Chapter 4 - William Stallings, Data and Computer ...
Data and Computer Communications Tenth Edition by William Stallings Data and Computer Communications, Tenth Edition by William Stallings, (c) Pearson Education,-2013 CHAPTER 4 Transmission Media Communication channels in the animal world include 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 Design Factors Determining Data Rate and Distance Bandwidth Higher bandwidth gives higher data rate Transmission impairments Impairments, such as attenuation, limit the distance Interference Overlapping frequency bands can distort or wipe out a signal Number of receivers
More receivers introduces more attenuation Frequency (Hertz) 102 ELF 103 VF 104 VLF 105 LF Power and telephone 106 MF 107 HF 108 VHF 109 UHF Radio Rotatinggenerators Musical instruments
Wavelength in space (meters) ELF VF VLF LF = = = = 106 105 104 Extremely low frequency Voice frequency Very low frequency Low frequency 103 102 Visible light Optical
Fiber Coaxial Cable AM Radio 1015 FM Radio Terrestrial and TV and Satellite Transmission 101 100 10-1 MF = Medium frequency HF = High frequency VHF = Very high frequency 102 103 104 105 106 UHF = Ultrahigh frequency
SHF = Superhigh frequency EHF = Extremely high frequency Figure4.1 Electromagnetic Spectrum for Telecommunications Table 4.1 Point-to-Point Transmission Characteristics of Guided Media Frequency Range Typical Attenuation Typical Delay Repeater Spacing Twisted pair (with loading) 0 to 3.5 kHz 0.2 dB/km @ 1 kHz 50 s/km 2 km Twisted pairs (multipair
cables) 0 to 1 MHz 0.7 dB/km @ 1 kHz 5 s/km 2 km Coaxial cable 0 to 500 MHz 7 dB/km @ 10 MHz 4 s/km 1 to 9 km Optical fiber 186 to 370 THz 0.2 to 0.5 dB/km 5 s/km 40 km THz = terahertz = 1012 Hz
twist length Separately insulated Twisted together Often "bundled" into cables Usually installed in building duringconstruction (a) Twisted pair Outer conductor Outer sheath Insulation Inner conductor Outer conductor isbraided shield Inner conductor issolid metal Separated by insulatingmaterial Covered by padding (b) Coaxial cable Buffer coating Core Cladding Glass or plastic core Laser or light emittingdiode
Small sizeand weight Light at less than critical angleis absorbed in bufer coating Angleof incidence (c) Optical fiber Figure4.2 Guided Transmission Media Angleof reflection Twisted Pair Reduce Electromagnetic Interference Twisted pair is the least expensive and most widely used guided transmission medium Consists of two insulated copper wires arranged in a regular spiral pattern A wire pair acts as a single communication link Pairs are bundled together into a cable Most commonly used in the telephone network and for communications within buildings
30 3/8" cable (9.5 mm) 15 10 5 0 105 1000 Wavelength in vacuum (nm) (a) Twisted pair (based on [REEV95]) 20 900 0.5 mm twisted pair 20 15 9.5 mm coax 10
typical optical fiber 5 106 107 Frequency (Hz) (b) Coaxial cable(based on [BELL90]) 108 0 103 1 kHz 106 1 MHz 109 1 GHz Frequency (Hz) (d) Compositegraph Figure4.3 Attenuation of Typical Guided Media 1012 1 THz 1015
Unshielded and Shielded Twisted Pair Unshielded Twisted Pair (UTP) Consists of one or more twisted-pair cables, typically enclosed within an overall thermoplastic jacket which provides no electromagnetic shielding Ordinary telephone wire Subject to external electromagnetic interference The tighter the twisting, the higher the supported transmission rate and the greater the cost per meter Shielded Twisted Pair (STP) Has metal braid or sheathing that reduces interference Provides better performance at higher data rates More expensive Table 4.2 Twisted Pair Categories and Classes UTP = Unshielded twisted pair FTP = Foil twisted pair S/FTP = Shielded/foil twisted pair Twisted Pair Near-End Crosstalk (NEXT) Coupling of signal from one pair of conductors to another
Conductors may be the metal pins in a connector or wire pairs in a cable Near end refers to coupling that takes place when the transmit signal entering the link couples back to the receive conductor pair at that same end of the link Greater NEXT loss magnitudes are associated with less crosstalk noise Received signal (power Pr) Rx SystemA Transmitted signal (power Pt) Tx NEXT (power Pc) Tx System B Rx Transmitted
signal (power Pt) Figure4.4 Signal Power Relationships (from SystemA viewpoint) 0 Attenuation decibels 20 ACR 40 NEXT 60 65 0 100 200 300 400 500 Frequency (MHz) NEXT =near-end crosstalk ACR =attenuation-to-crosstalk ratio Figure4.5 Category 6A Channel Requirements
Coaxial Cable Coaxial cable can be used over longer distances and support more stations on a shared line than twisted pair Consists of a hollow outer cylindrical conductor that surrounds a single inner wire conductor Is a versatile transmission medium used in a wide variety of applications Used for TV distribution, long distance telephone transmission and LANs Coaxial Cable - Transmission Characteristics Frequency characteristics superior to twisted pair Performance limited by attenuation and noise Analog signals Amplifiers are needed every few kilometers closer if higher frequency Usable spectrum
extends up to 500MHz Digital signals Repeater every 1km - closer for higher data rates 30 3.0 2.5 26-AWG (0.4 mm) 24-AWG (0.5 mm) 22-AWG (0.6 mm) 19-AWG (0.9 mm) 20 Attenuation (dB/km) Attenuation (dB/km) 25 15 10 5 0 102
