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Unformatted text preview: Data and Computer Communications Communications
Chapter 4 –Transmission Media Chapter Eighth Edition by William Stallings Lecture slides by Lawrie Brown Transmission Media Transmission
Communication channels in the animal world include Communication touch, sound, sight, and scent. Electric eels even use electric pulses. Ravens also are very expressive. By a combination voice, patterns of feather erection and body posture ravens communicate so clearly that an experienced observer can identify anger, affection, hunger, curiosity, playfulness, fright, boldness, and depression. —Mind of the Raven, Bernd Heinrich Overview Overview guided - wire / optical fibre unguided - wireless characteristics and quality determined by characteristics medium and signal medium iin unguided media - bandwidth produced by n the antenna is more important the in guided media - medium is more important key concerns are data rate and distance Design Factors Design bandwidth higher bandwidth gives higher data rate eg. attenuation transmission impairments interference number of receivers in guided media more receivers introduces more attenuation Electromagnetic Spectrum Electromagnetic Transmission Characteristics of Guided Media Twisted pair (with loading) Twisted pairs (multipair cables) Coaxial cable Optical fiber Frequency Range 0 to 3.5 kHz 0 to 1 MHz 0 to 500 MHz 186 to 370 THz Typical Attenuation 0.2 dB/km @ 1 kHz 0.7 dB/km @ 1 kHz 7 dB/km @ 10 MHz 0.2 to 0.5 dB/ km Typical Delay 50 µs/km 5 µs/km 4 µs/km 5 µs/km Repeater Spacing 2 km 2 km 1 to 9 km 40 km Twisted Pair Twisted Twisted Pair - Transmission Characteristics Characteristics analog analog needs amplifiers every 5km to 6km can use either analog or digital signals needs a repeater every 2-3km digital limited distance limited bandwidth (1MHz) limited data rate (100MHz) susceptible to interference and noise Unshielded vs Shielded TP Unshielded unshielded Twisted Pair (UTP) ordinary telephone wire cheapest easiest to install suffers from external EM interference metal braid or sheathing that reduces interference more expensive harder to handle (thick, heavy) shielded Twisted Pair (STP) in a variety of categories - see EIA-568 UTP Categories UTP
Category 3 Class C Bandwidth Cable Type Link Cost (Cat 5 =1) 16 MHz UT P 0.7 Category 5 Class D 100 MHz UTP/FTP 1 Category 5E 100 MHz UTP/FTP 1.2 Category 6 Class E 200 MHz UTP/FTP 1.5 Category 7 Class F 600 MHz SSTP 2.2 Comparison of Shielded and Unshielded Twisted Pair Unshielded
Attenuation (dB per 100 m) Frequency (MHz) 1 4 16 25 100 300 Category 3 UTP 2.6 5.6 13.1 Ñ Ñ Ñ Category 5 UTP 2.0 4.1 8.2 10.4 22.0 Ñ 150-ohm STP 1.1 2.2 4.4 6.2 12.3 21.4 Near-end Crosstalk (dB) Category 3 UTP 41 32 23 Ñ Ñ Ñ Category 5 UTP 62 53 44 41 32 Ñ 150-ohm STP 58 58 50.4 47.5 38.5 31.3 Near End Crosstalk Near coupling of signal from one pair to another occurs when transmit signal entering the occurs link couples back to receiving pair link ie. near transmitted signal is picked up by ie. near receiving pair near Coaxial Cable Coaxial Coaxial Cable - Transmission Characteristics Characteristics superior frequency characteristics to TP performance limited by attenuation & performance noise noise analog signals amplifiers every few km closer if higher frequency up to 500MHz repeater every 1km closer for higher data rates digital signals Optical Fiber Optical Optical Fiber - Benefits Optical greater capacity data rates of hundreds of Gbps smaller size & weight lower attenuation electromagnetic isolation greater repeater spacing 10s of km at least Optical Fiber - Transmission Characteristics Characteristics uses total internal reflection to transmit uses light light effectively acts as wave guide for 1014 to 1015 Hz Light Emitting Diode (LED)
• cheaper, wider operating temp range, lasts longer can use several different light sources Injection Laser Diode (ILD)
• more efficient, has greater data rate relation of wavelength, type & data rate Optical Fiber Transmission Modes Modes Frequency Utilization for Fiber Applications Fiber
