Transcript S1M03

Semester 1 Module 3
Networking Media
Andres, Wen-Yuan Liao
Department of Computer Science and
Engineering
De Lin Institute of Technology
[email protected]
http://www.cse.dlit.edu.tw/~andres
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Overview
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Discuss the electrical properties of matter
Define voltage, resistance, impedance,
current, and circuits
Describe the specifications and performances
of different types of cable
Describe coaxial cable and its advantages
and disadvantages compared to other types
of cable
Describe STP cable and its uses
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Describe UTP cable and its uses
Discuss the characteristics of straight-through,
crossover, and rollover cables and where
each is used
Explain the basics of fiber-optic cable
Describe how fiber-optic cables can carry
light signals over long distances
Describe multimode and single-mode fiber
Describe how fiber is installed
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Describe the type of connectors and
equipment used with fiber-optic cable
Explain how fiber is tested to ensure that it
will function properly
Discuss safety issues related to fiber optics
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Outline
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Copper Media
Optical Media
Wireless Media
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Atoms and electrons
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The atom(原子) is comprised of:
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Electrons(電子) – Particles with a negative charge that orbit
the nucleus.
Nucleus(核子) – The center part of the atom, composed of
protons and neutrons.
Protons(質子) – Particles with a positive charge.
Neutrons(中子) – Particles with no charge (neutral).
Coulomb‘s(庫侖) Electric Force Law
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The opposite charges react to each other with a force that
causes them to be attracted to each other.
Like charges react to each other with a force that causes
them to repel each other.
The force is inversely proportional to the square of the
separation distance.
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Bohr’s(波爾 ) model
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Protons are positive charges and electrons are negative
charges. There is more than 1 proton in the nucleus.
Loosened electrons that do not move and have a
negative charge are called static electricity (靜電).
If these static electrons have an opportunity to jump
to a conductor, this can lead to electrostatic
discharge (ESD).
ESD, though usually harmless to people, can create
serious problems for sensitive electronic equipment.
A static discharge can randomly damage computer
chips, data, or both.
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Voltage
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Voltage is sometimes referred to as electromotive
force (EMF)(電動勢).
EMF is related to an electrical force, or pressure.
Voltage can also be created in three other ways:
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By friction(摩擦), or static electricity.
By magnetism(磁力), or electric generator.
By light, or solar cell(太陽能蓄電).
Voltage is represented by the letter V, and
sometimes by the letter E.
The unit of measurement for voltage is volt (V).
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Resistance and impedance
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The materials through which current flows
offer varying amounts of opposition, or
resistance to the movement of the electrons.
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Conductors(導體) : The materials that offer very
little, or no, resistance.
Insulators(絕緣體) : Those materials that do not
allow the current to flow, or severely restrict its flow.
The letter R represents resistance.
The unit of measurement for resistance is the
ohm(歐姆).
The symbol comes from the Greek letter,
omega. (Ω)
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Semiconductors are materials where the
amount of electricity they conduct can be
precisely controlled.
The most important semiconductor which
makes the best microscopic-sized electronic
circuits is silicon (Si)(矽).
Silicon is very common and can be found in
sand, glass, and many types of rocks.
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Current
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In electrical circuits, the current is caused by a flow
of free electrons.
When voltage, or electrical pressure, is applied and
there is a path for the current, electrons move from
the negative terminal along the path to the positive
terminal.
The letter “I” represents current.
The unit of measurement for current is Ampere
(Amp or A)(安培).
Amp is defined as the number of charges per
second that pass by a point along a path.
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As an example,
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Static electricity has very high voltage, so much
that it can jump a gap of an inch or more.
However, it has very low amperage and as a
result can create a shock but not permanent injury.
The starter motor in an automobile operates at a
relatively low 12 volts but requires very high
amperage to generate enough energy to turn over
the engine.
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Circuits
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Current flows in closed loops called circuits.
Voltage causes current to flow.
Resistance and impedance oppose it.
An electric appliance(設備) has a plug with
three prongs(分支), one of the three prongs
serves as the ground, or zero volts.
The ground provides a conducting path for
the electrons to flow to the earth.
A water analogy helps to explain concepts of