1400 1500 1600 1700 (c) Optical fiber (based on [FREE02]) 30 3/8" cable (9.5 mm) 15 10 5 0 105 1000 Wavelength in vacuum (nm) (a) Twisted pair (based on [REEV95]) 20 900 0.5 mm twisted pair
20 15 9.5 mm coax 10 typical optical fiber 5 106 107 Frequency (Hz) (b) Coaxial cable(based on [BELL90]) 108 0 103 1 kHz 106 1 MHz 109 1 GHz Frequency (Hz)
(d) Compositegraph Figure4.3 Attenuation of Typical Guided Media 1012 1 THz 1015 Optical Fiber Optical fiber is a thin flexible medium capable of guiding an optical ray Various glasses and plastics can be used to make optical fibers Has a cylindrical shape with three sections core, cladding, jacket Widely used in long distance telecommunications Performance, price and advantages have made it popular to use Optical Fiber Optical Fiber - Benefits Greater Data rates of hundreds of Gbps over tens of kilometers have been demonstrated Smaller
capacity size and lighter weight Considerably thinner than coaxial or twisted pair cable Reduces structural support requirements Lower attenuation Electromagnetic isolation Not vulnerable to interference, impulse noise, or crosstalk High degree of security from eavesdropping Greater repeater spacing Lower cost and fewer sources of error Categories of Application Five basic categories of application have become important for optical fiber:
Long-haul trunks Metropolitan trunks Rural exchange trunks Subscriber loops Local area networks Electrical digital signal LED or Electronic laser interface light source Lightwave pulses Detector Electrical (light Electronic digital sensor) interface signal Optical fiber E/O Conversion O/E Conversion
Figure4.6 Optical Communication Input pulse Output pulse (a) Step-index multimode Input pulse Output pulse (b) Graded-index multimode Input pulse Output pulse (c) Single mode Figure4.7 Optical Fiber Transmission Modes Table 4.3 Frequency Utilization for Fiber Applications Fiber Type Application Multimode LAN
S Single mode Various 196 to 192 C Single mode WDM 192 to 185 L Single mode WDM Wavelength (in vacuum) range (nm) Frequency Range (THz) 820 to 900 366 to 333
1280 to 1350 234 to 222 1528 to 1561 1561 to 1620 WDM = wavelength division multiplexing Band Label Attenuation in Guided Media Wireless Transmission Frequencies 1GHz to 40GHz Referred to as microwave frequencies Highly directional beams are possible Suitable for point to point transmissions Also used for satellite communications Suitable for omnidirectional applications 30MHz to Referred to as the radio range
1GHz Infrared portion of the spectrum Useful to local point-to-point and multipoint applications within 11 3 x 10 to confined areas 2 x 1014 Antennas Electrical conductor or system of conductors used to radiate or collect electromagnetic energy Radio frequency electrical energy from the transmitter is converted into electromagnetic energy by the antenna and radiated into the surrounding environment Reception occurs when the electromagnetic signal intersects the antenna In two way communication, the same antenna can be used for both transmission and reception Radiation Pattern Power radiated in all directions Does not perform equally well in all directions Radiation
A graphical representation of the radiation properties of an antenna as a function of space coordinates Isotropic pattern antenna A point in space that radiates power all directions equally Actual radiation pattern is a sphere the antenna at the center in with y transmitting waves a directrix b c f
a b c f focus x sourceof electromagnetic energy (a) Parabola (b) Cross-section of parabolic antenna showingreflectivepr operty Figure4.8 Parabolic Reflective Antenna Antenna Gain A measure of the directionality of an antenna Effective area of an antenna is related to the physical size of the antenna and to its shape The increased power radiated in a given direction is at
the expense of other directions Defined as the power output in a particular direction versus that produced by an isotropic antenna Measured in decibels (dB) Terrestrial Microwave Most common type is the parabolic dish A series of microwave relay towers is used to achieve long-distance transmission Usually located at substantial heights above ground level Typical size is about 3 m in diameter Antenna is fixed rigidly and focuses a narrow beam to achieve line-of-sight transmission to the
receiving antenna Terrestrial Microwave Applications Used for long haul telecommunications service as an alternative to coaxial cable or optical fiber Used for both voice and TV transmission Fewer repeaters but requires line-of-sight transmission 1-40GHz frequencies, with higher frequencies having higher data rates Main source of loss is attenuation caused mostly by distance, rainfall and interference Table 4.4 Typical Digital Microwave Performance Band (GHz) Bandwidth (MHz) Data Rate(Mbps) 2
7 12 6 30 90 11 40 135 18 220 274 Satellite Microwave A communication satellite is in effect a microwave relay station Used to link two or more ground stations Receives transmissions on one frequency band, amplifies or repeats the signal, and transmits it on another frequency
