OSI Physical Layer Supplement

8/3/2019 OSI Physical Layer Supplement

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2007 Cisco Systems, Inc. All rights reserved. Cisco PublicITE PC v4.0Chapter 1 1

OSI Physical Layer

Supplement

Network Fundamentals Chapter 8


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2.3.7 Detailed Encapsulation Process

All

People

Seem

To

Need

Data

Processing


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Names for PDUs at Each Layer

Drippy

Sweet

Pancakes

For

Breakfast


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4.2.1 Signaling over Copper and Fiber

On copper cable, data signals are represented by voltage levels that represent

binary ones and zeros. The voltage levels are measured based on a reference

level of 0 volts at both the transmitter and the receiver. This reference level is

called the signal ground.

It is important for devices that transmit and receive data to have the same 0-volt

reference point. When they do, they are said to be properly grounded.

For a LAN to operate properly, the devices that receive data must be able to

accurately interpret the binary ones and zeros transmitted as voltage levels.

Since current Ethernet technology supports data rates of billions of bps, each bit

must be recognized and the duration of each bit is very small. This means that

as much of the original signal strength as possible must be retained as the signal

moves through the cable and passes through the connectors.

In anticipation of faster Ethernet protocols, new cable installations should be

made with the best cable, connectors, and interconnect devices such as punch-

down blocks and patch panels.


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Coaxial Cable

Coaxial cable is a type of shielded cable. It consists of a solid copper conductor

surrounded by insulating material and a braided conductive shield.

In LAN applications, the braided shielding is electrically grounded to protect the

inner conductor from external electrical noise. The shield also keeps the

transmitted signal confined to the cable, which reduces signal loss.

This helps make coaxial cable less noisy than other types of copper cabling, butalso makes it more expensive. The need to ground the shielding and the bulky

size of coaxial cable make it more difficult to install than other copper cabling.


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Shielded Twisted-Pair

STP cable contains an outer conductive shield that is electrically grounded to

insulate the signals from external electrical noise. STP also uses inner foil

shields to protect each wire pair from noise generated by the other pairs.


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Twisted-Pair

UTP contains no shielding and is more susceptible to external noise but is

the most frequently used because it is inexpensive and easier to install.


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Optical Fiber

Fiber-optic cable represents binary ones and zeros in two ways;

increases and decreases in the intensity of light, or light and no light.

The strength of a light signal does not diminish as much as the strength

of an electrical signal does over an identical run length. Optical signals

are not affected by electrical noise and optical fiber does not need to be

grounded unless the jacket contains a metal strength member.

Therefore, optical fiber

is often used between

buildings and between

floors within a building.

As costs decrease and

speeds increase, optical

fiber may become a

more commonly used

LAN media.


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2.3.3 OSI Model

The Open System Interconnection (OSI)reference model released in 1984 was the

descriptive network model that the ISO

created. It provided vendors with a set of

standards that ensured greater compatibility

and interoperability among various network

technologies produced by companies aroundthe world.

The OSI reference model has become the

primary model for network communications.

Although there are other models in existence,

most network vendors relate their products to

the OSI reference model. This is especiallytrue when they want to educate users on the

use of their products.

It is considered the best tool available for

teaching people about sending and receiving

data on a network.

All

People

Seem

To

Need

Data

Processing


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2.3.4 OSI Layers


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5.1.1 LAN and Physical Layer

Note here that Ethernet is a

family of technologies, which

have some differences in both

the physical and data link

layers, including media.

Token Ring and FDDI arementioned only as LAN

comparisons for Ethernet.

Any single network can be built

with a combination of many

different media types.

When designing a network, the

choice of media types should be

based on the following factors:Required length of cable runs

Cost of material & labor

Ease of installation

Susceptibility to interference


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LAN Physical Layer Implementation


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IEEE 802.3x

802.3 Ethernet (CSMA/CD) Standards for Media

Access Control (MAC), 10 BASE-5

802.3a 10 BASE-2

802.3ab 1000 BASE-T (UTP)

802.3ad Link Aggregation

802.3ae 10Gb Ethernet

802.3i 10 BASE-T

802.3u 100 BASE-TX/FX

802.3z 1000 BASE-X (fiber)

http://standards.ieee.org/getieee802/
http://standards.ieee.org/getieee802/http://standards.ieee.org/getieee802/


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5.1.2 Ethernet in the Campus


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Main Points

10BASE-T EthernetEnd user levelDevice to deviceLow to medium volume applications

Fast EthernetHigh performance connections for workstations100Mbps between workstations and to serversConnects workgroups to backboneConnects servers to backbone

Gigabit EthernetHigh performance1000Mbpsbackbone


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The Different Levels

End-User Level

End-User Level

Workgroup Level

Backbone Level


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5.1.3 Media & Connector Requirements

Unshielded Twisted Pair (UTP) is the most common cabling used in LANs.


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Is there a problem here?