electricity.
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Ohm’s law
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Two ways in which current flows are alternating
current (AC) and direct current (DC):
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V=I*R
Voltage (V) equals current (I) multiplied by resistance (R).
AC: flows in one direction, then reverses its direction and
flows in the other direction, and then repeats the process.
DC: always flows in the same direction.
Power lines carry electricity in the form of AC
because it can be delivered efficiently over large
distances.
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Cable specifications
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Cables have different specifications and expectations
pertaining to performance:
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What speeds for data transmission can be achieved using a
particular type of cable?
What kind of transmission is being considered? Digital?
Analog?
How far can a signal travel through a particular type of cable
before attenuation of that signal becomes a concern?
Some examples of Ethernet specifications which relate
to cable type include:
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10BASE-T (100m)
10BASE5 (500m)
10BASE2 (200m)
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Coaxial cable
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Coaxial cable consists of a copper conductor
surrounded by a layer of flexible
insulation.(銅包鋼導體)(發泡聚乙烯絕緣)
Over this insulating material is a woven
copper braid or metallic foil that acts as the
second wire in the circuit and as a shield for
the inner conductor. (編織屏蔽)
Covering this shield is the cable jacket.(聚氯
乙烯護套)
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Advantages:
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It can be run longer distances than STP, UTP,
ScTP cable without the need for repeaters.
Repeaters regenerate the signals in a network so
that they can cover greater distances.
Coaxial cable is less expensive than fiber-optic
cable and the technology is well known.
It has been used for many years for many types
of data communication such as cable television.
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Disadvantages:
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Coaxial cable is more difficult to work.
Coaxial cable is more expensive to install than
twisted-pair cable.
A solid electrical connection at both ends is
important to properly ground the cable. Poor
shield connection is one of the biggest sources of
connection problems in the installation of coaxial
cable.
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STP cable
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STP cable combines the techniques of
cancellation, shielded, and twisted wires.
Each pair of wires is wrapped in metallic
foil.(鋁箔屏蔽)
The two pairs of wires are wrapped in an
overall metallic braid or foil.(編織層)
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A new hybrid of UTP is Screened UTP
(ScTP), which is also known as foil screened
twisted pair (FTP).
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UTP cable
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UTP is a four-pair wire medium used in a variety
of networks.
Each of the eight copper wires in the UTP cable
is covered by insulating material.(聚乙烯絕緣)
In addition, each pair of wires is twisted around
each other.
This type of cable relies on the cancellation
effect produced by the twisted wire pairs to limit
signal degradation caused by EMI and RFI.
To further reduce crosstalk between the pairs in
UTP cable, the number of twists in the wire pairs
varies.
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TIA/EIA-568-B contains specifications that govern
cable performance.
Category 5 is the cable most frequently
recommended and implemented in installations.
However, Category 6 cable will supersede Category
5 cable in network installations.
The fact that Category 6 link and channel
requirements are backward compatible to Category
5e makes it very easy for customers to choose
Category 6 and supersede Category 5e in their
networks.
Applications that work over Category 5e will work
over Category 6.
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STP reduces electrical noise within the cable
such as pair to pair coupling and crosstalk.
STP also reduces electronic noise from
outside the cable such as electromagnetic
interference (EMI)(電磁波) and radio
frequency interference (RFI)(無線電波).
However, STP is more expensive and difficult
to install than UTP.
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Advantages
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It is easy to install and is less expensive than
other types of networking media.
Disadvantages
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UTP cable is more prone to electrical noise and
interference than other types of networking media
The distance between signal boosts is shorter for
UTP than it is for coaxial and fiber optic cables.
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Three types of cable connections used
between internetwork devices:
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Straight-through: The cable that connects from
the switch port to the computer NIC port.
Crossover: The cable that connects from one
switch port to another switch port. (pins #1, #2 to
pins #3, #6)
Rollover: The cable that connects the RJ-45
adapter on the COM port of the computer to the
console port of the router or switch. (RJ-45 to DB9)
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Outline
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Copper Media
Optical Media
Wireless Media
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The electromagnetic spectrum
