Chapter 4 Transmission Media. Transmission medium: physical path between transmitter and receiver Guided media

Transcription

1 CEN 342 Introduction to Data Transmission Chapter 4 Transmission Media Dr. Mostafa Hassan Dahshan Computer Engineering Department College of Computer and Information Sciences King Saud University mdahshan@ccis.ksu.edu.sa Transmission Media Transmission medium: physical path between transmitter and receiver Guided media em wave guided along solid medium copper, twisted pair, optical Unguided media wireless transmission atmosphere, outer space, water

2 Design Factors Bandwidth more bandwidth more data rate Transmission impairments limit distance twisted pair > coax > fiber (best) Interference competing signal in overlapping frequency band more concern in unguided Number of receivers shared link more attachments more attenuation Electromagnetic Spectrum

3 Guided Transmission Media Twisted Pair Coaxial Cable Optical Fiber Twisted Pair Least expensive, most widely used Two insulated copper wires Arranged in regular spiral pattern

4 Twisted Pair Applictions Residential phones connected to local phone exchange (subscriber loop) by TP Within office building, TP connect phones to a digital PBX Within building for LAN supporting PCs Wide range of data rates Transmission Characteristics Both analog and digital transmission Analog amplifiers required every 5-6 km Digital repeaters required every 2-3 km Limited in distance, bandwidth, data rate Attenuation is strong function of frequency

5 Transmission Characteristics Quite susceptible to interference, noise Impulse noise easily intrudes into TP Shielding, twisting reduces interference Different twist length reduce crosstalk Transmission Characteristics

6 Shielded and Unshielded TP STP TP is shielded with metallic braid or sheathing better performance more expensive, difficult to work with UTP TP is not shielded easy to install and work with Shielded and Unshielded TP

7 Shielded and Unshielded TP Cable Categories Max Max Twist Length Grade Bandwidth Data Rate Category 3 16 MHz 16 Mbps cm Voice Limited data Category MHz 100 Mbps cm Data

8 Coaxial Cable Hollow outer cylindrical conductor that surrounds single inner wire conductor Inner conductor insulated by solid dielectric material Outer conductor covered with jacket/shield Diameter between cm Coaxial Cable Applications Television distribution Long-distance telephone transmission Local area networks (deprecated)

9 Transmission Characteristics Analog and digital signals Frequency characteristics superior to TP Less susceptible to interference than TP Spectrum up to 500 MHz Main constraint is attenuation Repeaters needed at closer distances for digital transmission at high data rates Transmission Characteristics

10 Optical Fiber Thin, flexible medium capable of guiding an optical ray Consists of core, cladding and jacket Transmission Characteristics Light propagating between two materials with different refraction index is partially reflected, partially refracted Light going from high refraction index material to low index material refracts with higher angle Beyond critical angle, light is totally reflected

11 Transmission Characteristics Fiber acts as waveguide Frequencies in range Hertz Covers portion of infrared, visible light Transmission Characteristics

12 Fiber Types Step-index multimode Graded-index multimode Single mode Fiber Types

13 Step-Index Multimode Multiple reflection angles (modes) Multiple propagation paths: modal dispersion Different length and arrival time Signal elements spread out in time Increase time between pulses i.e. Limit data rate Core diameter 50 μm, cladding 125 μm Suitable for short distances Single Mode Core radius in the order of a wavelength Diameter 8-10 μm, cladding 125 μm Only single angle can pass (axial path) Used for long distance transmission

14 Graded-Index Multimode Refractive index of core center is higher than near cladding Light at center travel slower than edges Arrival time difference is shortened Modal dispersion is reduced Frequency Utilization Wavelength (in vacuum) range (nm) Frequency Range (THz) Band Label Fiber Type Application 820 to to 333 Multimode LAN 1280 to to 222 S Single mode Various 1528 to to 192 C Single mode WDM 1561 to to 185 L Single mode WDM

15 Frequency Utilization Frequency Utilization Four transmission windows in IR range Below visible light ( nm) Loss is lower at higher wavelengths Lower wavelength suffer loss due to absorption scattering: change in direction of light rays after they strike small particles in medium

16 Light Sources Light Emitting Diode (LED) less expensive operate at higher temperatures longer operational life Typical wavelength 850 nm Injection Laser Diode (ILD) operate on the laser principle more efficient can sustain higher data rates Typical wavelength nm Advantages of Optical Fiber Greater capacity Fiber: 100s of Gbps over 10s of kilometers Twisted pair: 1Mbps for few kilometers, or 100 Mbps for few meters Coaxial cable: 100s of Mbps for 1 km Smaller size and weight Lower attenuation

17 Advantages of Optical Fiber Advantages of Optical Fiber Electromagnetic isolation not affected by external electromagnetic fields do not radiate energy Greater security difficult to tap Greater repeater spacing 10s to 100s of kilometers