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6.1.2 IEEE Ethernet Naming Rules

When Ethernet needs to be expanded to add a new medium or capability, the IEEEissues a new supplement to the 802.3 standard. The new supplements are given a one

or two letter designation such as 802.3af. An abbreviated description, called an identifier

(refer to graphic) , is also assigned to the supplement.

The abbreviated description consists of the following elements:

- A number that indicates the number of Mbps transmitted

- The word base to indicate that baseband signaling is used

- One or more letters of the alphabet indicating the type of medium used.


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Baseband versus Broadband

basebandCharacteristic of a network technology where only one carrier frequency is used.

Ethernet is an example of a baseband network.

Also called narrowband.

broadbandTransmission system that multiplexes multiple independent signals onto one cable.

In telecommunications terminology, any channel having a bandwidth greater

than a voice-grade channel (4 kHz).

In LAN terminology, a coaxial cable on which analog signaling is used.Also called wideband.


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IEEE 802 Media Activity Solved


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4.1.1 Waves

A wave is energy that travels from one place to another. A wavelength is the

distance in the line of advance of a wave from any one point to the next point of

corresponding phase.

It is helpful to think of waves as disturbances. The ocean always has some sort of

detectable waves due to disturbances such as wind and tide.


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Describing Waves

Ocean waves can be described in terms of their height, or amplitude, which could

be measured in meters. They can also be described in terms of how frequently the

waves reach the shore, which relates to period and frequency.

The period of the waves is

the amount of time between

each wave, measured inseconds.

The frequency is the number

of waves that reach the

shore each second.


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Frequency, Amplitude, and Period

Networking professionals are specifically interested in voltage waves on copper

media, light waves in optical fiber, and alternating electric and magnetic fields

called electromagnetic waves.

Frequency: Frequency (F) is the numberof complete cycles per second. This is

measured in Hertz (Hz).

Amplitude: The amplitude (A) of anelectrical signal represents the height of

the wave, and it is measured in volts (V).

Period: The period (T) is the amount oftime that it takes to complete 1 cycle.

This is measured in seconds.


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Pulse

A pulse is a deliberately caused disturbance of a fixed,

predictable duration.

Pulses are an important part of electrical signals because

they are the basis of digital transmission. The pattern of the

pulses represents the value of the data being transmitted.


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4.1.2 Sine Waves and Square Waves

Sine waves are periodic, which means that theyrepeat the same pattern at regular intervals.

Sine waves vary continuously,

which means that no twoadjacent points on the graphhave the same value.

Sine waves are graphicalrepresentations of manynatural occurrences that

change regularly over time.

Sine waves are examples ofanalog waves, since they varycontinuously.


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Sine Waves and Square Waves

Square waves, like sine waves, are periodic, which meansthat they repeat the same pattern at regular intervals.

Square waves do notcontinuously vary with time;they maintain one value andthen suddenly change to adifferent value.

Square waves representdigital signals, or pulses.


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6.1.5 Layer 2 Framing

Framing provides essential information that could not be obtained from coded bit streamsalone. This information includes the following:

Which computers are in

communication with each other

When communication between

individual computers begins and

when it ends

Which errors occurred while the

computers communicated

Which computer will communicate

next

Framing is the Layer 2

encapsulation process.


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6.2.3 Ethernet Timing

Any station on an Ethernet network wishing to transmit a message first

listens to ensure that no other station is currently transmitting. If the cable is

quiet, the station will begin transmitting immediately.

The electrical signal takes time to travel down the cable (delay), and each

subsequent repeater introduces a small amount of latency in forwarding the

frame from one port to the next. Because of the delay and latency, it ispossible for more than one station to begin transmitting at or near the same

time. This results in a collision.

If the attached station is operating in full duplex then the station may send

and receive simultaneously and collisions should not occur. Full-duplex

operation also changes the timing considerations and eliminates the concept

of slot time.Full-duplex operation allows for larger network architecture designs since the

timing restriction for collision detection is removed.


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Waiting for a collision fragment

The actual calculated slot time is just longer than the theoretical amount of

time required to travel between the furthest points of the collision domain,

collide with another transmission at the last possible instant, and then have

the collision fragments return to the sending station and be detected. For the

system to work the first station must learn about the collision before it finishes

sending the smallest legal frame size (hence the 5-4-3 rule).

To allow 1000-Mbps Ethernet to operate in half duplex the extension field was

added when sending small frames purely to keep the transmitter busy long

enough for a collision fragment to return.

This field is present only on 1000-Mbps, half-duplex links and allows

minimum-sized frames to be long enough to meet slot time requirements.

Extension bits are discarded by the receiving station.


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Frame size versus length of cable

For CSMA/CD Ethernet to operate, the sending station must become aware

of a collision before it has completed transmission of a minimum-sized frame.

At 100 Mbps the system timing is barely able to accommodate 100 meter

cables.

At 1000 Mbps special adjustments are required as nearly an entire minimum-

sized frame would be transmitted before the first bit reached the end of thefirst 100 meters of UTP cable.

For this reason half duplex is not permitted in 10-Gigabit Ethernet.