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The light used in optical fiber networks is
one type of electromagnetic(電磁) energy.
This energy in the form of waves can
travel through a vacuum(真空), the air, and
through some materials like glass.
An important property of any energy wave
is the wavelength(波長).
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Radio, microwaves, radar, visible light, xrays, and gamma rays seem to be very
different things. However, they are all
types of electromagnetic energy.
If all the types of electromagnetic waves
are arranged in order from the longest
wavelength down to the shortest
wavelength, a continuum called the
electromagnetic spectrum(電磁波譜) is
created.
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Human eyes were designed to only sense
electromagnetic energy with wavelengths between
700 nanometers and 400 nanometers (nm). (visible
light)
A nanometer is one billionth of a meter
(0.000000001 meter) in length.
The longer wavelengths of light that are around 700
nm are seen as the color red.
The shortest wavelengths that are around 400 nm
appear as the color violet(紫).
This part of the electromagnetic spectrum is seen as
the colors in a rainbow.
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Wavelengths that are not visible to the human eye
are used to transmit data over optical fiber.
These wavelengths are slightly longer than red light
and are called infrared(紅外線) light.
Infrared light is used in TV remote controls.
The wavelength of the light in optical fiber is either
850 nm, 1310 nm, or 1550 nm.
These wavelengths were selected because they
travel through optical fiber better than other
wavelengths.
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Ray model of light
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When electromagnetic waves travel out from a
source, they travel in straight lines.
These straight lines pointing out from the source are
called rays(射線).
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(入射) ray,
crosses the boundary from one material to another,
some of the light energy in the ray will be
reflected(反射) back.
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The light energy in the incident ray that is not
reflected will enter the glass.
The entering ray will be bent at an angle from
its original path. This ray is called the
refracted(折射) ray.
The index of refraction(折射率) is defined as
the speed of light in vacuum divided by the
speed of light in the medium. .
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Reflection
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When a ray of light (the incident ray) strikes the
shiny(發光的) surface of a flat piece of glass, some
of the light energy in the ray is reflected.
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(入射角).
The angle between the reflected ray and the normal
is called the angle of reflection(反射角).
The Law of Reflection(反射率) states that the angle
of reflection of a light ray is equal to the angle of
incidence.
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Refraction
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If the incident ray strikes the glass surface at an
exact 90-degree angle, the ray goes straight into the
glass. The ray is not bent.
However, if the incident ray is not at an exact 90degree angle to the surface, then the transmitted ray
that enters the glass is bent. The bending of the
entering ray is called refraction(折射).
How much the ray is refracted depends on the index
of refraction of the two transparent materials.
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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.
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Total internal reflection
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A light ray that is being turned on and off to send
data (1s and 0s) into an optical fiber must stay
inside the fiber until it reaches the far end.
The ray must not refract into the material wrapped
around the outside of the fiber. The refraction would
cause the loss of part of the light energy of the ray.
The following two conditions must be met for the
light rays in a fiber to be reflected back into the fiber
without any loss due to refraction:
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The core of the optical fiber has to have a larger index of
refraction (n) than the material that surrounds it. The
material that surrounds the core of the fiber is called the
cladding.
The angle of incidence of the light ray is greater than the
critical angle(臨界角) for the core and its cladding.
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When both of these conditions are met, the entire
incident light in the fiber is reflected back inside the
fiber. This is called total internal reflection.(全反射)
A fiber that meets the first condition can be easily
created. In addition, the angle of incidence of the
light rays that enter the core can be controlled.
Restricting the following two factors controls the
angle of incidence:
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The numerical aperture(孔徑) of the fiber – The
numerical aperture of a core is the range of angles of
incident light rays entering the fiber that will be completely
reflected.
Modes – The paths which a light ray can follow when
traveling down a fiber.
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Multimode fiber
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There are a limited number of optical paths that a
light ray can follow through the fiber. These optical
paths are called modes.
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Multimode: If the diameter of the core of the fiber is large
enough so that there are many paths that light can take
through the fiber.
Single-mode: Fiber has a much smaller core that only allows
