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Semester 1 Module 3

Networking Media

Andres, Wen-Yuan Liao

Department of Computer Science and Engineering

De Lin Institute of Technologyandres@dlit.edu.tw

http://www.cse.dlit.edu.tw/~andres

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Overview

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

Copper Media Optical Media Wireless Media

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Atoms and electrons The atom( 原子 ) is comprised of:

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 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 th

em to repel each other. The force is inversely proportional to the square of the sep

aration distance.

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Bohr’s( 波爾 ) model Protons are positive charges and electrons are negative ch

arges. There is more than 1 proton in the nucleus. Loosened electrons that do not move and have a ne

gative 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

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:

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 The materials through which current flows off

er varying amounts of opposition, or resistance to the movement of the electrons. Conductors( 導體 ) : The materials that offer very lit

tle, or no, resistance. Insulators( 絕緣體 ) : Those materials that do not al

low 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, ome

ga. (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

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 (Am

p or A)( 安培 ). Amp is defined as the number of charges per secon

d that pass by a point along a path.

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As an example, 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

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 t

hree 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 V=I*R Voltage (V) equals current (I) multiplied by resistance (R).

Two ways in which current flows are alternating current (AC) and direct current (DC): 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

Cables have different specifications and expectations pertaining to performance: 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: 10BASE-T (100m) 10BASE5 (500m) 10BASE2 (200m)

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Coaxial cable

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: It can be run longer distances than STP, UTP, Sc

TP 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 ca

ble and the technology is well known. It has been used for many years for many types o

f data communication such as cable television.

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Disadvantages: 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

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

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 It is easy to install and is less expensive than

other types of networking media. Disadvantages

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: Straight-through: The cable that connects from th

e switch port to the computer NIC port. Crossover: The cable that connects from one swit

ch 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

Copper Media Optical Media Wireless Media

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The electromagnetic spectrum

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, x-rays, 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 8

50 nm, 1310 nm, or 1550 nm. These wavelengths were selected because they trav

el through optical fiber better than other wavelengths.

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Ray model of light When electromagnetic waves travel out from a sour

ce, 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 continuo

usly 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 When a ray of light (the incident ray) strikes the shin

y( 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

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 90-degree 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 A light ray that is being turned on and off to send dat

a (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: The core of the optical fiber has to have a larger index of re

fraction (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: The numerical aperture(孔徑 ) of the fiber – The numeric

al 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 There are a limited number of optical paths that a

light ray can follow through the fiber. These optical paths are called modes. 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: 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

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 90-degree 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: 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

Transmitter/Receiver: Convert the electricity to light and at the other end

of the fiber convert the light back to electricity. Light sources

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 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

The factors of energy loss in fibers Scattering( 分散 ) : The scattering of light in a fiber is c

aused 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 core-to-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

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

Copper Media Optical Media Wireless Media

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Wireless LAN organizations and standards

Organization: FCC (Federal Communications Commission) : cre

ated within the framework of the regulations( 規章 ).

IEEE : prime issuer of standards for wireless networks.

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802.11 Standard is Direct Sequence Spread Spectrum (D

SSS)( 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 complia

nt above 2 Mbps.

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802.11b 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 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 54 Mbps data throughput. Backwards compatibility with 802.11b devices. Orthogonal Frequency Division Multiplexing (OFD

M: 正交分頻多工 ) modulation technology. Cisco has developed an access point that per

mits 802.11b and 802.11a devices to coexist on the same WLAN.

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Wireless devices and topologies

The most of WLAN nodes are desktop workstations or notebook computers which equipped with wireless NICs.

Ad hoc mode Both devices act as servers and clients in this pee

r-to-peer environment. Provide connectivity. NICs from different manufacturers are not compati

ble.

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Infrastructure mode 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 wire

less 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 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 Nodes listen for beacon( 浮標 ) management frames (b

eacons), 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 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; howeve

r, 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 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 n

etwork. 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 – Allows only authorized users to connect, send

and receive data over the wireless network. Encryption –

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 !