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4.1.6 Analog and Digital Signals

Most modern telecommunications consists of modulating either amplitude, frequency,

or phase. Digital square waves that comprise the networking signals can be thought

of as a carefully constructed sum of sine waves.

Therefore, cable testing can

use sine waves at different

frequencies measured in

hertz, which is an analog

approach, to determine the

maximum data transfer

supported on a cable as

measured in bps, kbps,Mbps, Gbps, a digital

approach.


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2.2.1 Importance of Bandwidth

Bandwidth is defined as the amount of information that can flow through a

network connection in a given period of time. (2.2.1)

Bandwidth is the measure of how many bits of information can flow from one

place to another in a given amount of time. (2.2.3)

It is important to understand the concept of bandwidth for the following reasons.


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2.2.2 Bandwidth Pipe Analogy


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Bandwidth Highway Analogy


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2.2.3 Bandwidth Measurements

Kilo = thousand

Mega = million

Giga = billion

Tera = trillion

Although the terms bandwidth and speed are often used interchangeably, they are not

exactly the same thing. One may say, for example, that a T3 connection at 45 Mbps

operates at a higher speed than a T1 connection at 1.544 Mbps.

However, if only a small amount of their data-carrying capacity is being used, each ofthese connection types will carry data at roughly the same speed.

Therefore, it is usually more accurate to say that a T3 connection has greater bandwidth

than a T1 connection. This is because the T3 connection is able to carry more

information in the same period of time, not because it has a higher speed.


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2.2.3 Bandwidth Measurements

Kilo = thousand

Mega = million

Giga = billion

Tera = trillion

Although the terms bandwidth and speed are often used interchangeably, they are not

exactly the same thing. One may say, for example, that a T3 connection at 45 Mbps

operates at a higher speed than a T1 connection at 1.544 Mbps.

However, if only a small amount of their data-carrying capacity is being used, each ofthese connection types will carry data at roughly the same speed.

Therefore, it is usually more accurate to say that a T3 connection has greater bandwidth

than a T1 connection. This is because the T3 connection is able to carry more

information in the same period of time, not because it has a higher speed.


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2.2.4 Bandwidth Limitations


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2.2.5 Bandwidth Throughput

Throughput refers to actual measured bandwidth, at a specific time of day, using

specific Internet routes, and while a specific set of data is transmitted on the network.

Unfortunately, for many reasons, throughput is often far less than the maximum

possible digital bandwidth of the medium that is being used.

The following are some of the factors that determine throughput:

Internetworking devices

Type of data being transferred

Network topology

Number of users on the network

User computer

Server computer

Power conditions


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3.1.9 Unshielded Twisted Pair Cable (UTP)


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UTP Characteristics

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. 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 crosstalkbetween the pairs in UTP

cable, the number of twists

in the wire pairs varies.

Like STP cable, UTP cable

must follow precise

specifications as to howmany twists or braids are

permitted per foot of cable.


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UTP Pros

Pros:

It is easier to install than coaxial.

It is less expensive than other types of networking media.

Since it has such a small external diameter, UTP does notfill up wiring ducts as rapidly as other types of cable.

When UTP cable is installed with an RJ-45 connector,potential sources of network noise are greatly reduced anda good solid connection is almost guaranteed.


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UTP Cons

Cons:

UTP cable is more prone to electrical noise and interferencethan other types of networking media

The distance between signal boosts is shorter for UTP thanit is for coaxial and fiber optic cables.


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T568A and T568B Wiring Standards


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Straight-through Cable


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Use straight-through cables for the following connections:

Hub to a router

Switch to router

Hub to PC or server

Switch to PC or server


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Crossover Cable


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Crossover Cable


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Crossover Cable

Use crossover cables for the following connections:

Switch to switch

Switch to hub

Hub to hubRouter to router

PC to PC

Router to PC


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Exceptions for Hub-to-Hub


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Rollover Cable


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Rollover Cable


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Rollover Cable


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5.1.5 UTP Implementation

RJ-45 Connector

The letters RJ stand for

registered jack and the

number 45 refers to a

specific wiring sequence.


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RJ-45 Jack


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EIA/TIA-568-B.1 Color Code

For electricity to run between the connector and the jack, the order of the wires

must follow T568A or T568B color code found in the EIA/TIA-568-B.1 standard.


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Tip and Ring

Four of the wires, T1 through T4, carrythe voltage and are called tip.

The other four wires, R1 through R4, are

grounded and are called ring.

Tip and ring are terms that originated in

the early days of the telephone.Today, these terms refer to the positive

and the negative wire in a pair.


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Which is which?


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Hub Cabling Exceptions


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When to use which cable?

Use straight-throughcables for the followingconnections:Switch to routerSwitch to PC or serverHub to PC or server

Use crossover cables for the following connections:Switch to switch Switch to hub Hub to hubRouter to router Router to PC PC to PC


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3.1.6 Cable Specifications

The principle idea here is that the different types of media and their terminations aregoverned by standards. These standards are specified within the dominant LANtechnology, Ethernet. Therefore, we must understand the different specifications andexpectations of cables.