light rays to travel along one mode inside the fiber.
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The fibers are similar to two one-way streets going
in opposite directions.
This provides a full-duplex communication link.
Fiber-optic circuits use one fiber strand to transmit
and one to receive.
No light escapes when it is inside a fiber, this means
there are no crosstalk issues with fiber. One cable
can contain 2 to 48 or more separate fibers.
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Five parts make up each fiber-optic cable:
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Core: the light transmission element at the center
of the optical fiber.
Cladding: made of silica but with a lower index of
refraction than the core.
Buffer: shield the core and cladding from damage.
Strength material: preventing the fiber cable from
being stretched.
Outer jacket: protect the fiber against abrasion,
solvents, and other contaminants. The color of the
outer jacket of multimode fiber is usually orange.
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A standard multimode fiber-optic cable uses an
optical fiber with either a 62.5 or a 50-micron core
and a 125-micron diameter cladding.
This is commonly designated as 62.5/125 or 50/125
micron optical fiber. A micron is one millionth of a
meter (1µ).
Infrared Light Emitting Diodes (LEDs) or Vertical
Cavity Surface Emitting Lasers (VCSELs) are two
types of light source usually used with multimode
fiber.
Multimode fiber (62.5/125) can carry data distances
of up to 2000 meters (6,560 ft).
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Single-mode fiber
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Single-mode fiber consists of the same parts as
multimode.
The outer jacket of single-mode fiber is usually
yellow.
The single-mode core is eight to ten microns in
diameter. Nine-micron cores are the most common.
An infrared laser is used as the light source in
single-mode fiber.
The ray of light it generates enters the core at a 90degree angle. This greatly increases both the speed
and the distance that data can be transmitted.
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Single-mode fiber can carry LAN data up to 3000
meters.
Warming:
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Never look at the near end of a fiber that is connected to a
device at the far end.
Never look into the transmit port on a NIC, switch, or router.
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Other optical components
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Transmitter/Receiver:
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Convert the electricity to light and at the other end
of the fiber convert the light back to electricity.
Light sources
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A light emitting diode (LED): with wavelengths of either
850 nm or 1310 nm. These are used with multimode
fiber in LANs.
Light amplification by stimulated emission radiation
(LASER): with wavelengths of 1310nm or 1550 nm.
Lasers are used with single-mode fiber over the longer
distances involved in WANs or campus backbones.
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Connectors
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Attached to the fiber ends.
Subscriber Connector (SC): multimode.
Straight Tip (ST): single-mode.
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Fiber patch panels: increase the flexibility of
an optical network by allowing quick changes
to the connection of devices.
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Signals and noise in optical
fibers
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The factors of energy loss in fibers
 Scattering(分散) : The scattering of light in a fiber is
caused by microscopic non-uniformity (distortions變形)
in the fiber that reflects and scatters some of the light
energy.
 Absorption(吸收) : Some types of chemical
impurities(不純) in a fiber, the impurities absorb part of
the energy. This light energy is converted to a small
amount of heat energy. Absorption makes the light
signal a little dimmer(為暗).
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Irregularities(不規則) or roughness(粗糙) in the coreto-cladding boundary. Power is lost from the light
signal because of the less than perfect total internal
reflection in that rough area of the fiber.
Dispersion (離散) is the technical term for the
spreading of pulses of light as they travel down the
fiber.
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Installation, care, and testing
of optical fiber
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A major cause of too much attenuation in
fiber-optic cable is improper installation.
If the fiber is stretched or curved too tightly, it
can cause tiny cracks in the core that will
scatter the light rays.
Bending the fiber in too tight a curve can
change the incident angle (critical angle ) of
light rays striking the core-to-cladding
boundary.
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Improperly installed connectors, improper
splices(接合), or the splicing of two cables
with different core sizes will dramatically
reduce the strength of a light signal.
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The connectors and the ends of the fibers
must be kept spotlessly clean.
The ends of the fibers should be covered with
protective covers to prevent damage to the
fiber ends.
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Outline
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Copper Media
Optical Media
Wireless Media
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Wireless LAN organizations and
standards
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Organization:
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FCC (Federal Communications Commission) :
created within the framework of the regulations(規
