second  Period = time for one repetition (T)

T = 1/f  Phase (φ): Relative position in time

3-10 ©2005 Raj JainCSE473sWashington University in St. Louis

Wavelength

 Distance occupied by one cycle  Distance between two points of corresponding phase in two

consecutive cycles  Wavelength = λ  Assuming signal velocity v

 λ = vT  λf = v  c = 3*108 m/s (speed of light in free space) = 300 m/µs

Distance Amplitude

3-11 ©2005 Raj JainCSE473sWashington University in St. Louis

Time and Frequency Domains

Frequency

Amplitude

Frequency

Amplitude

Frequency

Amplitude

f

3f

A

A

f 3f

A/3

A/3

3-12 ©2005 Raj JainCSE473sWashington University in St. Louis

Frequency Domain Concepts  Fundamental Frequency: All other frequency

components are multiple of fundamental frequency f  Period = 1/f  Spectrum: Range of frequencies  Absolute Bandwidth: Width of the spectrum

Absolute Bandwidth = 3f-f = 2f  Effective Bandwidth: Narrow band of frequencies

containing most of the energy  DC Component: Constant or zero frequency

A+B sin (2πft+θ)

3-13 ©2005 Raj JainCSE473sWashington University in St. Louis

Signal with DC Component

3-14 ©2005 Raj JainCSE473sWashington University in St. Louis

Frequency Components of Square Wave

Freq.

Amplitude

Amplitude

Frequency

f

A

3f 5f

A/3 A/5

Freq.f 3f

A/3

5f

A/5

7f

A/7

Σκ=1,3,5,… A/k sin (2πkft)

3-15 ©2005 Raj JainCSE473sWashington University in St. Louis

Data vs Signal

Telephone

Modem

CODEC

Digital Transceiver

Analog

Digital

Data Signal Analog

Digital

Analog

Analog

Digital

Digital

Data Signal Medium

Data

3-16 ©2005 Raj JainCSE473sWashington University in St. Louis

Analog Data Example: Speech and Music

3-17 ©2005 Raj JainCSE473sWashington University in St. Louis

Analog Data Example 2: Television

30 Screens/sec Interlacing: Odd lines every 1/60 s and even lines every 1/60 s 483 lines/screen

3-18 ©2005 Raj JainCSE473sWashington University in St. Louis

Video Signal  USA - 483 lines scanned per frame at 30 frames per second

 525 lines but 42 lost during vertical retrace  So 525 lines x 30 scans = 15750 lines per second

 63.5µs per line  11µs for retrace, so 52.5 µs per video line

 Max frequency if line alternates black and white  Horizontal resolution is about 450 lines giving 225 cycles of

wave in 52.5 µs  Max frequency of 4.2MHz

3-19 ©2005 Raj JainCSE473sWashington University in St. Louis

Attenuation and Dispersion (Delay Distortion)

Distance

3-20 ©2005 Raj JainCSE473sWashington University in St. Louis

Digital Transmission  Repeaters are used to regenerate digital signal  Signal attenuation is overcome  Noise is not amplified  Low cost LSI/VLSI technology  Longer distances over lower quality lines  Capacity utilization

 High bandwidth links economical  High degree of multiplexing easier with digital

techniques  Security & Privacy: Encryption

3-21 ©2005 Raj JainCSE473sWashington University in St. Louis

Decibels

 Attenuation = Log10 Pin Pout

 Example 1: Pin = 10 mW, Pout=5 mW Attenuation = 10 log 10 (10/5) = 10 log 10 2 = 3 dB

 Example 2: Pin = 100mW, Pout=1 mW Attenuation = 10 log 10 (100/1) = 10 log 10 100 = 20 dB

Bel

Pin Pout

decibel Attenuation = 10 Log10

Vin Vout

decibel Attenuation = 20 Log10

3-22 ©2005 Raj JainCSE473sWashington University in St. Louis

Noise

 Additional signals inserted between transmitter and receiver

 Thermal Noise:  Due to thermal agitation of electrons  Uniformly distributed  White noise

 Intermodulation Noise:  Signals that are the sum and difference of original

frequencies sharing a medium

3-23 ©2005 Raj JainCSE473sWashington University in St. Louis

Noise (Cont)

 Crosstalk Noise: A signal from one line is picked up by another

 Impulse Noise:  Irregular pulses or spikes

e.g., External electromagnetic interference  Short duration  High amplitude

3-24 ©2005 Raj JainCSE473sWashington University in St. Louis

Channel Capacity

 Capacity = Maximum data rate for a channel  Nyquist Theorem: Bandwidth = B

Data rate < 2 B  Bi-level Encoding: Data rate = 2 × Bandwidth

0

5V

 Multilevel: Data rate = 2 × Bandwidth × log 2 M

Example: M=4, Capacity = 4 × Bandwidth

3-25 ©2005 Raj JainCSE473sWashington University in St. Louis

Shannon's Theorem

 Bandwidth = B Hz Signal-to-noise ratio = S/N

 Maximum number of bits/sec = B log2 (1+S/N)  Example: Phone wire bandwidth = 3100 Hz

S/N = 30 dB 10 Log 10 S/N = 30

Log 10 S/N = 3 S/N = 103 = 1000

Capacity = 3100 log 2 (1+1000) = 30,894 bps

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