Figure 3-11 Coaxial Cable—Thicknet Coaxial cable with a diameter of 0.35 cm, sometimes referred to as thinnet, was also frequently used in Ethernet networks at one time.. 134 Chapter 3:
Trang 1When working with cable, consider its size As the diameter of the cable increases,
so does the difficulty in working with it because cable must be pulled through existing
conduits that are limited in size Coaxial cable comes in several sizes The largest
diam-eter (1 cm) was once specified for use as Ethernet backbone cable because it had a
greater transmission length and better noise rejection characteristics than other types
of cable This type of coaxial cable is frequently referred to as thicknet, as shown in
Figure 3-11 As its nickname suggests, thicknet cable can be too rigid to install easily
in some situations because of its thickness The general rule is that the more difficult
the network medium is to install, the more expensive it is to install Coaxial cable is
more expensive to install than twisted-pair cable Thicknet cable is almost never used
except for special-purpose installations
Figure 3-11 Coaxial Cable—Thicknet
Coaxial cable with a diameter of 0.35 cm, sometimes referred to as thinnet, was also
frequently used in Ethernet networks at one time Thinnet, as shown in Figure 3-12,
was especially useful for cable installations that required the cable to make many twists
and turns Because it was easier to install, it was also cheaper to install Thus, it was
sometimes referred to as Cheapernet.
Figure 3-12 Coaxial Cable—Thinnet
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However, in both types of coaxial cable, the outer conductor must be carefully and properly grounded, which increases the complexity of the installation It is for this rea-son that, despite its advantages, coaxial cable is no longer commonly used in Ethernet networks
Although many bus topology networks still utilize coaxial cable in use throughout the world, the IEEE no longer recommends this cable or topology as a standard for use with Ethernet Nearly all new LANs use Ethernet extended star topology and a combi-nation of UTP and fiber
The following summarizes the features of coaxial cables:
Cable Specification and Termination
Specifications, or standards, are sets of rules or procedures that are widely used and serve as the accepted method of performing a task For example, the OSI reference model standards help to ensure that networking devices around the world are compatible and will work together Many specifications exist for cabling to ensure interoperability, safety, and performance
TheInstitute of Electrical and Electronic Engineers (IEEE)has outlined LAN cabling specifications IEEE 802.3 is a standard for Ethernet networks, and IEEE 802.5 is a Token Ring network standard The Underwriters Laboratories issues standards that are primarily concerned with safety
More Information: Plenum Cable Plenum cable is the cable that runs in plenum spaces of a building In building construction, a plenum (pronounced PLEH-nuhm, from Latin meaning “full”) is a separate space provided for air circulation for heating, ventilation, and air-conditioning (sometimes referred to as HVAC), typically in the space between the structural ceiling and a drop-down ceiling In buildings with computer installations, the plenum space often is used to house connecting communication cables Because ordinary cable introduces a toxic hazard in the event of fire, special plenum cabling is required in plenum areas.
Plenum cable sheathing is often is made of Teflon and is more expensive than ordinary cabling Its outer material is more resistant to flames and, when burning, produces less smoke than ordinary cabling Both twisted-pair and coaxial cable are made in plenum cable versions.
Trang 3TheTelecommunications Industry Association (TIA) and the Electronic Industries
Association (EIA) have jointly issued cabling standards often called TIA/EIA standards
The list that follows describes some of the TIA/EIA standards:
standard
building standard for telecommunications pathways and spaces
wiring standard
infrastructure of commercial buildings
require-ment for telecommunications
The specifications created by this organization have had the greatest impact on
net-working media standards and include standards for horizontal and backbone (vertical)
cabling, wiring closets and equipment rooms, work areas, and entrance facilities The
TIA/EIA standards allow for the planning and installation of LAN equipment in a way
that allows network designers the freedom to choose the devices needed while assuring
the operability of the LAN design
The TIA/EIA-568-B standard focuses on horizontal cabling, which is cabling that runs
from a wall outlet at a work area to a wiring closet There are five historic categories
for cable (CAT 1 through CAT 5), and of these, only CAT 3, CAT 4, and CAT 5 meet
the TIA/EIA-568-B standard CAT 5 cable is the most frequently installed Recent
stan-dards have been developed for CAT 5e and CAT 6, and a new standard for CAT 7
cabling is also underway These new standards offer improvements to CAT 5 and are
becoming more common
TIA/EIA-568-B calls for two cables to each work area outlet:
■ A telephone cable for voice
■ A network cable for data
The voice cable must be a 2-pair UTP cable with its correct connectors, or terminators
The network cable must be one of the following and must include the correct connectors,
or terminators:
■ 100 ohm UTP 4-pair cable (Ethernet LANs)
■ 62.5/125µ fiber-optic cable (Ethernet LANs)
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■ Coaxial cable (rarely used for new installations and expected to be removed from this list the next time the standard is updated)
Although not part of the standard, a coaxial 75 ohm RG-6 cable can also be run for cable TV connection, in addition to the minimum voice and data connections if desired
The standard also specifies the maximum length of each cabling run from the wall out-let to the wiring closet connections for UTP cabling A 3-meter patch cord is specified from the workstation to the wall outlet A 90-meter cable run is allowed from the wall outlet to the patch panel in the wiring closet A 6-meter patch cord is permitted from the patch panel to the horizontal cross connect in the wiring closet This standard ensures that the entire cable run does not exceed 100 meters
