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

Transmission Media

Classification of Transmission Media Type of path Guided Copper wiring, optical fibers, etc. Unguided Radio transmission, etc. .

A Taxonomy by Forms of Energy Copper based .

Why Twisted Pair? .

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? .

Noise induced is related to the spatial location of wires. Altering the spatial locations of the pair can cancel out the noise. Twisting two wires makes them less susceptible to electrical noise than leaving them parallel.

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 .

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 .

A Taxonomy by Forms of Energy .

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 Which travel mode do we want in signal transmission? .

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 High dispersion Least expensive Less dispersion Slightly more expensive Least dispersion Most expensive .

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 .

Submarine Cable Map http://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 .

Other media types that use light InfraRed (IR) Dispersing quickly Easily be blocked Hard to penetrate the wall. Commonly suitable for close communications Used in TV remote Point-To-Point Laser Communication Two devices communicate with a laser beam Cannot be obstructed Can achieve very high bandwidth Multi-Gigabit per second .

A Taxonomy by Forms of Energy .

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

Communication Satellites High-frequency signals must go direct. Cellular towers Satellites .

GEO (Geostationary Earth Orbit) Communication Satellites . Transmission delay at the speed of light: 0.238 sec

Low Earth Orbit (LEO) Satellites And Clusters 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 .

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 .

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.