Important considerations related to cable performance are as follows:

Speed of transmission: What speeds for data transmission can beachieved? The speed of bit transmission through the cable is extremelyimportant. The speed of transmission is affected by the kind of conduit used.

Digital or analog: Digital or baseband transmission and analog or broadbandtransmission requires different types of cable.

Distance of cable run: How far can a signal travel before attenuationbecomes a concern? If the signal is degraded, network devices might not beable to receive and interpret the signal. The distance the signal travelsthrough the cable affects attenuation of the signal. Degradation is directlyrelated to the distance the signal travels and the type of cable used.


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Cable Specifications

Speed of Transmission Type of Transmission Type of Cable & Max Length


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Cable Specifications

10BASE2

The speed of transmission at 10 Mbps

The type of transmission is baseband,or digitally interpreted

The 2 indicates that a signal can travel

for approximately 185 meters beforeattenuation could disrupt the ability ofthe receiver to interpret the signal

10BASE-T

The speed of transmission is 10 MbpsThe type of transmission is baseband,or digitally interpreted

The T stands for twisted pair

10BASE5

The speed of transmission at 10 Mbps

The type of transmission is baseband, ordigitally interpreted

The 5 indicates that a signal can travel for

approximately 500 meters beforeattenuation could disrupt the ability of thereceiver to interpret the signal

100BASE-T

The speed of transmission is 100 MbpsThe type of transmission is baseband, ordigitally interpreted

The T stands for twisted pair


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So which cable would you use?

4 2 1 Si li C d Fib


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4.2.1 Signaling over Copper and Fiber

On copper cable, data signals are represented by voltage levels that represent

binary ones and zeros. The voltage levels are measured based on a reference

level of 0 volts at both the transmitter and the receiver. This reference level is

called the signal ground.

It is important for devices that transmit and receive data to have the same 0-volt

reference point. When they do, they are said to be properly grounded.For a LAN to operate properly, the devices that receive data must be able to

accurately interpret the binary ones and zeros transmitted as voltage levels.

Since current Ethernet technology supports data rates of billions of bps, each bit

must be recognized and the duration of each bit is very small. This means that

as much of the original signal strength as possible must be retained as the signal

moves through the cable and passes through the connectors.In anticipation of faster Ethernet protocols, new cable installations should be

made with the best cable, connectors, and interconnect devices such as punch-

down blocks and patch panels.


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Coaxial Cable

Coaxial cable is a type of shielded cable. It consists of a solid copper conductorsurrounded by insulating material and a braided conductive shield.

In LAN applications, the braided shielding is electrically grounded to protect the

inner conductor from external electrical noise. The shield also keeps the

transmitted signal confined to the cable, which reduces signal loss.

This helps make coaxial cable less noisy than other types of copper cabling, but

also makes it more expensive. The need to ground the shielding and the bulky

size of coaxial cable make it more difficult to install than other copper cabling.


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Shielded Twisted-Pair

STP cable contains an outer conductive shield that is electrically grounded to

insulate the signals from external electrical noise. STP also uses inner foil

shields to protect each wire pair from noise generated by the other pairs.


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Twisted-Pair

UTP contains no shielding and is more susceptible to external noise but is

the most frequently used because it is inexpensive and easier to install.

O i l Fib


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Optical Fiber

Fiber-optic cable represents binary ones and zeros in two ways;increases and decreases in the intensity of light, or light and no light.

The strength of a light signal does not diminish as much as the strength

of an electrical signal does over an identical run length. Optical signals

are not affected by electrical noise and optical fiber does not need to be

grounded unless the jacket contains a metal strength member.

Therefore, optical fiber

is often used between

buildings and between

floors within a building.

As costs decrease andspeeds increase, optical

fiber may become a

more commonly used

LAN media.

4 2 2 I i L A i I d


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4.2.2 Insertion Loss = Attenuation + Impedance

A i


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Attenuation

Attenuation is the loss of signal strength as it is transmitted from the end of the cablewhich the signal is generated to the opposite end at which it is received.

Attenuation, also referred to as Insertion Loss, is measured in decibels (dB). For

attenuation, the lower the dB value, the better the performance, less signal is lost. This

decrease in performance is typically caused by absorption, reflection, diffusion,

scattering, deflection, or dispersion from the original signal and usually not as a result

of geometric spreading.

Attenuation is

measured by a

cable tester with

the highest

frequencies that

the cable is ratedto support.

Att ti


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Attenuation

Contributing factors to attenuation on network media include:

Long cable lengths lead to signal deterioration overthe length of a link, caused by the resistance toheat presented by the properties of the media.

If you have an improperly installed connector, it will

have a different impedance value than the cable.This is called an impedance mismatch.

Signal energy is also lost when it leaks through theinsulation of the cable.

I d


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Impedance

Impedance is a measurement of

the resistance of the cable to

alternating current (AC) and is

measured in ohms. The normal

impedance of a Category 5 cableis 100 ohms.

If a connector is improperly

installed on Category 5, it will

have a different impedance value

than the cable. This is called an

impedance discontinuity or animpedance mismatch.

