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Introduction to Information Technology

LECTURE 8: TRANSMISSION TECHNOLOGY

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Communications Media Free Space

Electromagnetic wave through free space via radio frequency and satellite systems.

Satellite Communications Microwave Transmission Cellular Radio

Copper Wire Electromagnetic variations through copper wires

Cable Television (Coaxial Cable) Copper Cable Into Homes (Twisted Pair Cable) LANs, WANs, Telephone System

Fiber Optic Cable Light transmitted through fiber-optic cable (glass)

Backbone of Telephone System and WANs

Wireless

Wired

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Some Transmission Concepts

Information Rate - The number of bits that can be sent per second at the transmission origin and received at the destination. Then what’s bandwidth?

Latency - The time delay involved in the movement of a message from one location to another.

Real Time Data Transmission - Nothing arrives instantaneously, but some applications receive data in adequate time to be considered “real time”

Even speech has a delay time, but is interpreted as real time.

Application-specific Speed of Light - No information can be relayed at speeds

that exceed the speed of light. (Einstein) 3x108 m/sInformation rate versus latency: Communication channels can move data at a rate of billions of bits per second, but still suffer huge latencies.

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Radio-Frequency Systems

Radio communication has been in existence for more than 90 years. We’re constantly surrounded by technology-generated radio waves:

Entertainment - AM and FM radio, broadcast TV, satellite TV. Communications - Cellular, global long distance, CB radio, wireless

LANs. Space exploration Garage door opener, baby monitor, wireless phone, car entry. Air traffic control, navigation systems, military applications, etc.

We’re constantly bathed in natural electromagnetic radiation The sun, lightning flashes, radioactive material in the earth, etc.

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Radio Waves

A radio wave travels from a transmitter to a receiver - What is it?

Continuous sine waves are used to transmit information

Electromagnetic wave Travels at the speed of light Unlike sound waves, which require air, radio waves

propagate through air or empty space, just like light travels to the earth from distant stars.

Visible light, radio, the X-Ray, ultraviolet light, etc. travel in a similar manner

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Radio Waves Each radio signal uses a different sine wave frequency

Frequency Modulation (FM): 88 MHz to 108 MHz Amplitude Modulation (AM): 535 kHz to 1,700 kHz Television stations

54 to 88 MHz for channels 2 through 6 174 to 220 MHz for channels 7 through 13

Garage door openers, alarm systems: 40 MHz Cordless phones: 40 to 50 MHz, 900 MHz Radio controlled cars: Around 75 MHz Air traffic control radar: 960 to 1,215 MHz

Electromagnetic Spectrum

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Calculating Wavelength

Electromagnetic Spectrum

Each frequency has an associated wavelength calculated as follows:

c/fwavelengthc = speed of light (3x108 m/s)f = frequency in Hz

What is the wavelength for a frequency of 8 MHz?

3x108/8x106 = 37.5 m

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Calculating Wavelength

c/fwavelengthc = speed of light (3x108 m/s)f = frequency in Hz

What is the wavelength for a frequency of 5 MHz?

3x108/5x106 = 60 m

=300,000,000/5,000,000 = 60 m

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How Can Waves Carry Information?

Through modulation: the process of controlling the properties of a signal so that it contains the information patterns to be transmitted.

Three types of modulation Amplitude Modulation (AM) Frequency Modulation (FM) Phase Modulation (PM)

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Amplitude Modulation (AM)

Uses a single frequency to convey information Varies only its amplitude to convey information Vulnerable to interference

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Frequency Modulation

Frequency Modulation (FM) – Information represented as changes in frequency rather than

amplitude. Less vulnerable to interference. Requires more of the frequency spectrum.

Phase Modulation (PM) A change of phase (direction) represents a change from one state to another,

such as from a 0 to a 1.

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Basic Parameters in the Design of RF systems

Transmitter power Transmitter frequency – Maintained by FCC Receiver sensitivity Desired bandwidth and/or bit rate Limitations on antenna size, locations Desired transmission distance

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Examples

Baby Monitor Modulation: Amplitude Modulation Frequency range: 49 MHz Transmitter power: .25 Watts

Cell Phone Modulation: Frequency Modulation Frequency range: 824 to 849 MHz Transmitter power: 3 Watts

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Radio Communications Example - GPS

Determines the position of a GPS receiver anywhere on (or above) the Earth

GPS has many applications: Aircraft and Marine Navigation Military Applications Hiking, Hunting, Other Outdoor Activities Driving (Route Finding, Emergencies) Surveying

Position defined in terms of latitude, longitude, and elevation above sea level

Limitation – Receiver must have a relatively clear view of the sky

3 Major Components Satellites, Receivers, and System Control Center

Who owns, operates, and funds the GPS system?

GPS:GLOBAL POSITIONING SYSTEM

GPS Satellite

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How Does GPS Work?

The principle of operation of the GPS system, triangulation, is very simple.