章).
IEEE : prime issuer of standards for wireless
networks.
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802.11
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Standard is Direct Sequence Spread Spectrum
(DSSS)(直接序列展頻技術 ).
Operating within a 1 to 2 Mbps range.
Up to 11 Mbps but will not be considered
compliant above 2 Mbps.
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802.11b
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802.11b may also be called Wi-Fi™.
Using a different coding technique from 802.11.
Operate at 1, 2, 5.5 and 11 Mbps.
Interoperate with DSSS WLANs for 1 and 2 Mbps
data rates.
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802.11a
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Operating in the 5 GHZ transmission band.
Disallows interoperability of 802.11b devices as
they operate within 2.4 GHZ.
Supplying data throughput of 54 Mbps.
108 Mbps with proprietary technology known as
"rate doubling“.
In production networks, a more standard rating is
20-26 Mbps.
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802.11g
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54 Mbps data throughput.
Backwards compatibility with 802.11b devices.
Orthogonal Frequency Division Multiplexing
(OFDM:正交分頻多工) modulation technology.
Cisco has developed an access point that
permits 802.11b and 802.11a devices to
coexist on the same WLAN.
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Wireless devices and topologies
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The most of WLAN nodes are desktop
workstations or notebook computers which
equipped with wireless NICs.
Ad hoc mode
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Both devices act as servers and clients in this
peer-to-peer environment.
Provide connectivity.
NICs from different manufacturers are not
compatible.
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Infrastructure mode
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An access point (AP) is commonly installed to act
as a central hub for the WLAN.
The AP is hard wired to the cabled LAN to provide
wireless internet access.
APs are equipped with antennae and provide
wireless connectivity over a specified area
referred to as a cell.
Most commonly, the wireless service range of the
antennae will be from 91.44 to 152.4 meters (300
to 500 feet).
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To service larger areas, multiple access points
may be installed with a degree of overlap.
The overlap permits "roaming" between cells. (ex:
cellular phone)
Overlap not addressed in the IEEE standards, a
20-30% overlap is desirable.
This rate of overlap will permit roaming between
cells, allowing for the disconnect and reconnect
activity to occur seamlessly without service
interruption.
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When a client is activated within the WLAN, it will start
"listening" for a compatible device with which to
"associate". This is referred to as "scanning" and may
be active or passive.
Active scanning
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It causes a probe request to be sent from the wireless
node seeking to join the network.
The probe request will contain the Service Set Identifier
(SSID) of the network it wishes to join.
When an AP with the same SSID is found, the AP will
issue a probe response. The authentication and
association steps are completed.
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Passive scanning
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Nodes listen for beacon(浮標) management frames
(beacons), which are transmitted by the AP
(infrastructure mode) or peer nodes (ad hoc).
When a node receives a beacon that contains the SSID
of the network it is trying to join, an attempt is made to
join the network.
Passive scanning is a continuous process and nodes
may associate or disassociate with APs as signal
strength changes.
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How wireless LANs communicate
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WLANs do not use a standard 802.3 frame.
Therefore, using the term wireless Ethernet is misleading.
There are three types of frames:
 control
 management
 data
Only the data frame type is similar to 802.3 frames.
The payload of wireless and 802.3 frames is 1500 bytes;
however, an Ethernet frame may not exceed 1518 bytes whereas
a wireless frame could be as large as 2346 bytes.
Usually the WLAN frame size will be limited to 1518 bytes as it is
most commonly connected to a wired Ethernet network.
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Since radio frequency (RF) is a shared medium,
collisions can occur just as they do on wired shared
medium.
The major difference is that there is no method by which
the source node is able to detect that a collision occurred.
For that reason WLANs use Carrier Sense Multiple
Access/Collision Avoidance (CSMA/CA).
When a source node sends a frame, the receiving node
returns a positive acknowledgment (ACK). This can
cause consumption of 50% of the available bandwidth.
The transmitting unit will drop the data rate from 11 Mbps
to 5.5 Mbps, from 5.5 Mbps to 2 Mbps or 2 Mbps to 1
Mbps.
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Authentication and association
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WLAN authentication occurs at Layer 2.
It is the process of authenticating the device not the
user.
The client will send an authentication request frame
to the AP and the frame will be accepted or rejected
by the AP.
The client is notified of the response via an
authentication response frame.
The AP may also be configured to hand off the
authentication task to an authentication server,
which would perform a more thorough credentialing
process.
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

