Ccnet Lec 05 Transmission Media

8/6/2019 Ccnet Lec 05 Transmission Media

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Lecture

05Computer Communications& Networks

Transmission Media

Muhammad Yousaf


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Transmission Media:

Transmission mediumPhysical path between transmitter and receiver

Transmission is in the form of electromagnetic waves

Guided/wired media

Signal is guided along a solid path

e.g. twisted pair, coaxial cable, fiber optic

Unguided/wireless media

Signal is not guided along a certain path, ratherpropagates around all the directions

e.g. air, water, vacuum


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Electromagnetic Spectrum:


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Communication Spectrum:


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Wired Media:

Most commonly used wired medias:Twisted Pair

Shielded Twisted Pair

Unshielded Twisted Pair

Coaxial cableOptical fiber


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Twisted Pair:

RJ-45 Jack


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TP physical description:

Two insulated copper wiresWire thickness = from 0.4 to 0.9mm

Arranged in spiral pattern

Pair of wires act as a single communication linkNumber of pairs can be bundles together

Twisting decreases the interference due tocrosstalk

Neighboring pairs have different twist length

Twist length = from 0.5 to 15cm


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Twisted Pair applications:

Most common mediumTelephone network

Between house and local exchange (subscriber loop)

Within buildings

To private branch exchange (PBX)

For local area networks (LAN)

10Mbps or 100Mbps


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TP Pros and Cons:

CheapEasy to work with

Low data rate

Short range


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TP Transmission Characteristics:

AnalogNeeds Amplifiers every 5 to 6km

Digital

Needs Repeater every 2 to 3km

Limited distance

Limited bandwidth

Limited data rate

Susceptible to interference and noise


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UTP & STP:

Unshielded Twisted Pair (UTP)Ordinary telephone wire

Cheapest

Easiest to install

Suffers from external

electromagnetic (EM)

interference


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UTP & STP: cont

Shielded Twisted Pair (STP)Metal braid or sheathing that reduces interference

More expensive

Harder to handle (thick, heavy)


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UTP Categories:

Bandwidth up to 600 MHz

Link cost = 2.2 (Normalized to cat 5)Cat-7

Bandwidth up to 250 MHz

Link cost = 1.5 (Normalized to cat 5)Cat-6

Bandwidth up to 100MHz

Twist length 0.6 cm to 0.85 cm

Data grade cable, most commonly used medium for LANs

Link cost = 1 (Normalized to cat 5)

Cat-5

Bandwidth up to 20 MHzNot so much used

Cat-4

Bandwidth up to 16MHz

Twist length of 7.5 cm to 10 cm

Voice grade cable, have been used for telephony networks

Link cost = 0.7 (Normalized to cat 5)

Cat-3

DescriptionCategory


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Coaxial Cable:


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Coax physical description:

Consists of two conductorsOuter conductor, cylindrical hollow

Inner conductor, surrounded by outer conductor

Both separated by some insulating materialOuter conductor also covered by protecting shield

Coax diameter = from 1 to 2.5cm

Less susceptible to interference and crosstalk


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Coax Applications:

Most versatile mediumTelevision distribution (Cable TV)

Can carry hundreds of TV channels up to 10s of Km

Long distance telephone transmission

Can carry 10,000 voice calls simultaneously

Now being replaced by fiber optic

Local area networks

Also, have been used for high speed I/O forcomputer systems


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Optical Fiber:


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Optical Fiber physical description:

Thin, flexible mediumData transmit as optical ray (light)

Various materials can be used to make fiber

Ultra pure fused Silica

Multi-component glass

Plastic fiber

Optical Fiber consists of 3 concentric components

Core (inner most section, diameter = 8 to 100m)Cladding (of different optical characteristics than core)

Jacket (surrounds one or more claddings)


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Optical Fiber Benefits:

Greater capacityData rates of hundreds of Gbps

Smaller size & weight

Lower attenuation

Repeater spacing up to 40Km

Reducing cost and error rate

Electromagnetic isolation

Not vulnerable to interference and noiseSecurity

Cant be taped


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Optical Fiber Applications:

Long-haul trunks of telecom networksIncreasingly replacing coax and microwave

Submarine optical fibers getting more popularity

FLAG = Fiber Link Around Globe

A global fiber optic connectivity project

Subscriber loops

NayaTel & COMSATS providing fiber connectivity tosubscribers

LANs

FDDI = Fiber Distributed Data InterfaceA LAN standard similar to token ring


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Optical Fiber - Trans Char:

Principle: Total internal reflectionPortions of infrared and visible light is used

Spectrum 1014 to 1015 Hz

Light sources for fiber:

Light Emitting Diode (LED)

Injection Laser Diode (ILD)

Wavelength Division Multiplexing:

Multiple light waves of different frequencies aretransmitted through single fiber

Bell Lab demonstration:

100 beams x 10Gbps fiber => 1Tbps data rate


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Wavelength Division Multiplexing:


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Wireless Media


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Wireless Transmission:

Antenna is required for every wireless

communicationFor transmission antenna radiates electromagnetic waves

For reception it picks these electromagnetic waves

OmnidirectionalSignal spreads in all directions

Can be received by many antennas

Directional

Focused beamCareful alignment is required

Higher the frequency, more it is possible to focusthe EM beam


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Omnidirectional Antenna:


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Directional Antenna:


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Sectorized Antenna:


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Wireless Propagation:

Wireless signals can travel in three ways:

Ground waveFollows contour of earth

Up to 2MHz

AM radio

Sky waveFrom 2MHz to 30MHz

Signal reflected from ionosphere layer of upper atmosphere

Ionosphere = Ionized region of atmosphere >50Km above earths surface

BBC world service, Voice of America

Line of sightAbove 30MHz

Line of Sight (LoS) limitation


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Ground Wave Propagation:


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Sky Wave Propagation:

Ionosphere > 50KmStratosphere >10Km,


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Line of Sight Propagation:


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Wireless Impairments:

Free space losses

Signal disperses with distance

Atmospheric Absorption

Water vapour and oxygen absorb radio signals

Water absorption, greatest at 22GHz, less below 15GHz

Oxygen absorption, greater at 60GHz, less below 30GHzRain & fog scatter radio waves

Multipath interference

Better to get line of sight if possible

Signal can be reflected causing multiple copies to be receivedMay be no direct signal at all

May reinforce or cancel direct signal


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Multipath Interference:


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Wireless Frequency spectrum:

Broadcast radio

1MHz to 1GHz

Omni-directional

Microwave

2GHz to 40GHzDirectional

Terrestrial / Satellite

Infrared

300GHz to 200THz

Highly directional

Short distance communication


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Broadcast Radio:

Omnidirectional

AM & FM radio

UHF and VHF television

Doesnt requires Line of Sight

Suffers from multipath interference

Less sensitive from attenuation due to rain


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Broadcast Radio: cont

Maximum distance between two antennas:

d = distance in kilometers

h = height in metersK = adjustment factor, = 4/3

Losses (L) due to attenuation:

d = distance

= wavelength

Kd 14.7

d

L

24

log10


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Effect of Height on Distance:


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Terrestrial Microwave:

Parabolic dishFocused beam

Line of sight requirements

Long haul telecommunications

Higher frequencies give higher data rates

Distance and losses equations same as broadcastradio


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Satellite Microwave:

Satellite acts as relay station

Receives on one frequency, amplifies it and transmits onanother frequency

4 / 6 GHz band:Angular spacing between satellites 4o, maximum 90 satellites

4.2 to 4.7GHz for downlink5.9 to 6.4GHz for uplink

12 / 14 GHz band:Angular spacing between satellites 3o, maximum 120 satellites

11.7 to 12.2GHz for downlink

14.0 to 14.5GHz for uplink

19 / 29 GHz band:For future requirements

17.7 to 21.2GHz for downlink

27.5 to 31.0GHz for uplink


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Satellite Point to Point Link:


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Satellite Broadcast Link:


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Satellite orbits:

Geostationary Earth Orbit (GEO)

Height = 35,784km

Latency = 240ms

Needs 3 satellites to cover the whole globe

Medium Earth Orbit (MEO)

Height = from 7000 to 18000km

Latency = from 35 to 85ms

Global Positioning System (GPS) comprises of 24 satellites

Low Earth Orbit (LEO)

Height =


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Satellite orbits: cont


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Infrared:

Modulated non-coherent infrared light

Line of sight (either directed or reflected)

Blocked by walls

Require no licensing

Remote control, IRDA port

IRDA = Infrared Data Association


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ISM band:

Industrial, Scientific, Medical (ISM) Band License free frequency spectrum WLAN IEEE 802.11 (uses both 2.4 and 5GHz bands) Bluetooth IEEE 802.15 (uses 2.4GHz band)


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