I d Mi t h


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Impedance Mismatch

Impedance mismatches cause attenuation because a portion of atransmitted signal is reflected back, like an echo, and does not reach thereceiver. This effect is compounded if multiple mismatches cause

additional portions of the signal to be reflected back to the transmitter.

When the reflected signal strikes the first mismatch, some of the signalrebounds in the original direction, which creates multiple echo effects.

The echoes strike the receiver at different intervals.

This makes it difficult for the receiver to detect data values.

This is called jitter and results in data errors.

I d


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Impedance

4 2 3 Noise on Copper Media


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4.2.3 Noise on Copper Media

Noise is any electrical energy on the transmission cable that makes it difficult for areceiver to interpret the data sent from the transmitter. We have already discussed

RFI and EMI noise, as well as laser noise. Our focus now will be on crosstalk.

Crosstalk involves the transmission of signals from one wire to a nearby wire. When

voltages change on a wire, electromagnetic energy is generated. This energy

radiates outward from the wire like a radio signal from a transmitter. Adjacent wires

in the cable act like antennas and receive the transmitted energy, which interfereswith data on those wires.

Measuring Crosstalk


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Measuring Crosstalk

Crosstalk can also be caused by signals on separate, nearby cables. When crosstalk is

caused by a signal on another cable, it is called alien crosstalk. In networks with higher

transmission frequencies, there is an increase in crosstalk, resulting in the destruction of

more of the data signal.

Cable testing instruments measure crosstalk by applying a test signal to one wire pair.

The cable tester then measures the amplitude of the unwanted crosstalk signals on the

other wire pairs in the cable.

Agilent Network Analyzer$59,000

Minimizing Noise on Twisted Pair


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Minimizing Noise on Twisted-Pair

Twisted-pair cable is designed to take

advantage of the effects of crosstalk

in order to minimize noise.

In twisted-pair cable, a pair of wires is

used to transmit one signal. The wire

pair is twisted so that each wire

experiences similar crosstalk.

Because a noise signal on one wire

will appear identically on the other

wire, this noise be easily detected andfiltered at the receiver.

Making a Good Connection


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Making a Good Connection

Twisted wire pairs in a cable are also more resistant to crosstalk or noise signals from

adjacent wire pairs. Higher categories of UTP require more twists on each wire pair in

the cable to minimize crosstalk at high transmission frequencies.

When connectors are attached to the ends of UTP cable, the wire pairs should be

untwisted as little as possible to ensure reliable LAN communications.

4 2 4 Types of Crosstalk


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4.2.4 Types of Crosstalk

This section defines the three typesof crosstalk:

Near-end Crosstalk (NEXT)

Far-end Crosstalk (FEXT)

Power Sum NEXT(PSNEXT)

For Example


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For Example,

An example of crosstalk on voice

channels is when extraneous

conversations can be heard in the

background over the phone line

while on a telephone conversation.Those signals are being induced

onto the voice channel from another

channel.

The same instance occurs in data

signal transmission. If the crosstalk

is great enough, it will interfere withsignals received across the circuit.

Crosstalk Test Parameters


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Crosstalk Test Parameters

Low decibel values ofattenuation aredesirable because then less of the signal is

lost on its way to the receiver. Higherdecibel values ofcrosstalk (NEXT, ELFEXT,etc.) and return loss are actually desirablebecause that means less signal has beenmeasured on adjacent wires.The way the testing is done, you measure

how much signal energy did not transfer tothe other pair. A pair (or pairs, in the case of

power-sum measurements) is energized with

a signal. This is the disturber. You listen

on another pair called the disturbed pair.Subtracting what you inserted on the

disturber from what measure on the

disturbed tells you how much signal stayed

with the disturber.

Near-End Crosstalk (NEXT)


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Near-End Crosstalk (NEXT) is computed as the

ratio of voltage amplitude between the test signal

and the crosstalk signal when measured from the

same end of the link.

In other words, Near-End Crosstalk (NEXT) measures the amount of signal

coupled from one pair to another within the cable caused by radiation

emission at the transmitting end (near end) of the cable.



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NEXT needs to be measured from each pair

to each other pair in a UTP link, and from

both ends of the link. To verify proper link

performance, NEXT should be measured

from both ends of the link with a high-quality

test instrument.Low negative numbers indicate more noise.

By tradition, cable testers do not show the

minus sign indicating the negative NEXT

values.

A NEXT reading of 30 dB (which actually

indicates -30 dB) indicates less NEXT noiseand a cleaner signal than does a NEXT

reading of 10 dB.

Far-End Crosstalk (FEXT)


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Far-End Crosstalk (FEXT)

FEXT is similar to NEXT except that it is detected at the

opposite end of the cable from where the signal was

sent. Due to attenuation, the signals at the far end of

the transmitting wire pair are much weaker than the

signals at the near end.

The noise caused by FEXT still travels back to the source, but it is attenuated as it

returns. Thus, FEXT is not as significant a problem as NEXT. However, more

FEXT will be seen on a shorter cable than a longer one because the signal at the

receiving side will have less distance over which to attenuate.

