This Lecture BUS3150 - Computer Facilities Network Management · 2011-06-25 · BUS3150 - Computer Facilities Network Management 4 - Transmission Media Faculty of Information Technology

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4 - Transmission Media

Faculty of Information Technology

BUS3150 - Computer Facilities Network Management


Faculty of Information Technology

Monash University

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2 Faculty of Information Technology

This Lecture

• Different transmission media.

– Guided transmission media:

· Twisted Pair· Coaxial Cable· Optical Fibre

– Wireless transmission:

· Terrestrial Microwave· Satellite Microwave· Broadcast Radio· Infrared

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Overview

• Guided - wire /optical fibre

• Unguided - wireless

• Characteristics and quality of a data transmission determined by mediumand signal.

– For guided, the medium is more important.

– For unguided, the bandwidth produced by the antenna is more impor-tant.

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Overview

• Key concerns are data rate and distance.

• These depend on:

– Bandwidth:

· Higher bandwidth gives higher data rate.

– Transmission impairments:

· Attenuation limits distance.· Interference and noise causes bit errors.

– Number of receivers in guided media.

· More receivers (multi-point) introduce more attenuation.

• Also need to consider cost (materials, labour and operating), security, op-erating environment, safety, reliability, maintenance and expansibility.

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Electromagnetic Spectrum for Telecommunications

102Frequency

(Hertz) 103 104 105 106 107 108 109 1010 1011 1012 1013 1014 1015

Power and telephoneRotating generatorsMusical instrumentsVoice microphones

MicrowaveRadarMicrowave antennasMagnetrons

InfraredLasersGuided missilesRangefinders

RadioRadios and televisionsElectronic tubesIntegrated circuitsCellular Telephony

ELF VF

ELF = Extremely low frequencyVF = Voice frequencyVLF = Very low frequencyLF = Low frequency

MF = Medium frequencyHF = High frequencyVHF = Very high frequency

UHF = Ultrahigh frequencySHF = Superhigh frequencyEHF = Extremely high frequency

VLF LF MF HF VHF UHF SHF EHF

Twisted Pair

Coaxial Cable

Visiblelight

OpticalFiber

FM Radioand TV

AM Radio Terrestrialand SatelliteTransmission

Wavelengthin space(meters)

106 105 104 103 102 101 100 10−1 10−2 10−3 10−4 10−5 10−6

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Guided Transmission Media

• It is possible to store data on magnetic (tapes or disks) or optical (CD orDVD) media, package the media and send using a courier.

• For extremely large sets of data, this is the cheapest method of transmis-sion. What is the down side?Andrew S. Tanenbaum, “Never underestimate the bandwidth of a station

wagon full of tapes hurtling down the highway”.

• For guided transmission, we typically consider:

– Twisted pair

– Coaxial cable

– Optical fibre

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Copper Twisted Pair

Outer insulatoror PVC

Solid copperconductors

Twist length

• Twisted pair cable consists of two insulated copper wires twisted together.

• Twisting allows each wire to have approximately the same noise level andreduces crosstalk.

• Typically installed in buildings during construction.

• Twisted pair comes in two varieties: STP and UTP.

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Effect of Noise on Parallel and Twisted-Pair Lines

Total effect is16 − 12 = 4

ReceiverSender

Noise effect = 12 units

Noise source

Noise effect = 16 units

16

12

Noise source

14

14Receiver

4

3

4 4 4

3 3 3

Sender

Total effect is14 − 14 = 0

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Twisted Pair - Applications, Pros and Cons

• Applications:

– Most common medium for both analog and digital.

– Telephone network - Between house and local exchange (subscriberloop).

– Within buildings - To private branch exchange (PBX).

– For local area networks (LAN) - 10Mbps to 1Gbps.

• Pros - Cheap and easy to work with.

• Cons - Low bandwidth/data rate and short range.

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Twisted Pair - Transmission Characteristics

• Analog:

– Needs amplifiers every 5km to 6km.

• Digital:

– Can use either analog or digital signals.

– Needs a repeater every 2 to 3km (depends on data rate).

• Limited distance.

• Limited bandwidth (1MHz).

• Limited data rate (100MHz).

• Susceptible to interference and noise.

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Unshielded and Shielded TP

• Unshielded Twisted Pair (UTP):

– Ordinary telephone wire.

– Cheapest.

– Easiest to install.

– Suffers from external EM interfer-ence.

• Shielded Twisted Pair (STP):

– Insulated twisted pairs encased in ametal braid or sheathing that reducesinterference.

– More expensive.

– Harder to handle and work with(thick, heavy).

Twisted pairs5 pairs

Plastic cover

Plastic cover Insulation CopperMetal shield

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UTP Categories

• Cat 3:

– Up to 16 MHz and 16 Mbps.

– Voice grade found in abundance in most offices.