Lab Activity Basic Cable Testing
In this lab, you use a simple cable tester to verify whether a straight-through
or crossover cable is good or bad You also use the Fluke 620 advanced cable tester to test cables for length and connectivity
Lab Activity Making a Straight-Through Cable
In this lab, you build a CAT 5 or CAT 5e UTP Ethernet network patch cable (or patch cord) You also test the cable for good connections (continuity) and correct pinouts (correct color of wire on the right pin)
Lab Activity Making a Rollover Cable
In this lab, you build a CAT 5 or CAT 5e UTP console rollover cable You also test the cable for good connections (continuity) and correct pinouts (correct wire on the right pin)
Lab Activity Making a Crossover Cable
In this lab, you build a CAT 5 or CAT 5e UTP Ethernet crossover cable to TIA-568-B and TIA-568-A standards You also test the cable for good connec-tions (continuity) and correct pinouts (correct wire on the right pin)
Trang 5Optical Media
Optical fiber is the most frequently used medium for the longer, high bandwidth,
point-to-point transmissions required on LAN backbones and on wide-area networks (WANs)
Very good reasons exist for the popularity of fiber
Optical fiber is used in networks because
■ Fiber is not susceptible to lightning, electromagnetic interference (EMI), or
radio frequency interference (RFI), and it does not generate EMI or RFI
■ Fiber has much greater bandwidth capabilities than other media
■ Fiber allows significantly greater transmission distances and excellent signal
quality because very little signal attenuation occurs
■ Fiber is more secure than other media because it is difficult to tap into a fiber
and easy to detect someone’s placing a tap on the fiber
■ Current fiber transmitter and receiver technologies can be replaced by newer,
faster devices as they are developed so that greater transmission speeds can be achieved over existing fiber links with no need to replace the fiber
■ Fiber costs less than copper for long distance applications
■ The raw material that fiber is made from is sand, a plentiful substance
■ With fiber, you have no grounding concerns as you have when signaling using
electricity
■ Fiber is light in weight and easily installed
■ Fiber has better resistance to environmental factors, like water, than copper wire
■ Lengths of fiber can easily be spliced together for very long cable runs
For these reasons, when very large numbers of bits need to be sent over distances
greater than 100 meters, fiber-optic fiber is often used
This section explains the basics of fiber-optic cable You learn about how fibers can
guide light for long distances You also learn about the types of cable used, how fiber is
installed, the type of connectors and equipment used with fiber-optic cable, and how
fiber is tested to ensure that it functions properly
Lab Activity UTP Cable Purchase
In this lab, you are introduced to the variety and prices of network cabling and related components in the market This lab looks specifically at patch cables and bulk cable
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The Electromagnetic Spectrum
The light used in optical fiber networks is one type of electromagnetic energy When an electric charge moves back and forth or accelerates, a type of energy called electromag-netic energy is produced This energy, in the form of waves, can travel through a vacuum, the air, and through some materials like glass An important property of any energy wave is its wavelength, as shown in Figure 3-13
Figure 3-13 Wavelength
To human beings, radio, microwaves, radar, visible light, X rays, and gamma rays seem to be very different things, but they are all types of electromagnetic energy If all the types of electromagnetic waves are arranged in order from waves with the longest wavelength down to those waves with the shortest wavelength, it creates a continuum
called the Electromagnetic Spectrum, as shown in Figure 3-14.
Figure 3-14 Electromagnetic Spectrum
The wavelength of an electromagnetic wave is determined by how frequently the elec-tric charge that generates the wave moves back and forth For example, if the charge moves back and forth slowly, it generates a long wavelength Visualize the movement
of the electric charge as like that of a stick in a pool of water If the stick is moved back
Wavelength
Trang 7and forth slowly, it generates ripples in the water with a long wavelength between the
tops of the ripples If the stick is moved back and forth more rapidly, the ripples have a
shorter wavelength
Because electromagnetic waves are all generated in the same way, all electromagnetic
waves share many of the same properties For example, they all travel at the same rate
of speed through a vacuum, about 300,000 kilometers per second or 186,000 miles
per second or the speed of light
Human eyes can sense electromagnetic energy only with wavelengths between 700
nanometers and about 400 nanometers A nanometer is one billionth of a meter
(0.000000001 m) in length and is abbreviated nm Electromagnetic energy with
wave-lengths between 700 nm and 400 nm is called visible light The longer wavewave-lengths of
light (those around 700 nm) are seen as the color red The shortest wavelengths (around
400 nm) appear as the color violet This part of the electromagnetic spectrum is seen as
the colors in a rainbow
To transmit data over optical fiber, wavelengths that are not visible to the human eye
are used These wavelengths are slightly longer than red light and are called infrared
light Infrared light is used in TV remote controls The wavelength of the light in
opti-cal fiber is one of the following wavelengths:
These wavelengths were selected because they travel better through optical fiber than
other wavelengths
The Ray Model of Light
When electromagnetic waves, including light, travel out from the source, they travel in
straight lines These straight lines pointing out from the source are called rays.