Pair-to-Pair Crosstalk


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Pair-to-Pair Crosstalk

The following six combinations

are tested in a four-pair cable:

Pair 1 to pair 2

Pair 1 to pair 3

Pair 1 to pair 4

Pair 2 to pair 3

Pair 2 to pair 4

Pair 3 to pair 4

For both NEXT and FEXT, one way of measuring crosstalk is the pair-to-pairmethod.In pair-to-pair measurement, one pair, the disturber, is energized with a signal, and

another pair, the disturbed, is measured to see how much signal transfer occurs.

The test is repeated from the opposite end of the cable, resulting in 12 pair-to-pair

combinations tested. The worst combination is what is recorded as the cables

crosstalk value.

Power Sum NEXT (PSNEXT)


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Power Sum NEXT (PSNEXT) measures the cumulative effect of NEXT from all

wire pairs in the cable.

PSNEXT is computed for each wire pair based on the NEXT effects of theother three pairs. The combined effect of crosstalk from multiple simultaneous

transmission sources can be very detrimental to the signal.

TIA/EIA-568-B certification now requires this PSNEXT test.



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When testing PSNEXT, all pairs

except one are energized as

disturbing pairs, and the remaining

pair, the disturbed pair, is measured

for transferred signal energy.

Notice that the energy from pairs 2,3, and 4 can all affect pair 1. The

sum of this crosstalk must be within

specified limits.

Because each pair affects each other

pair, this measurement will have to

be made four separate times, once

for each wire pair against the others.

Again, testing is done from both ends, raising the number of tested combinations to

eight. The worst combination is recorded as the cables power-sum crosstalk.

4 2 5 Cable Testing Standards


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4.2.5 Cable Testing Standards

The TIA/EIA-568-B standard specifies ten tests that a copper cable must pass if it willbe used for modern, high-speed Ethernet LANs. All cable links should be tested to themaximum rating that applies for the category of cable being installed.

The ten primary test parameters are:

Wire map

Insertion loss

Near-end crosstalk (NEXT)

Power sum NEXT(PSNEXT)

Equal-level FEXT (ELFEXT)

Power sum equal-level FEXT (PSELFEXT)

Return loss

Propagation delay

Cable length

Delay skew

Attenuation to Crosstalk Ratio (ACR)


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Attenuation to Crosstalk Ratio (ACR)

The first thing to understand about testing data cables is the ACR, which stands forAttenuation to Crosstalk Ratio. The pink area in the graph is the attenuation, and theblue area is the crosstalk.

Attenuation is the reduction in signalstrength over the length of the cableand frequency range.

Crosstalk is the external noise that isintroduced into the cable.

So, if the two areas meet, the datasignal will be lost because the crosstalknoise will be at the same level as theattenuated signal.

ACR is the most important result whentesting a link because it represents theoverall performance of the cable.

Wire Map


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Wire Map

Wire map is a continuity test. It assuresthat the conductors that make up the fourtwisted pairs in the cable are continuousfrom the termination point of one end ofthe link to the other. This test assuresthat the conductors are terminatedcorrectly at each end and that none of the

conductor pairs are crossed or short -circuited.

An open circuit

occurs if the wiredoes not attachproperly at theconnector.

A short circuitoccurs if two

wires areconnected toeach other.

Wire Map


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Wire Map

The reversed-pair faultoccurs when a wire pair is correctly installed on oneconnector, but reversed on the other connector. A split-pair wiring faultoccurs whenone wire from one pair is switched with one wire from a different pair at both ends.

ELFEXT and PSELFEXT


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ELFEXT and PSELFEXT

To compensate for this, and toprovide a more meaningful result, the

attenuation is subtracted from theFEXT test and the result is thencalled Equal Level FEXT (ELFEXT).

And of course, no test parameterthese days would be completewithout adding the results together

for each pair and calling it a PowerSum measurement, so now we havePower Sum Equal Level FEXT(PSELFEXT).

FEXT doesn't mean much because the length of the cable determines howmuch the signal is attenuated before it can affect the pairs at the far end.

Return Loss


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Return Loss

When a cable is manufactured there are slight imperfections in the copper. Theseimperfections all contribute to the Structural Return Loss (SRL) measurement becauseeach one causes an impedance mismatch which adds to the cables attenuation.

The significant problem is that signal echoes caused by the reflections from theimpedance mismatches will strike the receiver at different intervals causing signal jitter.

If the power transmitted by the source is PT and the power reflected back is PR, then

the return loss is given by PR divided by PT.

Expressed in dB, the return loss should be as large a negative number as possible.For example a return loss of -40dB is better than one of -20dB.

Propagation Delay


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Propagation Delay

With the emergence of several high-speedapplications the need for additionalperformance parameters (propagationdelay and delay skew) are required.

Propagation delay is a simple

measurement of how long it takes for asignal to travel along the cable beingtested. The delay in a wire pair dependson its length, twist rate, and electricalproperties.

