Group Research Assignment 2

• Due: Friday, 2/13 at 11:59pm• Use Friday’s class time to work on it

Transmission Media

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Classification of Transmission Media

• Type of path– Guided • Copper wiring, optical fibers, etc.

– Unguided• Radio transmission, etc.

A Taxonomy by Forms of Energy

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Copper based

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Why Twisted Pair?

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Radiation And Electrical Noise – Facts: • “Random” electromagnetic radiation, called noise, is

everywhere.– Light bulbs, electrical motors, paper shredders,…

• When it hits metal, electromagnetic radiation induces a small signal.

• “Random” noise can interfere with signals used for communication

How to prevent signals from being interfered?

Twisting two wires makes them less susceptible to electrical noise than leaving them parallel.

Noise induced is related to the spatial location of wires.

Altering the spatial locations of the pair can cancel out the noise.

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Which wire is used for phone lines and which is for computer networks?

Shielded vs. Unshielded Twisted Pair• Unshielded Twisted Pair (UTP)– Used in transmission where• Low-frequency signals are dominant.• Electrical noise are not very strong.• Wires are far away the source of noise.

– Phone lines

• Shielded Twisted Pair (STP)– Computer networks • Requiring high reliability

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Categories of Twisted Pair Cable

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Coaxial Cable• Twisted pair, even shielded, tends to have

problems with:– Very strong electrical noise – Close physical proximity to the source of noise

• Close to a factory that uses electric arc welding equipment• Close to a transmission tower

– High frequency signals• A fluorescent light fixture can interference signals.

• We need something more resistant to electrical noise.

Coaxial Cable

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A Taxonomy by Forms of Energy

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Optical Fibers

• Most popular type of media that uses light• Thin strand glass or transparent plastic encased

in a plastic cover

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Fiber optic transmission

Transmission: Laser or Light Emitting Diode (LED)

Reception: photo-sensitive cell or photodiode

The Nobel Prize in Physics 2014

“for the invention of efficient blue light-emitting diodes which has enabled bright and energy-saving white light sources".

Signal Transmission with Optical Fibers

• Basic optics– angle of incidence < critical angle: refraction– angle of incidence < critical angle: reflection

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Which travel mode do we want in signal transmission?

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Reflection in An Optical Fiber Is Not Perfect!

• Reflection absorbs a small amount of energy– The result is that a pulse of light sent at one end of a

fiber emerges with less energy and is dispersed (i.e., stretched) over time

Types of Fiber Optics

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High dispersionLeast expensive

Less dispersionSlightly more expensive

Least dispersionMost expensive

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Light and Equipment Must Match

• Transmission: LED (light emitting diode) or ILD (injection laser diode)– LEDs• Short distances• Slower bit rates• Multimode fibers

– ILDs• Single mode fibers• Long distance with high bit rates

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Submarine Cable Maphttp://www.submarinecablemap.com/

Interruption of Submarine Cable: 01/30/2008

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Optical Fiber Compared To Copper Wiring

• Optical fiber is more desirable than copper wiring.– Optical fiber• Immune to electrical noise• Higher bandwidth• Light traveling across a fiber does not attenuate as much as

electrical signals traveling across copper.

– Copper• Less expensive• No need special treatment on wires

– Ends of an optical fiber must be polished before being used.

• Installation is easy. • Less likely to break if accidentally pulled or bent

A Taxonomy by Forms of Energy

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Electromagnetic (Radio) Communication• Unguided– In the Radio Frequency (RF) range

• RF transmission offers advantages over light– Longer distance– Penetrate objects

RF: 3KHz – 300 GHz

What frequency do the following techniques use?

• Radio broadcasting– 87.5MHz – 108.0MHz– Example: Majic 99 99.5MHz

• Wi-Fi– 2.4GHz or 5GHz

• Global Positioning System (GPS)– 1575.42MHz

• High-frequency signals must go direct.– Cellular towers– Satellites

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Communication Satellites

GEO (Geostationary Earth Orbit) Communication Satellites

.Transmission delay at the speed of light: 0.238 sec

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Low Earth Orbit (LEO) Satellites And Clusters

• Low Earth Orbit (LEO) – Low delay

• The disadvantage of LEO is that the orbit of a satellite does not match the rotation of the earth– Adjusting the orientation of transmitters and

receivers

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How to Choose Appropriate Media?

• The choice of medium is complex– Cost

• materials, installation, operation, and maintenance

– Data rate• number of bits per second that can be sent

– Delay• time required for signal propagation or processing

– Affect on signal• attenuation and distortion

– Environment• susceptibility to interference and electrical noise

– Security• susceptibility to eavesdropping

Some Examples

• Landline phones vs. cellular phones– Mature markets vs. emerging markets

• Satellite vs. cable communications– Urban vs. suburb

• Last mile– Copper vs. Fiber • Comcast XFINITY vs. AT&T U-Verse and Verizon FiOS

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How fast can we send information over a link?

• Key channel properties: The bandwidth (B), single strength (S), and noise strength (N)– B limits the rate of transmissions– S and N limit how many signal levels we can

distinguish

Bandwidth B Signal S,Noise N

Nyquist Theorem

• The maximum symbol rate is 2B

• Thus, if there are K signal levels, ignoring noise, the maximum bit rate is:

0 1 0 1 0 1 0 1

The Effect of Noise on Communication

• In practice, the signal levels we can distinguish depends on S/N– Or SNR, the Signal-to-Noise Ratio– SNR often given on a log-scale in deciBels:• dB = 10log10(S/N)

• Shannon’s Theorem

Calculate Channel Capacity with S/N in dB

• The voice telephone system:– Signal-to-noise ratio: about 30 dB– An analog bandwidth: about 3 kHz

• Calculation– Step 1: Converting the S/N in dB into a simple

fraction: S/N = 10(dB/10)

• 30dB 1000

– Step 2: Applying Shannon's Theorem

about 30,000 bps

dB = 10log10(S/N)

Exercise• If two signal levels are used, what is

the data rate that can be sent over a coaxial cable that has an analog bandwidth of 6.2 MHz?

• If a system has an average power level of 100, an average noise level of 33.33, and a bandwidth of 100 MHz, what is the effective limit on channel capacity?

• If a telephone system can be created with a signal-to-noise ratio of 40 dB and an analog bandwidth of 3000 Hz, how many bits per second could be transmitted?

The Significance of Channel Capacity• Nyquist's work has provided an incentive to

explore complex ways to encode bits on signals.

• Shannon's Theorem is more fundamental because it represents an absolute limit derived from the laws of physics.

• Engineers recognize that faster transmission speeds will only be possible if the signal-to-noise ratio can be improved.