1

Chapter 2

Data Transmission and Media

• Transmission Terminology

• Analog and Digital Transmission

• Transmission Impairments

• Channel Capacity

• Transmission Media

Reference: William Stallings, Data and Computer Communications, 9th Edition

2

TRANSMISSION TERMINOLOGY

• Data transmission occurs between a transmitter & a receiver

via some medium

Guided medium

o twisted pair, coaxial cable, optical fiber

Unguided / wireless medium

o air, water, vacuum

Direct link

no intermediate devices

Point-to-Point

direct link

only 2 devices share link

Multi-Point

more than two devices share the link

3

TRANSMISSION TERMINOLOGY

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

4

FREQUENCY, SPECTRUM AND

BANDWIDTH

• 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

5

SINE WAVE, s(t) = A sin(2ft +)

• Peak amplitude (A)

• Frequency (f)

• Phase ()

• Wavelength ()

• Velocity ()

6

FREQUENCY DOMAIN CONCEPTS

• Signal are made up of

many frequencies

• Components are sine

waves

• Fourier analysis can

shown that any signal

is made up of

component sine waves

• Can plot frequency

domain functions

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FREQUENCY DOMAIN REPRESENTATION

8

SPECTRUM & BANDWIDTH

• Spectrum

– range of frequencies contained in signal

• Absolute Bandwidth

– width of spectrum

• Effective Bandwidth

– often just bandwidth

– narrow band of frequencies containing most

energy

• DC Component

– component of zero frequency

9

DATA RATE AND BANDWIDTH

• Any transmission system has a limited band

of frequencies

• This limits the data rate that can be carried

• Square have infinite components and hence

bandwidth

• Most energy in first few components

• Limited bandwidth increases distortion

• A direct relationship between data rate &

bandwidth

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ANALOG AND DIGITAL DATA

TRANSMISSION

• Data – entities that convey meaning

• Signals & Signaling – electric or electromagnetic representations of

data, physically propagates along medium

• Transmission – communication of data by propagation and

processing of signals

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ACOUSTIC SPECTRUM (ANALOG)

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AUDIO SIGNALS

• Frequency range 20 Hz – 20 kHz

(speech 100 Hz – 7 kHz)

• Easily converted into electromagnetic signals

• Varying volume converted to varying voltage

• Can limit frequency range for voice channel to

300-3400Hz

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VIDEO SIGNALS

• USA - 483 lines per frame, at 30 frames per

sec

– have 525 lines but 42 lost during vertical retrace

• 525 lines x 30 scans = 15750 lines per sec

– 63.5s per line

– 11s for retrace, so 52.5 s 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 s

• Max Frequency of 4.2MHz

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National Television System Committee (NTSC)

Phase Alternate Line (PAL)

Sequential Color with Memory (SECAM)

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DIGITAL DATA • as generated by computers etc.

• has two dc components

• bandwidth depends on data rate

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ANALOG SIGNALS

17

DIGITAL SIGNALS

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TRANSMISSION IMPAIRMENTS

• Signal received may differ from signal

transmitted causing:

– (Analog) degradation of signal quality

– (Digital) bit errors

• Most significant impairments are

– attenuation and attenuation distortion

– delay distortion

– noise

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ATTENUATION

• where signal strength falls off with distance

• depends on medium

• received signal strength must be: – strong enough to be detected

– sufficiently higher than noise to receive without error

• so increase strength using amplifiers/ repeaters

• is also an increasing function of frequency

• so equalize attenuation across band of frequencies used – eg. using loading coils or amplifiers

20

DELAY DISTORTION

• only occurs in guided media

• propagation velocity varies with frequency

• hence various frequency components arrive

at different times

• particularly critical for digital data

• since parts of one bit spill over into others

• causing intersymbol interference (ISI)

• Equalization can be used to overcome delay

distortion.

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NOISE

• additional signals inserted between transmitter and receiver

• Thermal (N0 = kT)

– due to thermal agitation of electrons

– uniformly distributed

– white noise

• intermodulation

– signals that are the sum and difference of original frequencies sharing a medium

22

NOISE

• crosstalk

– a signal from one line is picked up by another

• impulse

– irregular pulses or spikes • eg. external electromagnetic interference

– short duration

– high amplitude

– a minor annoyance for analog signals

– but a major source of error in digital data • a noise spike could corrupt many bits

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CHANNEL CAPACITY

• max possible data rate on comms

channel

• is a function of

– data rate - in bits per second

– bandwidth - in cycles per second or Hertz

– noise - on comms link

– error rate - of corrupted bits

• limitations due to physical properties

• want most efficient use of capacity

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NYQUIST BANDWIDTH

• Considering noise free channels, if rate of

signal transmission is 2B then can carry

signal with frequencies no greater than B

– ie. given bandwidth B, highest signal rate is 2B

• For binary signals, 2B bps needs bandwidth

B Hz

• Increase rate by using M signal levels

• Nyquist’s Formula: C = 2B log2M

• Increase rate by increasing signals

– at cost of receiver complexity

– limited by noise & other impairments

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SHANNON CAPACITY FORMULA

• consider relation of data rate, noise & error

rate

– faster data rate shortens each bit so bursts of noise

affects more bits

– given noise level, higher rates means higher errors

• Shannon developed formula relating these to

signal to noise ratio (in decibels)

• SNRdb= 10 log10 (signal/noise)

• Capacity C=B log2(1+SNR)

– theoretical maximum capacity

– get lower in practise

• Because only white noise is considered, and encoding

issues.