Delays are measured in hundredths of

nanoseconds. One nanosecond is one-billionth of a second, or 0.000000001second. The TIA/EIA-568-B standard setsa limit for propagation delay for the variouscategories of UTP.

Propagation Delay


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Propagation Delay

Propagation delays differ between

mediums, which affect the maximum

possible length of the Ethernet topology

running on that medium.

The maximum propagation delay through

the network can be calculated by dividing

the maximum length by the speed.

For 10Base2 thin coax network, this is

185 meters divided by 195,000 km/sec,

or 950 nanoseconds.

If the actual propagation delay from oneend of the network to the other is greater

than 950 nanoseconds, late collisions

may occur.

Cable Length & TDR


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Cable Length & TDR

Propagation delay measurements are the

basis of the cable length measurement.

TIA/EIA-568-B.1 specifies that the physical

length of the link shall be calculated using the

wire pair with the shortest electrical delay.Testers measure the length of the wire based

on the electrical delay as measured by a Time

Domain Reflectometry (TDR) test, not by the

physical length of the cable jacket. Since the

wires inside the cable are twisted, signals

actually travel farther than the physical length

of the cable.

Cable Length & TDR


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Cable Length & TDR

When a cable tester makes a TDRmeasurement, it sends a pulse signal down a

wire pair and measures the amount of time

required for the pulse to return on the same wire

pair.

The TDR test is used not only to determine

length, but also to identify the distance to wiringfaults such as shorts and opens. When the

pulse encounters an open, short, or poor

connection, all or part of the pulse energy is

reflected back to the tester.

This can be used to calculate the approximate

distance to the wiring fault. The approximatedistance can be helpful in locating a faulty

connection point along a cable run, such as a

wall jack.

Delay Skew


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e ay S e

The propagation delays of different wire

pairs in a single cable can differ slightly

because of differences in the number of

twists and electrical properties of each

wire pair.

The delay difference between pairs iscalled delay skew.

Delay skew is a critical parameter for high-

speed networks in which data is

simultaneously transmitted over multiple

wire pairs, such as 1000BASE-T Ethernet.

If the delay skew between the pairs is too

great, the bits arrive at different times and

the data cannot be properly reassembled.

3.1.7 Coaxial Cable


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Coaxial Cable Pros & Cons


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Pros:

It can be run longer distances than UTP without the need for repeaters.

Coaxial cable is less expensive to install than fiber-optic cable.

The technology is well known.

Cons:

It is hard to work with because of thickness, making it more expensive to installthan Ethernet.

Poor shield connection is one of the biggest sources of connection problems inthe installation of coaxial cable.

Connection problems result in electrical noise that interferes with signaltransmission.

3.1.8 STP & ScTP


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STP & ScTP


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EMI & RFI


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electromagnetic interference (EMI) - Interference by electromagnetic

signals that can cause reduced data integrity and increased error

rates on transmission channels.

radio frequency interference (RFI) - The radio frequencies that create

noise that interferes with information being transmitted across

unshielded copper cabling.

STP and ScTP cable combines the techniques of cancellation,

shielded, and twisted wires to reduce EMI and RFI.

UTP relies on cancellation and twisted wires, without the metallic

shielding that the other two offer.

3.1.2 Voltage


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g

Voltage, which is sometimes referred to as electromotive force, isrelated to an electrical force, or pressure, that occurs when electronsand protons are separated. The force that is created pushes towardthe opposite charge and away from the like charge.

In other words, voltage is the pressure that moves electrons through

a circuit from one place to another.

Voltage is measured in volts (V).

3.1.3 Resistance and Attenuation


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Resistance is opposition to the flow of electrons. It is measured in Ohms ().

The materials through which current flows vary in their resistance to the movementof the electrons. The amount of resistance depends on the chemical compositionof the materials.

All materials that conduct electricity have a measure of resistance to the flow of

electrons through them.

Attenuation is important in relation to networks. Attenuation refers to theresistance to the flow of electrons and explains why a signal becomes degraded asit travels along the conduit.

In networking terms, attenuation is the reduction of signal energy during

transmission. This affects data communication signals in the form of light patterns,electrical voltages, and modulated electromagnetic waves.

Insulators


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Insulators: Electrical insulators are materials that are most resistant to the flow ofelectrons through them.

Examples of electrical insulators include plastic, glass, air, dry wood, paper, rubber,and helium gas. These materials have very stable chemical structures and theelectrons are tightly bound within the atoms.

Conductors


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Conductors: Electrical conductors are materials that allow electrons to flow throughthem easily. The outermost electrons are bound very loosely to the nucleus and areeasily freed. At room temperature, these materials have a large number of free electronsthat can provide conduction. The introduction of voltage causes the free electrons tomove, which results in a current flow.

The best conductors are metals such as copper (Cu), silver (Ag), and gold (Au). These

metals have electrons that are easily freed. Other conductors include solder, which is amixture of lead (Pb) and tin (Sn), and water with ions.

Semiconductors


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Semiconductors: Semiconductors are materials that allow the amount of electricity theyconduct to be precisely controlled. These materials are listed together in one column ofthe periodic chart.