Frequency bands are called transponder channels Satellite antenna up lin k k lin wn do Earth station (a) Point-to-point link Satellite antenna uplink do wn lin k k lin wn
do k lin wn do Multiple receivers k in nl w do nk nli w o d k lin wn o d Transmitter Multiple receivers (b) Broadcast link
Figure4.9 SatelliteCommunication Configurations Satellite Microwave Applications Most important applications for satellites are: Is the optimum medium for high-usage international trunks Navstar Global Positioning System (GPS) Longdistance telephone transmission Private business networks Global positioning Television distribution Satellite providers can divide capacity into channels
and lease these channels to individual business users Programs are transmitted to the satellite then broadcast down to a number of stations which then distribute the programs to individual viewers Direct Broadcast Satellite (DBS) transmits video signals directly to the home user Ku-band satellite Remote site Server Hub PCs Remote site Remote site Point-of-sale Terminals
Figure4.10 Typical VSAT Configuration Transmission Characteristics The optimum frequency range for satellite transmission is 1 to 10 GHz Below 1 GHz there is significant noise from natural sources Above 10 GHz the signal is severely attenuated by atmospheric absorption and precipitation Satellites use a frequency bandwidth range of 5.925 to 6.425 GHz from earth to satellite (uplink) and a range of 3.7 to 4.2 GHz from satellite to earth (downlink) This is referred to as the 4/6-GHz band Because of saturation the 12/14-GHz band has been developed Broadcast Radio Broadcast radio is omnidirectional and microwave is directional Radiois the term used to encompass frequencies in the range of 3kHz to 300GHz Broadcast radio (30MHz - 1GHz) covers: FM radio and UHF and VHF television band Data networking applications Limited
to line of sight Suffers from multipath interference Reflections from land, water, man-made objects Infrared Achieved using transceivers that modulate noncoherent infrared light Transceivers must be within line of sight of each other directly or via reflection Does not penetrate walls No licensing is required No frequency allocation issues Table 4.5 Frequency Bands (Table can be found on page 160 in textbook) transmit antenna pro signal pag atio n
e er ph s no Io signal propagation Earth receive antenna (b) Sky-wavepropagation (2 to 30 MHz) transmit antenna receive antenna Earth signal propagation transmit antenna Earth receive
antenna (a) Ground-wavepropagation (below 2 MHz) (c) Line-of-sight (LOS) propagation (above30 MHz) Figure4.11 Wireless Propagation Modes pro signal pag atio n re e h propagation follows the contour of the Ground wave sp o n earth and Io can propagate distances well over the visual horizon This effect is found in frequencies up to about 2MHz The best transmitknown example of ground wave communication receive ante nna ante nna Earth
is AM radio he sp no o I pro signa pag l atio n (a) Ground-w avepropagation (below 2 MHz) re transmit antenna Earth receive antenna pro signal pag atio n re he p
s no o I (b) Sky-wavepropagation (2 to 30 MHz) transmit antenna receive antenna signal propagation Earth transmit antenna Earth receive antenna (b) Sky-wavepropagation (2 to 30 MHz) (c) Line-of-sight (LOS) propagation (above30 MHz) Figure4.11 Wireless Propagation Modes signal propagation
Sky wave propagation is used for amateur radio and internationaltrans broadcasts such as BBC and Voice of America mit receive antenna antenna Earth A signal from an earth based antenna is reflected from the ionized layer of the upper atmosphere back down to earth Sky wave signals can travel through a number of hops, (c) Line-of-sight (LOS) propagation (above30 MHz) bouncing back and forth between the ionosphere and the earths surface Figure4.11 Wireless Propagation Modes (b) Sky-wavepropagation (2 to 30 MHz) signal propagation transmit antenna Earth receive antenna
(c) Line-of-sight (LOS) propagation (above30 MHz) Figure4.11 Wireless Propagation Modes Ground and sky wave propagation modes do not operate above 30MHz - communication must be by line of sight Refraction Occurs because the velocity of an electromagnetic wave is a function of the density of the medium through which it travels 3 x 108 m/s in a vacuum, less in anything else The speed changes with movement between a medium of one density to a medium of another density Index of refraction (refractive index) The sine of the angle of incidence divided by the sine of the angle of refraction Is also equal to the ratio of the respective velocities in the two media
Varies with wavelength Gradual bending Density of atmosphere decreases with height, resulting in bending of radio waves toward the earth Radio horizon Antenna Optical horizon Earth Figure4.12 Optical and Radio Horizons Line-of-Sight Transmission Free space loss Loss of signal with distance Atmospheri c Absorption From water vapor and
oxygen absorption Multipath Multiple interferin g signals from reflection s Refractio n Bending signal away from receiver 180 Hz 00 G 3 = f 170 160 150 GHz 0 3
f= Loss (dB) 140 130 GHz 3 = f 120 Hz 0M 0 3 f= 110 100 90 Hz 0M 3 = f 80 70 60 1
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