Chapter 3 discusses Transmission Media, common types of media, and accessing public and private networks. The objectives important to this chapter are on page 3-1:
Concepts:Transmission Media provide pathways for networked computers to contact each other. Note that the concept of communication is not addressed in this definition, only contact. Communication is addressed in the next several chapters concerning protocols and protocol layers. (Protocols are communication rules.) Some basic physics will be helpful here. Remember that light and electricity can be thought of as either pulses or waves. Electricity is more commonly used in networks than light, but both can be used to pass signals. In either case, the pulse or wave is said to pass from one point to another across a medium, for example, a wire. (Note: your book is rather inconsistent in its usage of the words media and medium. "Medium" is singular, "media" is plural.) Think of a beam of energy as a wave. The wave travels at the speed of
light (about 186,000 miles per second in a vacuum). When traveling through
a medium such as glass or water, the speed is reduced slightly. For our
purposes, let's assume the speed of light is constant. (Professor Einstein
will be so relieved.) If the speed of light is represented as c,
then we can represent the frequency as F, and the wavelength as
W. (I know, your physics teacher did it differently. I am simplifying.)
Then we can say that F times W equals c. This always
works for energy waves. Back to practicality, consider the facts on page 3-4. The first paragraph
tells us that computers use electric currents and various forms
of electromagnetic waves to communicate. We can class networks
as being cable or wireless, for obvious reasons. We will
discuss five attributes for each type of medium in this chapter:
Page 3-6 mentions three types of cable media:
(For the purists among you, I will note that the speed of light through these media is about two thirds the speed of light in a vacuum.) The graphic on page 3-7 shows a twisted pair of wires. Each wire is covered
with an insulator, and the two wires in the pair complete a circuit. These
wires suffer from crosstalk, leakage of signal. The twists help
cancel out such leaks. The graphic on 3-8 shows a UTP cable with eight
wires in it, making four pairs. The wires in each pair are twisted around each other. This type of cable comes in several varieties: two pair, three pair and four pair are common. Also, each variety may be available in grades, such as CAT 1 (Category 1) and CAT 5 (Category 5). There are five such categories, and a major difference between them is the number of twists per foot in each pair. CAT 1 will have less than 5 twists per foot, CAT 5 will have 25 or more twists per foot (so it is better, and costs more). Note that the better the class of cable, the more bits per second can be passed across it. Connecting a system with twisted pair wiring is easy. It is illustrated on page 3-9. A possible problem is that the wiring closets in any building are often in need of being "cleaned up". The "closet" on each floor of a building contains punch-down blocks, patch panels, and hubs (or switches). Many are disorganized and messy. People who try to clean them up, however, must be careful not to disconnect circuits that are needed. The factors for UTP:
An STP (Shielded Twisted Pair) cable is illustrated on page 3-12. This cable is more expensive than unshielded cable, and is less flexible due to the stiff shielding. The shield, however, makes it more EMI resistant than UTP. The factors for STP:
Coaxial cable is called that because it has two conductor, one wire in center and a conductive sheath around it, that share a "common axis". Most people have seen this style of cable used with cable television. The wiring standards used for network coax are different from those
used for cable TV. You should know the list at the bottom of page 3-15:
The number associated with each RG specification tells you the relative size of the center conductor. Smaller numbers mean thicker wires. Since we don't want to take forever for this page to load, I'll refer to the book for a while. See the graphic on page 3-17. This shows a coaxial cable being used to connect computers on a net. The harder parts of doing this become apparent here. The coaxial line is essentially a single bus, going from one station to the next. At each end of the line, the cable has to have a terminator on it. At one end, it also has to be grounded. Workstations can be connected two ways. If using thin Ethernet, T-connectors are used. If using thick Ethernet, vampire taps are used, like the one on the left in the illustration. They are called vampire taps because little teeth bite into the cable when you screw the clamp down. Note that the vampire tap just provides a place to tap into the cable. The workstation also needs a patch cable to connect to the tap. The factors for Coax:
Fiber optic can be glass or plastic, and is meant to conduct light instead of electricity. The conductor is called a waveguide, and is covered with cladding, a material to reflect the signal back into the center of the conductor. Two configurations are depicted on page 3-19. Loose configuration has a liquid filler between the outer sheath and the conductor. Tight configuration is shown with wire around the conductor to add strength to the cable. Fiber optic comes in two modes: single mode conducts a single signal, while multi-mode conducts many signals simultaneously. You may want to know that the second listing at the top of page 3-21 is the most common type used: 62.5 micron core with 125 micron cladding, multimode. Fiber optic is much harder to install and splice than electrical conductors. As illustrated on page 3-21, this type of connection requires two connectors for each station, a line in and a line out. The factors for fiber optic:
The book then goes on to discuss wireless media. This means that there is no cable of any sort between certain parts of the network. (There are still wires inside lots of components). Radio is the label used for frequencies from 10 KHz to 1 GHz (page 3-28). Several bands are indicated on the chart on this page. Frequencies that are used for networks can be divided into regulated and unregulated frequencies. Only a few frequencies are unregulated in the United States. Your book observes that it is not possible to guarantee error free transmission in the unregulated frequencies. This is because anyone else can broadcast in those frequencies, causing errors in your transmissions. For this reason, broadcasts are usually limited to low power in unregulated bands, to minimize interference. Three types of radio usage are discussed:
The chart for this section is a bit different from the last one. The
factors for low power, single frequency:
The factors for high power, single frequency:
Spread spectrum radio usage puts the incoming data stream on
several frequencies at once. This discourages eavesdropping. Using direct
sequence modulation, the signal is put on several frequencies, some
of which may contain false signals. Using frequency hopping, the
frequency being used is changed on a preset pattern, which the sender
and receiver know. The factors for spread spectrum:
Microwave signals are used in two formats: terrestrial
(earth-based) and satellite systems. Terrestrial systems
are used in line of sight connections where it is not possible to put
a wire, such as across several city blocks. The factors for terrestrial
microwave:
Satellite systems are used to connect sites that are widely separated.
Usually, signals are sent to geosynchronous satellites, orbiting
22,300 miles above the earth. This orbit puts the satellite in
the same part of the sky relative to a ground based observer at all times.
The factors for satellite microwave:
Infrared systems come in two types: point-to-point and
broadcast. Point-to-point systems are like the remote controls
we use for televisions. Some systems also use lasers. The factors for
point-to-point infrared:
Broadcast infrared systems are used in single room settings,
as these waves will bounce off walls, but not penetrate them. The advantage
is that you can put a system in each room where required, and the users
may move their machines around as they like. The factors for broadcast
infrared:
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