18 Propagation Velocity Velocity of light in fiber less than vacuum Wavelength cited correspond to frequency of light in vacuum Light speed in fiber = m/s Light speed in vacuum = m/s λ vacuum = 1550 nm f = c/ λ = 193.4THz λ fiber = v fiber /f = m/s / 193.4THz = 1055 nm Wireless Transmission Antennas Terrestrial Microwave Satellite Microwave Broadcast Radio Infrared

19 Wireless Spectrum Antennas Electrical conductor used for radiating electromagnetic energy collecting electromagnetic energy For transmission electrical energy converted to electromagnetic radiated to surrounding environment For reception electromagnetic signal impinge antenna electromagnetic energy converted to electrical

20 Antennas Same antenna can be used both tx and rx Radiation pattern graphical representation of radiation properties of antenna, function of space coordinates characterize performance of antenna isotropic antenna: radiate equally in all directions Parabolic Reflective Antenna Paraboloid parabola revloved about its axis cross section parallel axis parabola cross section perpendicular to axis circle used in microwave antennas

21 Parabolic Reflective Antenna Transmission source placed at focus of paraboloid wave bound parallel to axis Reciption incoming waves parallel to axis concentrated at the focus Parabolic Reflective Antenna

22 Antenna Gain Measure of directionality of antenna Power output in particular direction compared to power output of isotropic antenna in any direction Not output compared to input power Increased power in one direction means reduced power in other directions Effective Area of Antenna Area of idealized (isotropic) antenna which absorbs as much net power from the incoming wave as the actual antenna Related to physical size and shape For parabolic antenna face area A = πr 2 effective area A e = 0.56 A

23 Antenna Gain & Effective Area Relationship between G and A e is 2 4π 4πf G = A 2 e = A 2 e c λ G = antenna gain A e = effective area f = carrier frequency c = speed of light ( m/s) λ = carrier wavelength Example For isotropic antenna, G=1 1 = 4πA e /λ 2 A e = λ 2 /4π For parabolic antenna with face area A A e = 0.56 A G = 4πA e /λ 2 = 4π 0.56 A /λ 2 = 7A/λ 2

24 Example Parabolic reflective antenna has diameter 2 m operating at 12 GHz. Calculate A e, G Answer A = πr 2 = π 1 = π A e = 0.56 A = 0.56 π = 1.76 λ = c/f = ( )/( ) = m G = 7A/λ 2 = (7 π/ ) = G db = 10 log 10 (35186) = db Terrestrial Microwave Use parabolic dish, about 3 m diameter Antenna focuses signal in small beam Line-of-sight transmission Usually located at substantial height extend range avoid obstacles Relay towers used to extend range

25 Applications Long-haul telecommunications alternative to coaxial and optical fiber require fewer amplifiers for same distance require line-of-sight Short point-to-point links between buildings closed-circuit TV data link between local area networks bypass local phone company to long-distance Transmission Characteristics Cover substantial range of EMC spectrum Common frequencies 1-40 GHz Higher frequencies higher potential bandwidth higher data rate

26 Transmission Characteristics Main source of loss is attenuation Loss can be expressed as L = (4πd/λ) 2 d = distance λ = wavelength Loss varies as square of distance TP and coax: loss exponentially with distance more repeater spacing: km Transmission Characteristics Common band for long-haul 4-6 GHz Bands are increasingly congested More bands are now used: GHz Higher frequencies used for short p2p links less useful in long distance due to attenuation antennas are smaller and cheaper

27 Satellite Microwave Satellite is effectively relay station Used to link two or more ground stations satellite receives tx at frequency band (uplink) amplifies or repeat signal transmit it on another band (downlink) Satellite operate on multiple frequency bands (transponders) Satellite Link Types

28 Satellite Position Communication satellite should remain in stationary position over earth Otherwise, it will not be within line-of-sight of its earth stations Period of rotation = earth s rotation period Match occurs at height of km at the equator Satellite Spacing Satellites using same freq band must be spaced far enough to avoid interference 4 o spacing for 4/6 GHz band 3 o spacing f or 12/14 GHz band Angular displacement measured from earth Number of possible satellites is limited

29 Applications TV distribution Long-distance telephone transmission Private business networks Global positioning (GPS) Transmission Characteristics Optimum frequency range is 1-10 GHz <1 GHz: noise from natural sources, atmosphere, interference with electronics >10 GHz: signal attenuation, absorption Most satellites providing p2p service use GHz for uplink GHz for downlink known as 4/6 GHz band different frequency band to allow continuous operation without interference

30 Transmission Characteristics 4/6GHz band within optimum Has become saturated 12/14 GHz band has been developed attenuation problems must be considered will also saturate Use is projected for 20/30 GHz band greater attenuation but allows more bandwidth Satellite Comm. Considerations Long distance implies propagation delay about 0.25 end-to-end Satellite microwave is inherently broadcast many stations can transmit to satellite transmission received by many stations