– Twist length of 7.5 cm to 10 cm.

• Cat 4:

– Up to 20 MHz.

• Cat 5:

– Up to 100 MHz and 100 Mbps.

– Data-grade commonly pre-installed in new office buildings.

– Tighter twist (more expensive): twist length 0.6 cm to 0.85 cm.

• Cat 5E (enhanced), Cat 6, Cat 7.

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Near End Crosstalk

• Coupling of signal from one pair to another.

• Coupling takes place when transmit signal entering the link couples backto receiving pair.

• i.e. near transmitted signal is picked up by near receiving pair.

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Twisted Pair - Category Characteristics

Attenuation (dB per 100 m) Near−end Crosstalk (dB)

Frequency(MHz)

Category 3UTP

Category 5UTP 150−ohm STP

Category 3UTP

Category 5UTP 150−ohm STP

1 2.6 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

Category 3Class C

Category 5Class D

Category 5E Category 6Class E

Category 7Class 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 twisted pairFTP = Foil twisted pairSSTP = Shielded screen twisted pair

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Coaxial Cable

Outer conductor(shield)

Plastic cover Inner conductor

Insulator

• A coax cable consists of the following:

– A centre conductor - usually copper.

– A metallic outer conductor (shield), serves as aground.

– An insulator covering the centre conductor.

– A plastic jacket.

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Coaxial Cable - Applications

• Most versatile transmission medium.

• Television distribution:

– Ariel to TV.

– Cable TV (hundreds of TV channels up to a few tens of km).

• Long distance telephone transmission:

– Can carry 10,000 voice calls simultaneously (FDM).

– Mostly replaced by fibre optic, microwave and satellite.

• Short distance computer systems links.

• Local area networks.

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Coaxial Cable - Transmission Characteristics

• Superior frequency characteristics to twisted pair.

• Performance limited by attenuation and noise.

• Analog signals:

– Amplifiers every few km, closer if higher frequency.

– Up to 500 MHz

• Digital signals:

– Repeater every 1km, closer for higher data rates.

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Refraction and Reflection (Snell’s law)

More dense medium(water or glass)

BeamLess dense medium (air)

I

R

(a) From less dense to more dense medium

More dense medium(water or glass)

Less dense medium (air)

I

R

Beam

(b) From more dense to less dense medium

angleCritical

Angle ofincidence reflection

Angle of

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Refraction and Reflection Examples

You cannot see the pavement

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Refractive Index

• The refractive index of a medium is a measure of the speed at which lighttravels in it.

• It is proportional to the density of the medium.

• Index of refraction, n =

Speed of light in vacuumSpeed of light in the medium.

• For example:

– Vacuum: n = 1.0 ←−Fast

– Air: n = 1.0002926

– Water: n = 1.333

– Glass: n = 1.5

– Diamond: n = 2.419 ←−Slow

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Optical Fiber

Light at less thancritical angle isabsorbed in jacket

Angle ofreflection

Angle ofincidence

Cladding

Plastic coating

Core

• Glass or plastic core surrounded by a cladding with different optical prop-erties and coated in acrylate (plastic) for protection.

• Refractive index (density) of the core is higher than the refractive index ofthe cladding. Acts as a wave guide (total internal reflection) to trap lightin the core.

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Optical Fiber - Advantages

• Mechanical characteristics of fiber:

– Small diameter, light weight.

– Flexible.

– Resistance to corrosive materials.

• Electrical characteristics of fiber:

– Low attenuation.

– Wide bandwidth.

– Electromagnetic immunity.

– Ground loop elimination.

– No electromagnetic radiation.

• Greater repeater spacing.

• Large capacity.

• Space efficiency.

• Easy installation.

• Higher security.

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Optical Fiber - Applications

• Long-haul trunks:

– Average about 1500 km with high capacity (20 - 60,000 voice chan-nels).

– Undersea optical fiber being used.

• Metropolitan trunks:

– Average about 12 km with 100,000 voice channels per trunk group.

– Join telephone exchanges.

• Rural exchange trunks:

– Ranging from 40 to 160 km with fewer than 5000 voice channels.

• Cable TV networks.

• Subscriber loops: telephone, data, video, etc.

• LANs: 100 Mbps to 1 Gbps.

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Optical Fiber - Transmission Characteristics

• Act as wave guide (total internal reflection) for 1014 to 1015 Hz.

– Portions of infrared and visible spectrum.

• Light Emitting Diode (LED):

– Cheaper.

– Wider operating temperature range.

– Last longer.

• Injection Laser Diode (ILD):

– More efficient.

– Greater data rate.

• Wavelength division multiplexing: multiple light beams at different fre-quencies (colours).