Think of light rays as narrow beams of light like those produced by lasers In the vacuum
of empty space, light travels continuously in a straight line at 300,000 kilometers per
second However, light travels at different, slower speeds through other materials like
air, water, and glass When a light ray (called the incident ray) crosses the boundary
from one material (air, for example) to another (glass, for example), some of the light
energy in the ray is reflected back That is why you can see yourself in a mirror The
light that is reflected back is called the reflected ray.
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The light energy in the incident ray that is not reflected enters the glass The entering
ray is usually bent at an angle from its original path This ray is called the refracted ray How much the incident light ray is bent depends on two factors:
■ The angle at which the incident ray strikes the surface of the glass
■ The different rates of speed at which light travels through the two substances (air and glass in this example)
The bending of light rays at the boundary of two substances is the reason why light rays are able to travel through an optical fiber even if the fiber curves in a circle How much rays of light in glass bend is determined by the optical density of the glass Optical density refers to how much a light ray is slowed down by passing through a substance The greater the optical density of a material, the more it slows light down from its speed in a vacuum The ratio of the speed of light in a material to the speed of light in a vacuum is called the material’s index of refraction (IR) and is expressed as follows:
Therefore, the measure of a material’s optical density is that material’s index of refrac-tion A material with a large index of refraction is more optically dense and slows light down more than a material with a smaller index of refraction
Table 3-2 shows the IR of air, glass, diamond, and water
For a substance like glass, the index of refraction (the optical density) can be made larger by adding chemicals to the glass The index of refraction can be made smaller
by making the glass very pure
In the next two sections, you learn more about the reflection and refraction that help you to understand the design and operation of optical fibers
Table 3-2 Index of Refraction
IR Speed of light in a vacuum Speed of light in a material
-=
Trang 9The Law of Reflection
When light travels through a medium like air and strikes another medium like glass,
the light either reflects off the surface or passes into or through the second medium, as
shown in Figure 3-15 It depends on the angle it strikes the surface The angle between
the incident ray and a line perpendicular to the surface of the glass at the point where
the incident ray strikes the glass is called the angle of incidence When this angle of
incidence reaches a certain point, called the critical angle, all the light is reflected back
into the original medium, as illustrated in Figure 3-16
Figure 3-15 Reflection
Figure 3-16 Critical Angel
The perpendicular line is called the normal It is not a light ray but a tool to allow the
measurement of angles The angle between the reflected ray and the normal is called
the angle of reflection The Law of Reflection states that the angle of reflection of a
light ray is equal to its angle of incidence In other words, the angle at which a light ray
strikes a reflective surface determines the angle that the ray reflects off the surface
Air
Glass
Air
Glass
Light incident at any angle smaller than the critical angle is not completely reflected.
Light incident at any angle greater than the critical angle is completely reflected.
Light Source Critical Angle
c
c
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The Law of Refraction (Snell’s Law)
When light strikes the interface between two transparent materials like air and glass, the light divides into two parts Part of the light ray is reflected back into the first substance (the air), with the angle of reflection equaling the angle of incidence The remaining energy in the light ray crosses the interface and enters into the second sub-stance (the glass)
If the incident ray strikes the glass surface at exactly a 90-degree angle, the ray goes straight into the glass The ray is not bent But, if the angle of incidence is not exactly
90 degrees, then the ray that enters the glass (the transmitted ray) is bent We call this bending of the entering ray refraction How much the ray is bent (refracted) depends
on the index of refraction of the two transparent materials If the light ray travels from
a substance whose index of refraction is smaller into a substance where the index of refraction is larger, the refracted ray is bent towards the normal If the light ray travels from a substance where the index of refraction is larger into a substance where the index
of refraction is smaller, the refracted ray is bent away from the normal Figure 3-17 illustrates an example of refraction
Figure 3-17 Refraction
For example, consider a light ray moving at some angle other than 90 degrees through the boundary between glass and a diamond, as shown in Figure 3-18 The glass has an index of refraction of about 1.523 The diamond has an index of refraction of about 2.419 Therefore, the ray that continues into the diamond is bent towards the normal When that light ray crosses the boundary between the diamond and the air at some angle other than 90 degrees, it is bent away from the normal The reason for this is that air has a lower index of refraction (about 1.000) than does diamond
Air
Glass