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Example

Suppose that the spectrum of a channel is 1MHz and

SNR = 24 dB.

So, SNR = 10(24/10) = 251

C = B log2 (1+SNR) = 8 Mbps. (Theoretical rate)

C = 2B log2 M M = 16 signaling level is required.

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Eb/N0

• Ratio of signal energy per bit to noise power. 𝐸𝑏

𝑁0=

𝑆/𝑅

𝑁0=

𝑆

𝑘𝑇𝑅

𝐸𝑏 = 𝑆𝑇𝑏 = 𝑆/𝑅 ; Tb = time required to send 1 bit

𝐸𝑏

𝑁0 𝑑𝐵

= 𝑆𝑑𝐵𝑊 − 10𝑙𝑜𝑔𝑅 − 10 log 𝑘 − 10𝑙𝑜𝑔𝑇

= 𝑆𝑑𝐵𝑊 − 10𝑙𝑜𝑔𝑅 + 228.6𝑑𝐵𝑊 − 10 𝑙𝑜𝑔𝑇

Also,

𝐸𝑏

𝑁0=

𝑆

𝑁0𝑅=

𝑆

𝑁

𝐵

𝑅 &

𝑆

𝑁= 2𝐶/𝐵 − 1

𝐸𝑏

𝑁0=

𝐵

𝑅2𝐶/𝐵 − 1

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TRANSMISSION MEDIA: DESIGN

FACTORS

• Higher bandwidth gives higher data rate

Bandwidth

• Impairments, such as attenuation, limit the distance

Transmission impairments

• Overlapping frequency bands can distort or wipe out a signal

Interference

• More receivers introduces more attenuation

Number of receivers

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ELECTROMAGNETIC SPECTRUM

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GUIDED MEDIA

Frequency Range

Typical

Attenuation

Typical

Delay

Repeater

Spacing

Twisted pair (with loading)

0 – 3.5 kHz 0.2 dB/km @ 1 kHz

50 µs/km 2 km

Twisted pairs (multi-pair cables)

0 – 1 MHz 0.7 dB/km @ 1 kHz

5 µs/km 2 km

Coaxial cable 0 – 500 MHz 7 dB/km @ 10 MHz

4 µs/km 1 – 9 km

Optical fiber 186 to 370 THz

0.2 to 0.5 dB/km

5 µs/km 40 km

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TWISTED PAIR

• analog – needs amplifiers every 5km to 6km

• digital – can use either analog or digital signals

– needs a repeater every 2-3km

• limited distance

• limited bandwidth (1MHz)

• limited data rate (100MHz)

• susceptible to interference and noise

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(a) Twisted pair

(b) Coaxial cable

—Outer conductor is braided shield

—Inner conductor is solid metal

—Separated by insulating material

—Covered by padding

Light at less than

critical angle is

absorbed in bufer

coating

Angle of

incidence

Angle of

reflection

Outer sheathOuter conductor

Insulation

Inner

conductor

—Glass or plastic core

—Laser or light emitting diode

—Small size and weight

(c) Optical fiber

Figure 4.2 Guided Transmission Media

Core

Buffer

coating

Cladding

twist

length—Separately insulated

—Twisted together

—Often "bundled" into cables

—Usually installed in building

during construction

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UNSHIELDED VS SHIELDED TP

• Unshielded Twisted Pair (UTP) – ordinary telephone wire

– cheapest

– easiest to install

– suffers from external EM interference

• Shielded Twisted Pair (STP) – metal braid or sheathing that reduces interference

– more expensive

– harder to handle (thick, heavy)

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Cat 3

Class C

Cat 5

Class D

Cat 5E Cat 6

Class E

Cat 7

Class F

Bandwidth 16 MHz 100 MHz 100 MHz 200 MHz 600 MHz

Cable Type UTP UTP/FTP UTP/FTP UTP/FTP SSTP

Link Cost

(cat 5 = 1) 0.7 1 1.2 1.5 2.2

UTP : Unshielded Twister Pair; FTP : Foil Twisted Pair; SSTP : Shielded Screen Twisted Pair

Frequency

(MHz)

Attenuation (db/100 m) Near-End Crosstalk (dB)