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Optical Fiber - Transmission Modes

Source Destinationindex (n)

Refractive

(a) Step-index Multimode


Refractive

(b) Graded-index Multimode


Refractive

(c) Single Mode

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Optical Fiber - Dimensions

125

50

Multi-mode Fiber

125

7

Single-mode FiberDimensions in Micrometers (µm)

• Due to the dimensions:

– Multi-mode enables the use of cheap LED light sources.

– Single-mode requires more expensive laser sources.

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Optical Fiber - Cable� �

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Optical Fiber - Attenuation

800 900

2.0

1.8

1.6

1.4

1.2

1.0

0.8

0.6

0.4

0.2

1000 1100 1200 1300Wavelength (nanometer)

850nmBand

1300nmBand

1550nmBand

Atte

nuat

ion

(dB

/km

)

1400 1500 1600 1700 1800

• Depends on the light wavelength and the physical properties of the glass.

• Three low attenuation windows centred on 850, 1300 and 1550nm.

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Attenuation of Typical Guided Media

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Wireless Transmission

• Unguided media.

• Transmission and reception via antenna.

• Two basic configurations:

– Directional:

· Focused beam.· Careful alignment required.

– Omnidirectional:

· Signal spreads in all directions.· Can be received by many antennae.

– The higher the signal frequency, the more possible it is to focus.

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Frequencies

• 30 MHz to 1 GHz:

– Omnidirectional.

– Broadcast radio and television.

• 2 GHz to 40 GHz:

– Microwave.

– Highly directional.

– Point to point.

– Satellite.

• 3×1011 to 2×1014:

– Infrared.

– Point-to-point and multipoint ina confined area.

MF and HF

300 KHz 30 MHz3 MHz

CB radio

AM radio

530 1700

VHF

2 − 6Channels

7 − 13Channels

30 MHz 300MHz

TVAircraftFMTV

Paging

2161741088854

UHF

14 − 69Channels

Mobiletelephone

Cellularradio

3GHz

MicrowaveUHF TV

Paging

806470300 MHz

SHF and EHF

Microwave

3 GHz 300 GHz30 GHz

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Terrestrial Microwave

• Used for long haul telecommunications and short point-to-point links.

• Parabolic dish (typical size of 3 m diameter) creates a focused beam.

• Line of sight to receiving antenna (substantial height above ground).

• Microwave relay towers used to achieve distance.

• Higher frequencies give higher data rates and require smaller antennas.

• Attenuation is increased with rainfall (especially above 10 GHz).

Earth

Modulated signals

80 km4 − 10 GHz

100 m

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Satellite Microwave

• Satellite is a microwave relay station.

• Satellite receives on one frequency, amplifies or repeats signal and trans-mits on another frequency.

– e.g. uplink 5.925-6.425 GHz and downlink 3.7-4.2 GHz.

• Operates best in the range of 1 to 10 GHz.

• May require geo-stationary orbit:

– To maintain line of sight, requires a height of 35,784km.

– High propagation delay (0.25s).

• Typical Uses:

– Television.

– Long distance telephone.

– Private business networks.

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Satellite - Point-to-Point Link

Satelliteantenna

Earthstation

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Satellite - Broadcast Link

Satelliteantenna

Multiplereceivers

Transmitter

Multiplereceivers

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Orbit Classifications

Altitude (km)

35,000

30,000

25,000

20,000

15,000

10,000

5,000

0

GEO

MEO

Upper Van Allen belt

Lower Van Allen belt

LEO

• GEO: geo-stationary earth orbit

• MEO: medium earth orbit (e.g. 24 Global Positioning System (GPS) satellites).

• LEO: low earth orbit (e.g. Iridium (66 satellites) and Teledesic (288 satellites)).

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Example LEO: Iridium System

• Iridium system consists of 66 satellites providing global communication.

• Has an orbit height of 780 km and period of around 100 minutes.

(a) (b)

Fig. 2-18. (a) The Iridium satellites form six necklaces around theearth. (b) 1628 moving cells cover the earth.

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Broadcast Radio

• Omnidirectional: no need for rigidly mounted dish antennas.

• FM radio and UHF and VHF television.

• Line of sight.

• Less affected by rainfall and longer wavelength suffers relatively less attenuation.

• Suffers from multipath interference:

– Reflections (e.g. ghosting on TV).

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Infrared

• Modulate non-coherent infrared light.

• Line of sight (or reflection from light-coloured surface).

• Blocked by walls.

• e.g. TV remote control, IRD port.

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Further Reading

• Stallings, W., “Data and Computer Communications”, Prentice Hall. Chap-ter 4.

• Forouzan, B. A., “Data Communication and Networking”, McGraw-Hill.Chapter 7. Satellites covered in Chapter 17.

Documents

This Lecture BUS3150 - Computer Facilities Network Management · 2011-06-25 · BUS3150 - Computer Facilities Network Management 4 - Transmission Media Faculty of Information Technology