Chapter 4 - William Stallings, Data and Computer ...
Data and Computer Communications Transmission Media 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
Overview transmission medium is the physical path between transmitter and receiver guided media guided along a solid medium unguided media atmosphere, space, water characteristics and quality determined by medium and signal guided media - medium is more important unguided media - bandwidth produced by the
antenna is more important key concerns are data rate and distance 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 Electromagnetic Spectrum Transmission Characteristics of Guided Media Frequency Range Twisted pair (with loading) 0 to 3.5 kHz Twisted
pairs (multipair cables) 0 to 1 MHz Coaxial cable Optical fiber Typical Attenuatio n 0.2 dB/km @ 1 kHz Typical
Delay Repeater Spacing 50 s/km 2 km 0.7 dB/km @ 1 kHz 5 s/km 2 km
0 to 500 MHz 7 dB/km @ 10 MHz 4 s/km 1 to 9 km 186 to 370 THz 0.2 to 0.5
dB/km 5 s/km 40 km infrared portion of the spectrum useful to local point-to-point and multipoint applications within confined areas suitable for omnidirectional applications referred to as the radio range
referred to as microwave frequencies highly directional beams are possible suitable for point to point transmissions also used for satellite 3 x 1011 to 2 x 1014 30MHz to 1GHz 1GHz to
fed to receiver converted to electromagnetic energy by antenna converted to radio frequency electrical energy radio frequency energy from transmitter
electromagnetic energy impinging on antenna electrical conductors used to radiate or collect electromagnetic energy same antenna is often
used for both purposes Radiation Pattern power radiated in all directions does not perform equally well in all directions as seen in a radiation pattern diagram an isotropic antenna is a point in space that
radiates power in all directions equally with a spherical radiation pattern Parabolic Reflective Antenna Antenna Gain measure of the directionality of an antenna power output in particular direction verses that produced by an isotropic antenna
measured in decibels (dB) results in loss in power in another direction effective area relates to physical size and shape Terrestrial Microwave most common type is a parabolic dish with an antenna focusing a narrow beam onto a receiving antenna located at substantial heights above ground to extend range and transmit over obstacles
uses a series of microwave relay towers with point-to-point microwave links to achieve long distance transmission Terrestrial Microwave Applications used for long haul telecommunications, short
point-to-point links between buildings and cellular systems 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 Microwave Bandwidth and Data Rates Satellite Microwave
a communication satellite is in effect a microwave relay station used to link two or more ground stations receives on one frequency, amplifies or repeats signal and transmits on another frequency frequency bands are called transponder channels requires geo-stationary orbit
rotation match occurs at a height of 35,863km at the equator need to be spaced at least 3 - 4 apart to avoid interfering with each other spacing limits the number of possible satellites Satellite Point-to-Point Link Satellite Broadcast Link
Satellite Microwave Applications uses: private business networks satellite providers can divide capacity into channels to lease to individual business users television distribution programs are transmitted to the satellite then broadcast down to a number of stations which then distributes the programs to individual viewers Direct Broadcast Satellite (DBS) transmits video signals
directly to the home user global positioning Navstar Global Positioning System (GPS) Transmission Characteristics the optimum frequency range for satellite transmission is 1 to 10 GHz lower has significant noise from natural sources higher is 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 (uplink: 14 - 14.5 GHz; downlink: 11.7 - 12.2 GH Broadcast Radio radio is the term used to encompass frequencies in the range of 3kHz to 300GHz broadcast radio (30MHz - 1GHz) covers
FM radio UHF and VHF television data networking applications omnidirectional 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 licenses required no frequency allocation issues typical uses: TV remote control Frequency Bands Wireless Propagation Ground Wave
ground wave propagation follows the contour of the earth and can propagate distances well over the visible horizon this effect is found in frequencies up to 2MHz the best known example of ground wave communication is AM radio Wireless Propagation Sky Wave sky wave propagation is used for amateur radio, CB radio, and
international broadcasts such as BBC and Voice of America 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, bouncing back and for the between the ionosphere and the earths surface Wireless Propagation Line of Sight ground and sky wave propagation modes do not operate above 30 MHz - - communication must be by line of sight
Refraction velocity of electromagnetic wave is a function of the density of the medium through which it travels ~3 x 108 m/s in vacuum, less in anything else speed changes with movement between media index of refraction (refractive index) is
sine(incidence)/sine(refraction) varies with wavelength gradual bending density of atmosphere decreases with height, resulting in bending of radio waves towards earth Line of Sight Transmission Free Space Loss
Multipath Interference Summary transmission Media physical path between transmitter and receiver bandwidth, transmission impairments, interference, number of receivers guided Media twisted pair, coaxial cable, optical fiber
wireless Transmission microwave frequencies antennas, terrestrial microwave, satellite microwave, broadcast radio wireless Propagation ground wave, sky wave, line of sight
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