1. CS 313 Introduction to Computer Networking & Telecommunication
Physical Layer – Transmission Media
Chi-Cheng Lin, Winona State University

2. Topics Guided Transmission Media Wireless Transmission
Communication Satellites

3. Transmission Media Physical layer: Transport a raw bit stream
Physical media
Guided media
Information transmitted on wires by varying some physical property such as voltage or current
Copper wire, fiber optics
Unguided media
Information transmitted wirelessly by electromagnetic waves
Radio, lasers

4. Guided Media
Twisted pairs
Coaxial cable
Power lines
Fiber optics

5. Twisted Pair Cable Oldest, but still most common
Two twisted insulated copper wires
Why twisted?
To reduce electrical interference
Telephone system, Ethernet
Repeater needed for longer distances
Repeater: device that extends the distance a signal can travel by regenerating the signal
Adequate performance at low cost

6. Twisted Pair
Category 5 UTP cable with 4 twisted pairs

7. Transmission Direction Modes
100-Mbps Ethernet uses two pairs – one for each direction
1-Gbps Ethernet uses all four pairs in both directions simultaneously.
Terminologies: transmission direction modes
Simplex
Data only travel in one direction
Half-duplex
Data can travel in either direction, but not simultaneously
Full-duplex
Data can travel in both directions simultaneously

8. Coaxial Cable Better shielding than twisted pairs Widely used for
Span longer distances at higher speeds
Lower error rate
Widely used for
Cable TV
WAN (Internet over cable)

9. Fiber Optics Light Electromagnetic energy traveling at 3108 m/s
Refraction
Critical angle
Reflection

10. Fiber Optics
(Less dense)
cladding
core I
(critical
angle)
cladding
(More dense)
(a) Three examples of a light ray from inside a silica fiber impinging on the air/silica boundary at different angles.
(b) Light trapped by total internal reflection.

11. Fiber Cables (a) Side view of a single fiber.
(b) End view of a sheath with three fibers.

12. Fiber Optics Optical transmission system: Propagation modes
Light source: LED or lasers
Transmission medium: fiber optic cable
Detector: converting detected light to electrical pulse
Propagation modes
Multimode
Step-index
Grade-index
Single mode

13. Core diameter 200 micrometers 50 - 100 micrometers ~ 10 micrometers
Modes
Core diameter
200 micrometers
micrometers
~ 10 micrometers
The McGraw-Hill Companies, Inc., 2004

14. Single Mode
All beams received “together” and signal can be combined with little distortion
Widely used for longer distance (over 550 m)
More expensive
Currently 100 Gbps for 100 km w/o amplification

15. Fiber Optics Vs. Copper Wire
Pros
Higher bandwidth
Less attenuation  less repeater needed (about every 50 km, copper 5 km)
Noise resistance: no interference, surge, ...
Thin and lightweight
Excellent security, as wiretapping is harder
Cons
Fiber interface costs more
Fragility

16. Wireless Transmission
Electromagnetic Spectrum
Electron movement creates electromagnetic wave
Frequency: number of oscillations per second of a electromagnetic wave measured in Hertz (Hz)
Wavelength: distance between two consecutive maxima (or minima)
Speed of light: C = 3  108 m/sec
C = wavelength  frequency, i.e., C = λf

17. Electromagnetic Spectrum
Ground Sky Line-of-sight

18. Radio Transmission (1)
In the VLF, LF, and MF bands, radio waves follow the curvature of the earth
Computer Networks, Fifth Edition by Andrew Tanenbaum and David Wetherall, © Pearson Education-Prentice Hall, 2011

19. In the HF band, they bounce off the ionosphere.
Radio Transmission (2)
In the HF band, they bounce off the ionosphere.
Computer Networks, Fifth Edition by Andrew Tanenbaum and David Wetherall, © Pearson Education-Prentice Hall, 2011

20. Radio Transmission Easy to generate Travel long distance Penetration
Interference

21. Microwave Transmission
MCI?
Straight line travel
Higher towers for longer distances
Multipath fading problem, absorption by rain
Advantages:
Right of way not needed
Inexpensive
Industrial/Scientific/Medical (ISM) bands
No license needed
Garage door opener, cordless phone, etc
Bluetooth, wireless LANs

22. The Politics of the Electromagnetic Spectrum
ISM and U-NII bands used in the United States by wireless devices
Computer Networks, Fifth Edition by Andrew Tanenbaum and David Wetherall, © Pearson Education-Prentice Hall, 2011

23. Infrared and Millimeter Waves
Remote control
Directional, cheap, easy to build
Cannot pass through solid walls
Good or bad?
Limited use on desktop

24. Applications of Wireless Media
Radio waves
Multicast communications
Radio, television, and paging systems
Microwaves
Unicast communication
Cellular telephones, satellite networks, and wireless LANs.
Infrared signals
Short-range communication in a closed area using line-of-sight propagation
Wireless keyboards, mice, printers

25. Lightwave Transmission
Lasers
High bandwidth, low cost, easy to install
Aiming is hard
No penetration through rain or thick fog

26. Communication Satellite
Big microwave repeater in the sky
Transponders, each
Listens to some portion of spectrum
Earth to satellite: Uplink
Amplifies incoming signal
Rebroadcast it at another frequency
Satellite to Earth: Downlink
 Bent pipe mode

27. Communication Satellites
(Geostationary Earth Orbit)
(Medium Earth Orbit), app.: GPS
(Low Earth Orbit), voice/data communication
Communication satellites and some of their properties, including altitude above the earth, round-trip delay time and number of satellites needed for global coverage.

28. Geostationary Satellites (1)
The principal satellite bands
Computer Networks, Fifth Edition by Andrew Tanenbaum and David Wetherall, © Pearson Education-Prentice Hall, 2011

29. Communication Satellites
VSATs using a hub.
VSATs: Very Small Aperture Terminals

30. Communication Satellite
Low-Earth Orbit Satellites
Iridium: 66 satellites
Goal:
Provide worldwide telecommunication service using hand-held devices that communicates directly with the Iridium satellites
Current status?
Broke, auctioned, restarted
Globalstar: 48 LEOs using bent-pipe design
Teledisc:
Goal: provide Internet users with high bandwidth using VSAT-like antenna

31. Low-Earth Orbit Satellites (1)
The Iridium satellites form six necklaces around the earth.
Computer Networks, Fifth Edition by Andrew Tanenbaum and David Wetherall, © Pearson Education-Prentice Hall, 2011

32. Iridium vs. Globalstar (a) Iridium: Relaying in space.
(b) Globalstar: Relaying on the ground.

33. Satellites Vs. Fiber Availability Mobility Broadcasting
Geographically issue
Right of way
Rapid deployment
Future?
“It's tough to make predictions, especially about the future.” Yogi Berra (?)