31 Broadcast Radio Omnidirectional, unlike microwave Doesn t require dish-shaped antenna Antennas do not need to be aligned tightly Applications Radio general term, frequencies: 3 khz GHz Broadcast radio cover VHF, part of UHF: 30 MHz - 1 GHz This range covers FM radio, UHF & VHF TV Also used for some data networking applications

32 Transmission Characteristics Band 30 MHz - 1 GHz not affected by ionosphere Distant transmitters do not interfere due to reflections from atmosphere Less sensitive to attenuation from rainfall Attenuation with distance: L = (4πd/λ) 2 High λ means less attenuation Main impairment is multipath interference Infrared Transmitters modulate infrared light Transceivers must be within line-of-sight either directly or via reflection Does not penetrate walls unlike microwave better security No frequency allocation required

33 Frequency Bands GW = Ground Wave SW = Sky Wave Wireless Propagation Three routes Ground wave (GW) propagation Sky wave (SW) propagation Line of sight (LOS) propagation

34 Ground Wave Propagation Follows curvature of the earth Can propagate to large distance Frequency range up to 2 MHz Do not penetrate upper atmosphere Best known example: AM radio Sky Wave Propagation Signal from earth station reflected by ionosphere to earth Can travel through number of hops Frequency range 2 30MHz Can be picked up 1000s of kilometers away from transmitter Used for amateur radio, international radio

35 Line-of-Sight Propagation Above 30 MHz neither GW nor SW operate Effective LOS is longer than optical LOS Microwaves are refracted by atmosphere Refraction EMC wave speed depends on medium Slower in air, glass than vacuum Density change speed change direction bending occurs at boundary Gradual density change continuous gradual direction bending

36 Optical and Radio LOS Optical LOS with no obstacles d = 3.57 h Effective or radio LOS is d = 3.57 Kh d = distance between antenna and horizon (km) h = height of antenna (m) K = adjustment factor to account for refraction typically, K = 4/3 Maximum distance between two antennas ( ) 1 2 d = 3.57 Kh + Kh Optical and Radio LOS

37 Example Two antennas, one is 100m high. Other is ground level, calculate max distance Answer d = 3.57 Kh + Kh ( ) 1 2 ( ) = = 41km Example Suppose receiving antenna is 10m high. Calculate transmitting antenna height to achieve same distance Answer 41 = 3.57 Kh Kh h 1 1 ( ) 1 41 = 13.3 = = = 46.2 m 50m saving; better to increase receiver s h

38 Free Space Loss Regardless of other impairment Signal attenuates over distance Because signal spread over larger area For ideal isotropic antenna, free space loss Pt P r ( 4πd ) ( 4πfd ) = = λ P t = transmitted signal power P r = received signal power d = propagation distance between antennas c Free Space Loss For non-isotropic antenna, gain must be considered ( 4 ) ( 4 ) ( 4 Ar )( 4 At ) G t = gain of transmitting antenna G r = gain of receiving antenna A t = effective area of transmitting antenna A r = effective area of receiving antenna ( ) πd πd λd t = = = r r tλ π λ π λ λ r t P P GG AA ( λ ) ( ) ( ) L = 20log + 20log d 10log A A db r t

39 Free Space Loss Initially, FSL increases with frequency Gain compensate for the loss Net result free space loss is higher for lower frequencies all other factors remaining constant Free Space Loss

40 Example Determine isotropic free space loss (db) at 4 GHz for shortest path to a synchronous satellite from earth (35,863 km) Answer λ = c/f = / = L db = -20 log(0.075) +20 log( ) = db Example Now, consider antenna gain of both antennas: 44 db for satellite antenna, 48 db for ground antenna. Calculate L db Answer L db = = db

41 Example Now assume transmit power of 250 W of earth station. What is received power at satellite antenna? Answer P t in dbw = 10 log(250) = 24 dbw P r = P t L db = = dbw Atmospheric Absorption Water vapor peak attenuation near 22 GHz Oxygen absorption peak near 60 GHz Rain and fog cause scattering of radio waves attenuation Keep short path lengths or low frequency bands in areas with precipitation

42 Multipath Obstacles cause signals to reflect Receiver might get multiple copies of the signal with different delays Might reinforce or cancel each other Sometimes, no direct signal Can be controlled for fixed antennas Hard to control for mobile antennas Multipath

43 Additional References 1. Andew Tanenbaum, Computer Networks, Prentice-Hall, 4 th Edition, Michael Palmer, Hands-On Networking Fundamentals, Thomson Course Technology, Wikipedia 4. Richard Fitzpatrick, Antenna directivity and effective area, farside.ph.utexas.edu/teaching/jk1/lectures/node83.html