If you knows your distance from a given point, you know you are somewhere on a circle with that point in the center.

If, at the same time, you also know your distance from a different point, you know you are on another given circle that intersects.

With a known distance from a third point, you can identify your exact location.

GPS triangulation Satellite positions always known. Distance from satellite determined using speed of

light. 24 GPS satellites in orbit at an altitude of 11,000

miles with an orbital period of 12 hours. General Example of Triangulation

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Triangulation

With signals from four (or more) satellites, the altitude can also be known, so that the position in 3D space may be determined.

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GPS Receiver

5 Major Elements Antenna Sensitive radio receiver Microprocessor

Distance calculations, speed of travel, direction of travel, route to follow, time required.

Database – street address, maps Display

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Geosynchronous Satellites

A geosynchronous satellite appears to remain stationary to the earth.

Orbits the Earth at a height of 22,300 miles At this height, the time for one orbit of the Earth is 24 hours. Because the Earth also rotates once every 24 hours, the

satellite appears to stay still over the same spot on Earth. At this distance, a signal travelling at the speed of light takes

270 milliseconds to reach the Earth. The delay between when you say something and when you

hear the other person’s response is 540 milliseconds - over half a second.

What’s a geosynchronous satellite?

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Historical Milestone: SPUTNIK

First satellite Launched by the Soviet Union in 1957 Prompted a surge in U.S. investment in technology U.S. formed the Advanced Research Projects Agency

(ARPA) Responsible for ARPANET, precursor to

the Internet.

Sputnik 1957

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Wire as a Transmission Medium

The most common type of cable is “twisted-pair.” Pair of wires twisted around each other. Each wire covered in “jacket.” Why twisted? Shields the cable from interference. The more twists the better the shield.

Copper wire is still the most common communications medium.

Wire based transmission schemes guide electromagnetic waves, either between a pair of separate wires, or inside a coaxial arrangement.

“TWISTED PAIR”

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General Types of Twisted Pair Two kinds of twisted pair:

Unshielded Twisted Pair (UTP) Found in walls of most buildings Local Loop, ISDN, DSL, LANs (10Base-T Ethernet), T1 lines

(1.544 Mbps) Wiring connecting homes to the telephone companies

local switch All over the George Mason Campus

Shielded Twisted Pair (STP) - Special purpose cable

Today the most common type of twisted pair installed is called “CAT 6” or category 6 twisted pair. (Also CAT 5)

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“COAX”

Another type of commonly used copper cable is coax. Main Components

“Center” Conductor “Shield” Conductor Insulated material

Cable TV 70% of U.S. homes have CATV

High bandwidth

COAXIAL CABLE

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Cable Characteristic - Attenuation

Attenuation - Loss of energy and related reduction in the size of transmitted pulses

Longer the cable, greater the attenuation Measured in dB

37.5STP

20Coax, RG-8/U

56UTP Category 3

Attenuation per 1000 Feet in dB at 100 MHz

Cable Type

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Cable Susceptibility to Noise

Cables tend to be routed next to each other and near sources of electromagnetic interference

Unwanted noise affects all copper cables to various extents Parallel wire – Greatly affected UTP – Less affected because of the twists STP – Less affected because of the metal shielding Coaxial cable – Most resistant (out of copper cables) to

loss and noise problems

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Fiber Optics

Explosive, rapidly evolving technology Coaxial arrangement of glass The glass has to be extremely pure

Imagine a 50 mile thick window you could see perfectly through. Main components

Core - low index of refraction Cladding- higher index of refraction Teflon Jacket – protects and stiffen the fiber

Core and Cladding have different indices of refraction. Index of refraction is a measure of speed of light in a material, which

affects the angle by which a light ray is bent on passing through the material

Transmission of LIGHT over GLASS or PLASTIC fiber.

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Advantages of Fiber

Much Higher Bandwidth (Gbps) - Thousands of channels can be multiplexed together over one strand of fiber.

Immunity to Noise - Immune to electromagnetic interference (EMI).

Safety - Doesn’t transmit electrical signals, making it safe in environments like a gas pipeline.

High Security - Difficult to “tap into.” Less Loss - Repeaters can be spaced 75 miles apart. Reliability - More resilient than copper in extreme environmental

conditions. Size - Lighter and more compact than copper. Flexibility - Unlike impure, brittle glass, fiber is physically very

flexible.

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Disadvantages of Fiber

Cost of interfacing equipment necessary to convert electrical signals to optical signals. (optical transmitters, receivers)

Splicing fiber optic cable is also more difficult.

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Two Types of Fiber Optic Cable

Multimode fiber Transmits multiple signals per fiber Larger core – 62.5 microns in

diameter

Single-mode fiber Transmits one signal per fiber Small core – 9 microns in diameter

Industry Term: “DARK FIBER”

An important related technology “OPTICAL SWITCHING”