1. CIS 321- Fall 2004 Data Communications & Networking
Chapter 7 – Transmission Media

2. Relation to Internet Model
Actually located below the physical layer
Directly controlled by the physical layer
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3. Introduction
Data must be converted into electromagnetic signals to be transmitted from device to device
Signals can travel through a vacuum, air, or other media
May be in the form of power, voice, radio waves, infrared light, and X, gamma, and cosmic rays
Each of these forms constitutes a portion of the electromagnetic spectrum
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4. Electromagnetic Spectrum
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5. Categories of Media Broad categories:
Guided Media – media with a physical boundary
Twisted pair, coaxial, and fiber-optic
Unguided Media – no physical boundaries
Radio waves, infrared light, visible light, and X, gamma, and cosmic rays
Sent by microwave, satellite, and cellular transmission
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6. Guided Media
Signal is directed and contained by physical limits of medium
Twisted-pair and coaxial use copper conductors to accept and transport signals in form of electrical current
Optical fiber is glass or plastic cable that accepts and transports signals in form of light
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7. Twisted-Pair Cable Two conductors surrounded by insulating material
One wire used to carry signals; other used as a ground reference
Twisting wires reduces the effect of noise interference or crosstalk since both wires will likely be equally affected
More twists = better quality
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8. Unshielded Twisted Pair (UTP)
Most common type; suitable for both voice and data transmission
Frequency range is from 100 Hz to 5 MHz
Categories are determined by cable quality
Cat 3 commonly used for telephone systems (up to 10 Mbps)
Cat 5 usually used for data networks (up to 100 Mbps)
Performance is measured by attenuation versus frequency and distance
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9. UTP example University of South Alabama
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10. UTP (cont) Adv: cheaper, flexible, easy to install
UTP connectors attach to wire; most commonly used is RJ45, which have 8 conductors, one for each of four twisted pairs
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11. Shielded Twisted Pair (STP)
A metal foil or braided-mesh covering encases each pair of insulated conductors to prevent electromagnetic noise called crosstalk
Crosstalk occurs when one line picks up some of the signals traveling over another line
Uses same connectors
More expensive but less susceptible to noise
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12. STP example University of South Alabama
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13. Coaxial Cable
Has a central core conductor enclosed in an insulating sheath, encased in an outer conductor of metal foil
Frequency range is between 100 KHz and 500 MHz
RG numbers denote physical specs such as wire gauge, thickness and type of insulator, construction of shield and size/type of outer casing
RG-8, RG-9, and RG-11 used in thick Ethernet
RG-58 used in thin Ethernet
RG-59 used for TV
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14. Coaxial Cable example University of South Alabama
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15. Coaxial Cable Connectors
Most common is barrel connector (BNC)
T-connectors are used to branch off to secondary cables
Terminators are required for bus topologies to prevent echoing of signals
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16. Coaxial Performance Higher bandwidth than twisted-pair
However, attenuation is higher and requires frequent use of repeaters
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17. Coaxial Applications Analog and digital phone networks
Cable TV networks
Traditional Ethernet LANs
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18. Fiber-Optic Cable
Made of glass or plastic; signals are transmitted as light pulses from an LED or laser
Light is also a form of electromagnetic energy
Speed depends on density of medium it is traveling through; fastest when in a vacuum, 186,000 miles/second
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19. Refraction and Reflection
Refraction often occurs when light bends as it passes from one medium to another less dense medium
When this angle results in a refraction great enough, reflection occurs and the light no longer passes into the less dense medium
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20. Refraction University of South Alabama
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21. Critical Angle University of South Alabama
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22. Reflection University of South Alabama
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23. Reflection
Optical fibers use reflection to guide light through a channel
Information is encoded onto a beam of light as a series of on-off pulses representing 1s and 0s
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24. Propagation Modes Method for transmitting optical signals: Multimode
Multimode step-index fiber
Multimode graded-index fiber
Single Mode
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25. Multimode
Multiple beams from light source move through core at different paths
Multimode step-index fiber
Density remains constant from center to edges
Light moves in a straight line until it reaches the cladding
Some beams penetrate the cladding and are lost, while others are reflected down the channel to the destination