Cat 3

UTP

Cat 5

UTP

150-ohm

STP

Cat 3

UTP

Cat 5

UTP

150-ohm

STP

1 2.5 2.0 1.1 41 62 58

4 5.6 4.1 2.2 32 53 58

16 13.1 8.2 4.4 23 44 50.4

25 10.4 6.2 41 47.5

100 22.0 12.3 32 38.5

300 21.4 31.3

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NEAR END CROSSTALK

• coupling of signal from one pair to

another

• occurs when transmit signal entering the

link couples back to receiving pair

• ie. near transmitted signal is picked up by

near receiving pair

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COAXIAL CABLE • superior frequency characteristics to TP

• performance limited by attenuation & noise

• analog signals – amplifiers every few km

– closer if higher frequency

– up to 500MHz

• digital signals – repeater every 1km

– closer for higher data rates

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OPTICAL FIBER

• greater capacity

– data rates of hundreds of Gbps

• smaller size & weight

• lower attenuation

• electromagnetic isolation

• greater repeater spacing

– 10s of km

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• Uses total internal reflection to transmit light

– effectively acts as wave guide for 1014 to 1015 Hz

• Can use several different light sources

– Light Emitting Diode (LED) • cheaper, wider operating temp range, lasts

longer

– Injection Laser Diode (ILD) • more efficient, has greater data rate

• Relation of wavelength, type & data rate

OPTICAL FIBER

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FREQUENCY UTILIZATION FOR FIBER

APPLICATIONS

Wavelength

(vacuum) (nm)

Frequency

Range (THz)

Band Label Fiber Type Application

820 to 900 366 to 333 Multimode LAN

1280 to 1350 234 to 222 S Single mode Various

1528 to 1561 196 to 192 C Single mode WDM

1561 to 1620 192 to 185 L Single mode WDM

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ATTENUATION IN GUIDED MEDIA

41

WIRELESS TRANSMISSION

FREQUENCIES

1GHz to 40GHz

• Referred to as microwave frequencies

• Highly directional beams are possible

• Suitable for point to point transmissions

• Also used for satellite communications

30MHz to 1GHz

• Suitable for omnidirectional applications

• Referred to as the radio range

3 x 1011 to 2 x 1014

• Infrared portion of the spectrum

• Useful to local point-to-point and multipoint applications within confined areas

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43

TUTORIAL 1. (a) Suppose that a digitized TV picture is to be transmitted from a source that

uses a matrix of 480*500 picture elements (pixels), where each pixel can take

on one of 32 intensive values. Assume that 30 pictures are sent per second.

(This digital source is roughly equivalent to broadcast TV standards that have

been adopted.) Find the source rate R (bps).

(b) Assume that the TV picture is to be transmitted over a channel with 4.5-

MHz bandwidth and a 35 dB signal-to-noise ratio. Find the capacity of the

channel (bps).

(c) Discuss how the parameters given in part (a) could be modified to allow

transmission of color TV signals without increasing the required value for R.

2. Given an amplifier with an effective noise temperature of 10,000 K and a 10-

MHz bandwidth, what thermal noise level, in dBW, may we expect at its

output?

3. What is the channel capacity for a teleprint channel with a 300-Hz bandwidth

and a signal-to-noise ratio of 3 dB, where the noise is white thermal noise?

4. A digital signaling system is required to operate at 9600 b/s.

(a) If a signal element encodes a 4-bit word, what is the minimum required

bandwidth of the channel?

(b) Repeat part (a) for the case of 8-bit words.

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5. Consider a channel with a 1-MHz capacity and an SNR of 63.

(a) What is the upper limit to the data rate that the channel can carry?

(b) The result of part (a) is the upper limit. However, as a practical matter, better

error performance will be achieved at a lower data rate. Assume we choose a

data rate of 2/3 the maximum theoretical limit. How many signal levels are

needed to achieve this data rate.

6. Given a channel with an intended capacity of 20 Mbps, the bandwidth of the

channel is 3 MHz. Assuming white thermal noise, what signal-to-noise ratio is

required to achieve this capacity?

7. Suppose that data are stored on 1.4-Mbyte floppy diskettes that weigh 30 g

each. Suppose that an airliner carries 10,000 kg of these floppies at a speed of

1000 km/h over a distance of 5000 km. What is the data transmission rate in bits

per second of this system?

8. A telephone line is known to have a loss of 20 dB. The input signal power is

measured as 0.5W, and the output noise level is measured as 4.5 uW.

Calculate the output SNR in dB.

9. Given a 100 W power source, what is the maximum allowed length for the

following transmission media if a signal of 1 W is to be received? (a) 24-gauge

(0.5 mm) twisted pair operating at 300 Hz. (b) 24-gauge (0.5 mm) twisted pair

operating at 1MHz. (c) 0.375-in (9.5 mm) coaxial cable operating at 1 MHz. (d)

0.375-in (9.5 mm) coaxial cable operating at 25 MHz. (e) optical fiber operating

at its optimal frequency. (attenuation graph in slide 41)

TUTORIAL