Chapter 2

Data Communication2.1 Data Transmission

Key Points

Data and Computer Communications 2

Terminology (1)Introduction of some concepts & terms used.• Transmitter• Receiver• Medium

—Com. in form of electromagnetic waves—Guided medium

• Along a physical path• e.g. twisted pair, optical fiber

—Unguided medium• Means for transmitting electromagnetic

waves but not guide them• e.g. air, water, vacuum

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Terminology (2)• Direct link

—No intermediate devices—Transmission path between 2 devices—Can apply to both guided & unguided

media• Point-to-point

—Direct link —Only 2 devices share link

• Multi-point—More than two devices share the link

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Terminology (3)Transmission mode :• Simplex

—One direction• e.g. Television

• Half duplex—Either direction, but only one way at a time

• e.g. police radio• Full duplex

—Both directions at the same time• e.g. telephone

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Frequency• Time domain concepts

—Analog signal•Various in a smooth way over time

—Digital signal•Maintains a constant level then

changes to another constant level—Periodic signal

•Pattern repeated over time—Aperiodic signal

•Pattern not repeated over time

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Analog and Digital Data Transmission• Data

—Entities that convey meaning or info• Signals

—Electric or electromagnetic representations of data

• Transmission—Communication of data by propagation

and processing of signals

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Analog and Digital Data• Analog

—Continuous values within some interval—e.g. sound, video

• Digital—Discrete values—e.g. text, integers

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Analogue & Digital Signals

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PeriodicSignals

Amplitude

Period = T = 1/f

Period = T = 1/f

Amplitude

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Sine WaveFundamental periodic signal• Peak Amplitude (A)

—maximum strength of signal—volts

• Frequency (f)—Rate of change of signal—Hertz (Hz) or cycles per second—Period = time for one repetition (T)—T = 1/f

• Phase ()—Relative position in time

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Varying Sine Wavess(t) = A sin(2ft +)

A = 0.5

T 1360rad2ππ

45rad 4π

Φ

f = 1 Hz, T =1 s

f = 2 Hz, T =0.5 s

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Components of Speech• Frequency range (of hearing) 20Hz-20kHz

—Speech 100Hz-7kHz

• Easily converted into electromagnetic signal for transmission

• Sound frequencies with varying volume converted into electromagnetic frequencies with varying voltage

• Limit frequency range for voice channel—300-3400Hz

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Conversion of Voice Input into Analog Signal

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Binary Digital Data• Generated by computer terminals etc.

—Converted into digital voltage pulses for transmission

• Two dc components—Voltage levels – 1s and 0s

• Bandwidth of signal depends on data rate—Bandwidth approximation of digital

pulse stream

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Conversion of PC Input to Digital Signal

-

-

1 signal = 0.02 msec1 sec = 1000 msec = 50,000 bits

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Data and Signals• Usually use digital signals for digital data

and analog signals for analog data• Can use analog signal to carry digital data

—Modem (modulator/demodulator)

Digitalsignal

modulator

Analogsignal

demodulatorDigitalsignal

Transmissionmedium

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Data and Signals(2)• Usually use digital signals for digital data

and analog signals for analog data• Can use digital signal to carry analog data

—Compact Disc audio—Codec (coder-decoder)

Analogsignal

codec

digitalsignal

receiverTransmission

medium

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Analog Signals Carrying Analog and Digital Data

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Digital Signals Carrying Analog and Digital Data

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Analog Transmission• Analog signal transmitted without regard to

content• May be analog or digital data

—Eg: Analog data: voice, Digital data: binary data that passed through a

modem

• Attenuated over distance • Use amplifiers to boost signal• Also amplifies noise• Disadvantage: distance distortion

—Analog data (eg. voice) :distortion can be tolerated—Digital data :introduce errors

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Digital Transmission• Concerned with content(binary)• Integrity endangered by noise, attenuation etc. -limited distance• Repeaters used

—Repeater receives digital signal—Extracts bit pattern

• recover the patterns of 1s and 0s

—Retransmits• a new signal

• Attenuation is overcome• Noise is not amplified

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Advantages of Digital Transmission• Digital technology

— Low cost LSI/VLSI technology• Data integrity

— Longer distances over lower quality lines • with the use of repeater(regenerate) rather than amplifier

• Capacity utilization— High bandwidth links economical.

