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

Topics Guided Transmission Media Wireless Transmission Communication Satellites

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

Guided Media Twisted pairs Coaxial cable Power lines Fiber optics

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

Twisted Pair Category 5 UTP cable with 4 twisted pairs

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

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)

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

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.

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

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

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

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

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

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

Electromagnetic Spectrum Ground Sky Line-of-sight

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

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

Radio Transmission Easy to generate Travel long distance Penetration Interference

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, 802.11 wireless LANs

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

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

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

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

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

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.

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

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

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

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

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

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 (?)