Introduction to Information Technology LECTURE 8: TRANSMISSION TECHNOLOGY

Communications Media Free Space Copper Wire Fiber Optic Cable 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

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/s Information rate versus latency: Communication channels can move data at a rate of billions of bits per second, but still suffer huge latencies.

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.

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

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

Calculating Wavelength Each frequency has an associated wavelength calculated as follows: l = wavelength c = speed of light (3x108 m/s) f = frequency in Hz l = c/f What is the wavelength for a frequency of 8 MHz? l = 3x108/8x106 = 37.5 m Electromagnetic Spectrum

Calculating Wavelength l = wavelength c = speed of light (3x108 m/s) f = frequency in Hz l = c/f What is the wavelength for a frequency of 5 MHz? = 3x108/5x106 = 60 m =300,000,000/5,000,000 = 60 m

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)

Amplitude Modulation (AM) Uses a single frequency to convey information Varies only its amplitude to convey information Vulnerable to interference

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.

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

Examples Baby Monitor Cell Phone 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

Radio Communications Example - GPS GLOBAL POSITIONING SYSTEM 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 Satellite

General Example of Triangulation 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

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

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

Geosynchronous Satellites What’s a geosynchronous satellite? 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.

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

Wire as a Transmission Medium 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” 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.

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)

“COAX” Another type of commonly used copper cable is coax. COAXIAL CABLE 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

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 37.5 STP 20 Coax, RG-8/U 56 UTP Category 3 Attenuation per 1000 Feet in dB at 100 MHz Cable Type

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

Fiber Optics Transmission of LIGHT over GLASS or PLASTIC fiber. 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

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.

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.

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”