• eg: satellite channels, optical fiber— High degree of multiplexing easier with digital techniques

• Time division multiplexing (TDM) rather than frequency division multiplexing (FDM)

• Security & Privacy— Encryption

• To digital data and analog data that have been digitized

• Integration— Can treat analog and digital data similarly

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Transmission Impairments• Signal received may differ from signal

transmitted• Analog - degradation of signal quality• Digital - bit errors• Caused by

—Attenuation and attenuation distortion—Delay distortion—Noise

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Attenuation• Signal strength falls off with distance• Depends on medium• Received signal strength:

—must be enough to be detected—must be sufficiently higher than noise to be

received without error—Can be deal by using amplifier/repeater

• Attenuation is an increasing function of frequency—Noticeable for analog signal—Use equalizer to smooth out attenuation effect—Use amplifier to amplify high freq. more than low

freq.

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Delay Distortion• Only in guided media• Propagation velocity varies with frequency• Signal arrived distorted due to varying

delays experienced at its partial freq.

For voice communication, this would probablynot be noticeable but for datacommunication using modems, this could affect the phaseof the carrier or the modulationtechnique used to encode the data.

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Noise (1)• Additional signals inserted between

transmitter and receiver1. Thermal

—Due to thermal agitation of electrons—Uniformly distributed—White noise, cannot be eliminated

2. Intermodulation—Signals that are the sum and difference of original frequencies sharing a medium

Intermodulation

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Noise (2)3. Crosstalk

—A signal from one line is picked up by another

4. Impulse—Irregular pulses or spikes

• e.g. External electromagnetic interference

—Short duration—High amplitude—Sharp spike could change a 1 to 0 or a 0 to 1.

Thermal Noise• The amount of thermal,noise to e found in a

bandiwdth of 1 Hz in any device or conductor is:

• N0 = kT (W/Hz)

• N0 = noise power density in watt per 1 Hz of bandwidth

• k = boltzman constant = 1.38 x 10

• T = temp, in Kelvins

• Thermal noise in watt present in a bandwidth of B

• N = kTB = 10 log k + 10 log T + 10 log BData and Computer Communications 28

Data and Computer Communications 29

Channel Capacity Def. :Max. rate at which data can be

transmitted over a given com. path/channel under given condition

Concept of channel capacity:• Data rate

—In bits per second—Rate at which data can be communicated

• Bandwidth—Range of frequency—In cycles per second or Hertz,

(unit for frequency, f= 1/T)—Constrained by transmitter and medium

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Channel Capacity (cont)• Noise

—Average level of noise over com. path

• Error rate—Rate at which errors occur—Reception of a 1 when 0 was transmitted or

the other way

• Com. facilities are expensive—Bandwidth cost—Make efficient use of given bandwidth—Main constraint in achieving this efficiency is

noise

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Nyquist Bandwidth• In the case of a channel that is noise free,

limitation on data rate is simply the bandwidth of the signal.

• If rate of signal transmission is 2B then signal with frequencies no greater than B is sufficient to carry signal rate—Given bandwidth B, highest signal rate is 2B—Given binary signal, data rate supported by B Hz

is 2B bps

• Can be increased by using M signal levelsC= 2B log2M

C = Channel capacity

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Nyquist Bandwidth (2)• Can be increased by using M signal levels

C= 2B log2M

• M is the number of discrete signal or voltage levels

• Eg: 1 bit value (0 to 1) = 21 (M= 2 levels)2 bit value (00 to 11) =22 (M= 4

levels)3 bit value (000 to 111) =23 (M= 8

levels)

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Nyquist Bandwidth (3)• Nyquist’s formula : C= 2B log2M

Ex. 1:Consider a voice channel being used to transmit digital data. Assume a bandwidth of 3100 Hz.

So the data rate, C of the channel is 2B = 6200bps For M= 8 (23 :bit value from 000 to 111), C becomes 18,600 bps for a bandwidth of

3100 Hz.

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Nyquist Bandwidth (4)• Nyquist’s formula : C= 2B log2MEx. 2:For sampling rate for analog digital signal.