Wavelength (in vacuum) range (nm) 820 to 900 1280 to 1350 1528 to 1561 1561 to 1620 Frequency Range (THz) 366 to 333 234 to 222 196 to 192 192 to 185 S C L Band Label Fiber Type Application Multimode Single mode Single mode Single mode LAN Various WDM WDM Attenuation in Guided Media Attenuation Wireless Transmission Frequencies Frequencies 2GHz to 40GHz microwave highly directional point to point satellite omnidirectional broadcast radio infrared local 30MHz to 1GHz 3 x 1011 to 2 x 1014 Antennas Antennas electrical conductor used to radiate or collect electrical electromagnetic energy electromagnetic transmission antenna radio frequency energy from transmitter converted to electromagnetic energy byy antenna radiated into surrounding environment electromagnetic energy impinging on antenna converted to radio frequency electrical energy fed to receiver reception antenna same antenna is often used for both purposes Radiation Pattern Radiation power radiated in all directions not same performance in all directions as seen in a radiation pattern diagram as radiation an isotropic antenna is a (theoretical) point an in space in radiates in all directions equally with a spherical radiation pattern Parabolic Reflective Antenna Parabolic Antenna Gain Antenna measure of directionality of antenna power output in particular direction verses power that produced by an isotropic antenna that measured in decibels (dB) results in loss in power in another direction effective area relates to size and shape related to gain Terrestrial Microwave Terrestrial used for long haul telecommunications and short point-to-point links requires fewer repeaters but line of sight use a parabolic dish to focus a narrow beam use onto a receiver antenna onto 1-40GHz frequencies higher frequencies give higher data rates main source of loss is attenuation distance, rainfall also interference Satellite Microwave Satellite satellite is relay station receives on one frequency, amplifies or repeats receives signal and transmits on another frequency signal eg. uplink 5.925-6.425 GHz & downlink 3.7-4.2 GHz height of 35,784km spaced at least 3-4° apart television long distance telephone private business networks global positioning typically requires geo-stationary orbit typical uses Satellite Point to Point Link Satellite Satellite Broadcast Link Satellite Broadcast Radio Broadcast radio is 3kHz to 300GHz use broadcast radio, 30MHz - 1GHz, for: FM radio UHF and VHF television is omnidirectional still need line of sight suffers from multipath interference reflections from land, water, other objects Infrared Infrared modulate noncoherent infrared light end line of sight (or reflection) are blocked by walls no licenses required typical uses TV remote control IRD port Wireless Propagation Wireless Ground Wave Wireless Propagation Wireless Sky Wave Wireless Propagation Wireless Line of Sight Refraction Refraction velocity of electromagnetic wave is a function of velocity density of material density
~3 x 108 m/s in vacuum, less in anything else speed changes as move between media Index of refraction (refractive index) is sin(incidence)/sin(refraction) varies with wavelength density of atmosphere decreases with height results in bending towards earth of radio waves hence optical and radio horizons differ have gradual bending if medium density varies Line of Sight Transmission Line Free space loss loss of signal with distance from water vapour and oxygen absorption multiple interfering signals from reflections bending signal away from receiver Atmospheric Absorption Multipath Refraction Free Space Loss Free Multipath Interference Multipath Summary Summary looked at data transmission issues frequency, spectrum & bandwidth analog vs digital signals transmission impairments ...
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This note was uploaded on 04/06/2011 for the course EE 5363 taught by Professor Kang during the Spring '09 term at NYU Poly.
- Spring '09