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Transmission Media

Overview

• Characteristics and quality determined by:
• Medium
• Signal

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• Medium
• Guided - wire
• Unguided - wireless

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• For Guided Medium
• The medium is more important

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• For Unguided
• The bandwidth produced by the antenna is more important

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• Key concerns are data rate and distance

Design Factors

• Bandwidth
• Higher bandwidth gives higher data rate

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• Transmission impairments
• Attenuation

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• Interference
• Issue especially in case of unguided medium

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• Number of receivers
• Unicast (one sender, one receiver)
• Multicast (multiple receivers can introduce more errors)

Guided Transmission Media

• Twisted Pair
• Coaxial Cable
• Optical Fiber

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Transmission Characteristics of Guided Media

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Twisted Pair

Twisted Pair 🡺 Architecture

Two Insulated copper wires

Issues:

(1) Interference due to unwanted electrical coupling of two copper

(2) Interference due to unwanted electrical coupling between the neighboring twisted pairs

Twisted Pair 🡺 Applications

• Most commonly used medium
• Telephone 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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Twisted Pair - Pros and Cons

• Advantages
• Less expensive
• Easy to work with

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• Disadvantages
• Low data rate
• Short range

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Twisted Pair (TP) 🡺Characteristics

• Analog transmission
• Amplifiers every 5km to 6km
• Digital transmission
• Use either analog or digital signals
• repeater every 2km or 3km

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• TP is Limited
• Distance
• Bandwidth
• Data rate

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• Susceptible to interference and noise
• Easy coupling of electromagnetic fields

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Unshielded and Shielded TP

• Unshielded Twisted Pair (UTP)
• Ordinary telephone wire
• Less expensive
• Weak immunity against noise and interference
• Suffers from external EM interference

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• Shielded Twisted Pair (STP)
• An extra metallic sheath on each pair
• Relatively more expensive
• Provide better performance than UTP
• Increased Data rate
• Increased Bandwidth

UTP Categories

• Cat 3
• up to 16MHz
• Voice grade found in most offices
• Twist length of 7.5 cm to 10 cm
• Cat 4
• up to 20 MHz
• Cat 5
• up to 100MHz
• Commonly pre-installed in new office buildings
• Twist length 0.6 cm to 0.85 cm
• Cat 5E (Enhanced) –see tables
• Cat 6
• Cat 7

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Coaxial Cable

Coaxial Cable 🡺 Architecture

Coaxial Cable 🡺Applications

• Television (TV) signals distribution
• Ariel to TV
• Cable TV

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• Long distance telephone transmission
• Can carry 10,000 voice calls simultaneously
• Being replaced by fiber optic

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• Short distance computer systems links
• Local area networks (LAN)
• Metropolitan area network (MAN)

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Coaxial Cable 🡺Characteristics

• Analog
• Amplifiers every few km
• Closer if higher frequency
• Up to 500MHz

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• Digital
• Repeater every 1km
• Closer for higher data rates

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• Problem
• Inter-modulation noise
• Thermal noise

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Optical Fiber

Optical Fiber 🡺 Architecture

Optical Fiber 🡺 Benefits

• Greater capacity
• Data rates of hundreds of Gbps

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• Smaller size & weight
• Made up of extremely thin fibers

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• Lower attenuation
• Electromagnetic isolation

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• Greater repeater spacing
• 10s of km at least

Optical Fiber - Transmission Characteristics

• Operational range
• 10¹⁴ to 10¹⁵ Hz

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• Light source
• Light Emitting Diode (LED)
• Cheaper
• Wider operating temperature range
• Last longer
• Injection Laser Diode (ILD)
• Operates on laser principle
• More efficient
• Greater data rate

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• Wavelength Division Multiplexing (WDM)

Wireless Transmission Frequencies

• 2GHz to 40GHz (Microwave Frequency)
• Highly directional
• Point to point devices
• Microwave communications
• 30MHz to 1GHz (Radio Frequency)
• Omnidirectional
• Broadcast radio
• 3 x 10¹¹ to 2 x 10^{14 (Local Frequency)}
• For Local applications

Antennas

• By definition
• Is a electrical device

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• Transmission
• Radio frequency energy from transmitter
• Converted to electromagnetic energy
• By antenna
• Radiated into surrounding environment
• Reception
• Electromagnetic energy impinging on antenna
• Converted to radio frequency electrical energy
• Fed to receiver
• Same antenna often used for both

Radiation Pattern

• Antenna might radiate power in all direction
• Not same performance in all directions

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• How can we determine the performance of an antenna?
• Solution is “Radiation Pattern”
• Graphical representation of the radiated power

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• Isotropic antenna is an ideal antenna
• Radiates in all directions equally
• Use as a reference to characterize the power

Antenna Gain

• Measure of directionality of antenna

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• Power output in particular direction compared with that produced by isotropic antenna

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• Measured in decibels (dB)

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• Gain could be +ve or -ve

Terrestrial Microwave (TMW)

1. Parabolic antenna

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• Small beam

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• Line of sight

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• Use especially for P2P applications

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• Usually use for long distance communications

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Satellite Microwave (SM)

1. Satellite is relay station
2. Satellite
• receives on one frequency
• amplifies or repeats signal
• transmits on another frequency
3. Requires geo-stationary orbit
• Height of 35,784km
5. Applications
• Television
• Long distance telephone
• Private business networks

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Satellite Point to Point Link

ground based microwave transmitter

ground based microwave receiver

Satellite Broadcast Link

Is it really broadcast??

Broadcast Radio

1. Omnidirectional (travel in all directions)

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• Line of sight is over

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• Doesn’t need parabolic antenna

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• Example🡪 FM radio

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Wireless Propagation

• Signal travels along three routes

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• Ground wave
• Follows contour of earth
• Up to 2MHz
• AM radio
• Sky wave
• Signal reflected from ionize layer of upper atmosphere
• BBC world service, Voice of America
• Line of sight
• Above 30Mhz
• Antennas must be physically aligned
• Atmosphere can reflect the microwave signal

Sky Wave Propagation

Ground Wave Propagation

Line of Sight Propagation

Transmission Impairments in Wireless Transmission

• Free Space Loss
1. Signal dispersion is a function of distance
2. Ratio between power-radiated to power-received
3. Greater for lower wavelength
4. Antenna gain can be used to compensate the losses
5. Also known as near far problem

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• Refraction
• Each wireless medium has its own density
• Propagation speed is a function of density of the medium
• When medium changes, the result is refraction
• Refraction means change of direction

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