What data rate is needed for a signal with a bandwidth of 10,000Hz (1,000 to 11,000 Hz)?

Solution:The sampling rate must be twice the highest frequency in the signal:Data rate = 2(11,000)=22,000 data/sec

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Shannon Capacity Formula• Nyquist’s formula indicates that doubling

the bandwidth doubles the data rate.• Consider data rate, noise and error rate• Faster data rate shortens each bit, so

burst of noise affects more bits—At given noise level, high data rate means

higher error rate—Greater signal strength would improve ability

to receive data correctly in the presence of noise.

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Shannon Capacity Formula (2)• Formula developed by a mathematician

Claude Shannon.• Signal to noise ratio (in decibels)

SNRdb=10 log10 (signal/noise)

• Ex.: Suppose that Vs = 10.0 μv

and Vn = 1.00 μv . Then

S/N = 20 log(10(1.00)) = 20.0 dB

• Max. channel capacity C=B log2(1+SNR)

• This is error free capacity

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Shannon Capacity Formula (3)• Ex.2:• Consider an extremely noisy channel in which

the value of the signal-to-noise ration is almost zero. In other words, the noise is so strong that the signal is faint. For this channel, the capacity is calculated as

• C = B log2 (1 + S/N) = B log2 (1 + 0)

= B log2 (1) = B x 0 = 0

• This means that the capacity of this channel is zero regardless of the bandwidth. In other words, we cannot send any data through this channel.

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Shannon Capacity Formula (3)• Let us consider an ex. that relates te

Nyquist and Shannon formulations. • Ex.: Suppose that the spectrum of a

channel is between 3 MHz and 4 MHz and SNRdB=24 dB

• B = 4 MHz – 3 MHz = 1 MHz

• SNRdB = 24 dB = 10 log10 (SNR)=251

• Using Shannon’s formula,

• C = 106 x log2 (1+251)≈106 x 8 = 8 Mbps

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Shannon Capacity Formula (4)Based on Nyquist’s formula, how many signalinglevels are required?• B = 4 MHz – 3 MHz = 1 MHzUsing Shannon’s formula,

• C = 106 x log2 (1+251)≈106 x 8 = 8 Mbps

C = 2B log2 M

8 x 106 = 2 x (106) x log2 M

4 = log2 M

M = 16

END!!!

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CONCLUSION

• Successful transmission of data depends principally on 2 factors

— Quality of the signal being transmitted— Characters of the transmission medium

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Key Points• All forms of info. can be represented by

electromagnetic signals

• Analog or Digital signals can be used to convey info.

• The greater the bandwidth of the signal, the greater its info.-carrying capacity

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Key Points (2)• Major problem in designing a com. facility is

transmission impairment—Attenuation, delay attenuation, noise (thermal

noise, intermodulation noise, crosstalk, impulse noise)

• Designer of com. facility must deal with 4 factors—Bandwidth of signal, data rate that is used for

digital info., amount of noise & other impairments, and the level of error rate that is acceptable.

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• The first integrated circuits contained only a few transistors. Called "Small-Scale Integration" (SSI), they used circuits containing transistors numbering in the tens.

• The next step in the development of integrated circuits, taken in the late 1960s, introduced devices which contained hundreds of transistors on each chip, called "Medium-Scale Integration" (MSI).They were attractive economically because while they cost little more to produce than SSI devices, they allowed more complex systems to be produced using smaller circuit boards, less assembly work (because of fewer separate components), and a number of other advantages.

• Further development, driven by the same economic factors, led to "Large-Scale Integration" (LSI) in the mid 1970s, with tens of thousands of transistors per chip.Integrated circuits such as 1K-bit RAMs, calculator chips, and the first microprocessors, that began to be manufactured in moderate quantities in the early 1970s, had under 4000 transistors. True LSI circuits, approaching 10,000 transistors, began to be produced around 1974, for computer main memories and second-generation microprocessors.

• The final step in the development process, starting in the 1980s and continuing on, was "Very Large-Scale Integration" (VLSI), with hundreds of thousands of transistors, and beyond (well past several million in the latest stages).For the first time it became possible to fabricate a CPU on a single integrated circuit, to create a microprocessor.

LSI and VLSI

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FDM and TDM

FDM TDM