Data and Computer Communications. Chapter 4 –Transmission Media. Eighth & Ninth Editions by William Stallings. Transmission Media.
Data and Computer Communications
Chapter 4 –Transmission Media
Eighth & Ninth Editions
by William Stallings
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
media that are used to convey information can be classified as guided or unguided
frequencies at which various guided media and unguided transmission techniques operate
Twisted pair (with loading)
0 to 3.5 kHz
0.2 dB/km @ 1 kHz
Twisted pairs (multi-pair cables)
0 to 1 MHz
0.7 dB/km @ 1 kHz
0 to 500 MHz
7 dB/km @ 10 MHz
1 to 9 km
186 to 370 THz
0.2 to 0.5 dB/km
Based on this chart a decision can be made on what media to choose!
In response to the need to support higher speeds, Standard EIA-568-A was issued in 1995.
The new standard reflects advances in cable and connector design and test methods
Table 4.2 summarizes the performance of Category 3 and 5 UTP, as well as the STP specified in EIA-568-A
The strength of a signal falls off with distance over any transmission medium
Near-end crosstalk see next slide
like twisted pair, consists of two conductors, but is constructed differently to permit it to operate over a wider range of frequencies
used in a wide variety of applications, including:
•Television distribution - aerial to TV & CATV systems
•Long-distance telephone transmission - traditionally used for inter-exchange links, now being replaced by optical fiber/microwave/satellite
•Short-run computer system links
•Local area networks
An optical fiber is a thin flexible medium capable of guiding an optical ray.
Various glasses and plastics can be used to make optical fibers.
An optical fiber cable has a cylindrical shape and consists of three concentric sections:
Both single mode and multimode can support several different wavelengths of light and can employ laser or LED light sources.
attenuation versus wavelength for the various types of wired media
attenuation for twisted pair is a very strong function of frequency
coaxial cable has frequency characteristics that are superior to those of twisted pair and can be used effectively at higher frequencies and data rates.
The unusual shape of the curve is due to the combination of a variety of factors that contribute to attenuation.
absorption and scattering, which is the change in direction of light rays after they strike small particles or impurities in the medium.
used for LAN
For unguided media, transmission and reception are achieved by means of an antenna
Paraboloid surfaces are used in headlights, optical and radio telescopes, and microwave antennas
parabolic reflective antenna, used in terrestrial microwave and satellite applications.
A parabola is the locus of all points equidistant from a fixed line (the directrix) and a fixed point (the focus) not on the line,
If a parabola is revolved about its axis, the surface generated is called a paraboloid.
Figure 4.6 depicts in a general way two common configurations for satellite communication. In the first, the satellite is being used to provide a point-to-point link between two distant ground-based antennas.
the satellite provides communications between one ground-based transmitter and a number of ground-based receivers.
A signal radiated from an antenna travels along one of three routes: ground wave, sky wave, or line of sight
Ground wave propagation more or less follows the contour of the earth and can propagate considerable distances, well over the visual horizon. This effect is found in frequencies up to about 2 MHz.
The best-known example of ground wave communication is AM radio.
Sky wave propagation is used for amateur radio, CB radio, and international broadcasts such as BBC and Voice of America.
A sky wave signal can travel through a number of hops, bouncing back and forth between the ionosphere and the earth's surface (Refraction).
~3 x 108 m/s in vacuum, less in anything else
Some impairments specific to wireless line-of-sight transmission.
Free space loss can be expressed in terms of the ratio of the radiated power Pt to the power Pr received by the antenna or, in decibels, by taking 10 times the log of that ratio.
Figure 4.11 illustrates in general terms the types of multipath interference typical in terrestrial, fixed microwave and in mobile communications.
For fixed microwave, in addition to the direct line of sight, the signal may follow a curved path through the atmosphere due to refraction and the signal may also reflect from the ground.
For mobile communications, structures and topographic features provide reflection surfaces.