Data Transmission 484i33

Data Transmission

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Outline • Introduction • Transmission methods • Simplex, half duplex and full duplex transmission • Frequency, Spectrum and Bandwidth • Noise • Data rate • References • Tutorial 07/02/17

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Introduction • Data transmission occurs between transmitter and receiver over some transmission medium. Transmission media may be classified as guided or unguided. • With guided media, the waves are guided along a physical path; examples of guided media are twisted pair, coaxial cable, and optical fiber. • Unguided media, also called wireless, provide a means for transmitting electromagnetic waves but do not guide them • In both cases, communication is in the form of electromagnetic waves. 07/02/17

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Transmission methods • direct link – no intermediate devices

• point-to-point – direct link – only 2 devices share link

• multi-point – more than two devices share the link 07/02/17

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Simplex, half duplex and full duplex Transmission • simplex – one direction • eg. television

• half duplex – either direction, but only one way at a time • eg. police radio

• full duplex – both directions at the same time • eg. telephone 07/02/17

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Digital Data transmission System

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Frequency • Frequency is defined as number of cycles per second. • Each cycle forms a period. Thus : 1 f  Hz.........................(1) T

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Spectrum • The spectrum of a signal is the range of frequencies that it contains. Vs1

1

t

Vs2 1

2

t

Vs3 1

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Spectrum • The range of frequencies can be calculated from: 1 f  Hz T

• Where :

T

is the period for each

signal 07/02/17

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Spectrum • Thus:

1 1 f1    0.5Hz T1 2 1 1 f2    1Hz T2 1 1 1 f3    2 Hz T3 0.5 07/02/17

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Bandwidth • The absolute bandwidth of a signal is the width of the spectrum. • Thus:

B  f 3  f1  2  0.5  1.5Hz

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Noise • Define as an unwanted signals within the wanted signal

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Noise categories • Noise may be divided into four categories: – Thermal noise – Intermodulation noise – Crosstalk – Impulse noise

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Crosstalk • Crosstalk it is an unwanted coupling between signal paths. • It can occur by electrical coupling between nearby twisted pairs • experienced by anyone who, while using the telephone, has been able to hear another conversation.

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Impulse noise • Non-continuous noise consisting of irregular pulses or noise spikes of short duration and of relatively high amplitude. • It is generated from a variety of causes: – external electromagnetic disturbances, such as lightning – faults and flaws in the communications system.

• Impulse noise is generally only a minor annoyance for analog data. • However, impulse noise is the primary source of error in digital data 07/02/17

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Intermodulation Noise • Noise due to signals of different frequencies sharing the same transmission medium, • The effect of intermodulation noise is to produce signals at a frequency that is the sum or difference of the two original • This derived signal could interfere with an intended signal at the frequency

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Thermal noise • Thermal noise is due to thermal agitation of electrons. • It is present in: – all electronic devices and – transmission media

• It is a function of temperature.

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Thermal noise • Thermal noise power:

Pn  kTB..............( 2) • Where:

PT B T

is the noise power in watts is the bandwidth in Hz is the temperature in kelvins is Boltzmann’s constant

1.38 10

k 07/02/17

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Thermal noise • Thermal noise voltage is therefore: 2 n

V Pn  R

• Show derivation of equation (3)

Vn  4kTBR ......................(3) 07/02/17

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Data rate • The rate, in bits per second (bps), at which data can be communicated. • Also called channel capacity • Data rate can be computed taking into the channel noise condition.

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Data rate for noise free channel • For noise free channel condition, data rate can be computed using Nyquist formula:

C  2 B log 2 M ................( 4)

• Where:M number of 07/02/17

is the voltage levels or discrete signal EEE6211

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Data rate for noise channel • For noise channel condition, data rate can be computed using shannon formula:

C  B log 2 1  SNR .....................(5)

• Where: SNR  10 log

10

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PS .......................(6) Pn

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Reference [1] William Stallings, 2010, “Data and Computer Communications, 9th Edition”, Pearson, ISBN-10:0132172178 or ISBN13: 978-0132172172. [2] Michael P. Fitz, 2007, "Fundamentals of Communication Systems, 1st Edition”, McGraw-Hill Professional, ISBN-10: 0071482806 or ISBN-13: 978- 0071482806. [3] John G. Proakis and Masoud Salehi , 2013, “Fundamentals of Communication Systems, 2nd Edition”, Prentice Hall, ISBN10: 0133354857 or ISBN-13: 9780133354850 07/02/17

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Tutorial Question one a) State the difference between frequency spectrum and frequency bandwidth. b) A noise free channel has a bandwidth of 4kHz. i.

If the voltage levels are 6, calculate the channel capacity. ii. Suppose the channel capacity and bandwidth remains the same but the channel experiences some noise. Calculate the signal to noise ratio in decibels 07/02/17

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Tutorial Question two a) briefly explain four categories of noise b)Given a receiver with an effective noise temperature of 294 K and a 10-MHz bandwidth. If the effective resistance of the receiver is 100 ohms, calculate: i. ii. 07/02/17

The noise power The noise voltage EEE6211

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