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26. Multimode (cont)
As a result, beams reach the destination at different times and the signal may not be the same as that which was transmitted
To address this problem and to allow for more precise transmissions, multimode graded-index fiber may be used
Index refers to the index of refraction
Graded-index refers to varying densities of the fiber; highest at center and decreases at edge
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27. Multimode Graded-Index Fiber (cont)
Since the core density decreases with distance from the center, the light beams refract into a curve
Eliminates problem with some of the signals penetrating the cladding and being lost
Also signals intersect at regular intervals
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28. Single Mode
Only one beam from a light source is transmitted using a smaller range of angles
Smaller diameter and lower density
Makes propagation of beams almost horizontal; delays are negligible
All beams arrive together and can be recombined without signal distortion
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29. Propagation Modes University of South Alabama
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30. Light Sources Light source is light-emitting diode (LED) or a laser
LEDs are cheaper but not as precise (unfocused); limited to short-distance use
Lasers can have a narrow range, better control over angle
Receiving device needs a photosensitive cell (photodiode) capable of receiving the signal
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31. Fiber Construction University of South Alabama
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32. Applications of Fiber Optics
Backbone networks due to wide bandwidth and cost effectiveness
Up to 1600 Gbps with WDM (SONET)
Cable TV
LANS
100Base-FX (Fast Ethernet)
1000Base-X
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33. Advantages of Fiber Optics
Higher bandwidth than twisted-pair and coaxial cable; not limited by medium, but by equipment used to generate and receive signals
Noise resistance
Less signal attenuation
More resistant to corrosive materials
Lightweight
Greater security
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34. Disadvantages of Fiber Optics
Installation/maintenance
Unidirectional
Cost
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35. 7.2 Unguided Media: Wireless
Wireless communication; transporting electromagnetic waves without a physical conductor
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36. Wireless Propagation Methods
Ground – radio waves travel through lowest portion of atmosphere, hugging the Earth
Sky – higher-frequency radio waves radiate upward into ionosphere and then reflect back to Earth
Line-of-sight – high-frequency signals transmitted in straight lines directly from antenna to antenna
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37. Propagation Methods University of South Alabama
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38. Bands Band Range Propagation Application VLF 3–30 KHz Ground
Long-range radio navigation LF
30–300 KHz
Radio beacons and navigational locators MF
300 KHz–3 MHz
Sky
AM radio HF
3–30 MHz
Citizens band (CB), ship/aircraft communication
VHF
30–300 MHz
Sky and line-of-sight
VHF TV, FM radio
UHF
300 MHz–3 GHz
Line-of-sight
UHF TV, cellular phones, paging, satellite
SHF
3–30 GHz
Satellite communication
EHF
30–300 GHz
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39. Wireless Transmission Waves
Radio Waves
Microwave
Infrared
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40. Radio Waves Frequency ranges: 3 KHz to 1 GHz Omnidirectional
Susceptible to interference by other antennas using same frequency or band
Ideal for long-distance broadcasting
May penetrate walls
Apps: AM and FM radio, TV, maritime radio, cordless phones, paging
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41. Microwaves Frequencies between 1 and 300 GHz Unidirectional
Narrow focus requires sending and receiving antennas to be aligned
Issues:
Line-of-sight (curvature of the Earth; obstacles)
Cannot penetrate walls
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42. Parabolic Dish Antenna
Incoming signals - Signal bounces off of dish and is directed to focus
Outgoing signals – transmission is broadcast through horn aimed at dish and are deflected outward
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43. Horn Antenna
Outgoing transmissions broadcast through a stem and deflected outward
Received transmissions collected by a scooped part of the horn and deflected downward into the stem
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44. Microwave Applications
Unicasting – one-to-one communication between sender and receiver
Cellular phones
Satellite networks
Wireless LANs
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45. Infrared Frequencies between 300 GHz and 400 THz
Short-range communication
High frequencies cannot penetrate walls
Requires line-of-sight propagation
Adv: prevents interference between systems in adjacent rooms
Disadv: cannot use for long-range communication or outside a building due to sun’s rays
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46. Infrared Applications
Wide bandwidth available for data transmission
Communication between keyboards, mice, PCs, and printers
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47. Coming Up… Chapter 8 – Circuit Switching and the Telephone Network
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48. Credits
All figures obtained from publisher-provided instructor downloads
Data Communications and Networking, 3rd edition by Behrouz A. Forouzan.  McGraw Hill Publishing, 2004
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