Association, performed after authentication, is the
state that permits a client to use the services of the
AP to transfer data.
Authentication and Association types



Unauthenticated and unassociated
 The node is disconnected from the network and not
associated to an access point.
Authenticated and unassociated
 The node has been authenticated on the network but has
not yet associated with the access point.
Authenticated and associated
 The node is connected to the network and able to transmit
and receive data through the access point.
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
Methods of authentication


SSID is the first authentication process is the
open system.
Wireless Equivalency Protocol (WEP)
encryption.



WEP is a fairly simple algorithm using 64 and 128
bit keys.
The AP and nodes are configured with an encrypted
matching key.
Statically assigned WEP keys provide a higher level
of security.
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The radio wave and microwave
spectrums





Changing electric currents in the antenna of a
transmitter generates the radio waves.
In a WLAN, a radio signal measured at a distance of
just 10 meters (30 feet) from the transmitting antenna
would be only 1/100th of its original strength.
Like light, radio waves can be absorbed by some
materials and reflected by others.
When passing from one material, like air, into another
material, like a plaster wall, radio waves are refracted.
Radio waves are also scattered and absorbed by
water droplets in the air.
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


The process of evaluating a location for the
installation of a WLAN is called making a Site
Survey.
When radio waves hit the antenna of a receiver,
weak electric currents are generated in that antenna.
In a transmitter, the electrical (data) signals from a
computer or a LAN are not sent directly into the
antenna of the transmitter. Rather, these data signals
are used to alter a second, strong signal called the
carrier signal.
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

The process of altering the carrier signal that will
enter the antenna of the transmitter is called
modulation.
Three basic modulation ways:



Amplitude Modulated (AM) radio stations modulate the
height (amplitude) of the carrier signal.
Frequency Modulated (FM) radio stations modulate the
frequency of the carrier signal as determined by the
electrical signal from the microphone.
Phase modulation (PM) is used to superimpose the data
signal onto the carrier signal that is broadcast by the
transmitter.
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Signals and noise on a WLANs




All band interference affects the entire spectrum
range.
Bluetooth™ technologies hops across the entire 2.4
GHz many times per second and can cause
significant interference on an 802.11b network.
In homes and offices, a device that is often
overlooked as causing interference is the standard
microwave oven.
Fog or very high moisture conditions can and do
affect wireless networks.
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Wireless security

Security solutions:


Virtual Private Networking (VPN)
Extensible Authentication Protocol (EAP): the
access point does not provide authentication to
the client, but passes the duties to a more
sophisticated device.
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
EAP-MD5 Challenge –



The earliest authentication type.
Similar to CHAP password protection on a wired
network.
LEAP (Cisco) –


Lightweight Extensible Authentication Protocol is
the type primarily used on Cisco.
Provides security during credential(憑據)
exchange, encrypts using dynamic WEP keys,
and supports mutual authentication.
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
User authentication –


Encryption –


Allows only authorized users to connect, send
and receive data over the wireless network.
Provides encryption services further protecting the
data from intruders.
Data authentication –

Ensures the integrity of the data, authenticating
source and destination devices.
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Good luck in your exams !
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