Examples include carbon (C), germanium (Ge), and the alloy gallium arsenide (GaAs).Silicon (Si) is the most important semiconductor because it makes the best microscopic-sized electronic circuits.

3.1.4 Current Flow


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Current is the flow ofcharged particles.

Currents flow in closedloops called circuits,

which must be composedof conducting materialsand must have sourcesof voltage.

Voltage causes current to flow. Resistance and impedance oppose it. Currentconsists of electrons that flow away from negative terminals and toward positive

terminals. These facts allow people to control the flow of current.

Current (I) is measured in amperes (A).

3.1.5 (Circuits) Electricity


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Electricity will naturally flow to

the earth if there is a path.

Current also flows along the

path of least resistance.

If a human body provides the

path of least resistance, the

current will flow through it.

When an electric appliance has

a plug with three prongs, one of

the prongs acts as the ground,

or 0 volts. The ground provides

a conductive path for the

electrons to flow to the earth.The resistance of the body

would be greater than the

resistance of the ground.

Well Grounded


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Water Analogy for Electricity


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A water analogy can help

explain the concept of

electricity.

The higher the water and the

greater the pressure, the more

the water will flow.

The water current also depends

on the size of the space it must

flow through.

Similarly, the higher the voltage

and the greater the electricalpressure, the more current will

be produced.

Water Analogy for Electricity


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The electric current then

encounters resistance that, like

the water tap, reduces the flow.

If the electric current is in an AC

circuit, then the amount of

current will depend on howmuch impedance is present.

If the electric current is in a DC

circuit, then the amount of

current will depend on how

much resistance is present.

The pump is like a battery. Itprovides pressure to keep the

flow moving.

Measuring the flow: Ohms Law


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The relationship among voltage, resistance, and current is voltage (V) equalscurrent (I) multiplied by resistance (R). In other words, V=I*R.

This is Ohms law, named after the scientist who explored these issues.

For current, I=V/R.

For resistance, R=V/I.

Series Circuit: Flashlight


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Electrons flow in closed circuits, or complete loops. The chemical processes in the

battery cause charges to build up. This provides a voltage, or electrical pressure, that

enables electrons to flow through various devices. The lines in the graphic represent

a conductor, which is usually copper wire.

Think of a switch as two ends of a single

wire that can be opened or broken to

prevent the flow of electrons. When the

two ends are closed, fixed, or shorted,

electrons are allowed to flow.

Finally, a light bulb provides resistance

to the flow of electrons, which causes

the electrons to release energy in the

form of light.

The circuits in networks use a much

more complex version of this simple

circuit.

3.2.6 Multimode Fiber


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Core: The part of an optical fiber through

which light rays travel is called the core ofthe fiber.

Cladding: Surrounding the core is thecladding. Cladding is also made of silica

but with a lower index of refraction than the

core. Light rays traveling through the fiber

core reflect off this core-to-cladding

interface as they move through the fiber bytotal internal reflection.

Coating or Buffer: Surrounding thecladding is a buffer material that is usually

plastic. The buffer material helps shield the

core and cladding from damage.

Strength Material: The strength material surrounds the buffer, preventing the fiber cable from beingstretched when installers pull it. The material used is often Kevlar, the same material used to produce

bulletproof vests.

Outer Jacket: The final element is the outer jacket. The outer jacket surrounds the cable to protect thefiber against abrasion, solvents, and other contaminants. The color of the outer jacket of multimode

fiber is usually orange, and singlemode fiber is usually yellow.

Single-mode versus Multimode Fiber


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Full Duplex in Optical Fiber


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Every fiber-optic cable used for networking consists of two glass fibers encased inseparate sheaths. One fiber carries transmitted data from device A to device B. The

second fiber carries data from device B to device A.

The fibers are similar to two one-way streets going in opposite directions. This

provides a full-duplex communication link. Copper twisted-pair uses a wire pair to

transmit and a wire pair to receive. Fiber-optic circuits use one fiber strand to

transmit and one to receive. Typically, these two fiber cables will be in a single outerjacket until they reach the point at which connectors are attached.

No Crosstalk


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Until the connectors are

attached, there is no need for

shielding, because no lightescapes when it is inside a fiber.

This means there are no

crosstalk issues with fiber.

Multiple Pairs in One Case


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One cable can contain

2 to 48 or more

separate fibers.

With copper, one UTP

cable would have to be

pulled for each circuit.

It is very common to see multiple fiber pairs encasedin the same cable. This allows a single cable to be

run between data closets, floors, or buildings.

Optical Cable Design


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3.2.7 Single-mode Fiber


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Single-mode fiber consists of the same parts as multimode. The outer jacket ofsingle-mode fiber is usually yellow. The major difference between multimode

and single-mode fiber is that single-mode allows only one mode of light to

propagate through the smaller, fiber-optic core.

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. As a result, the data

carrying light ray pulses in single-mode fiber are essentially transmitted in astraight line right down the middle of the core. This greatly increases both the

speed and the distance that data can be transmitted.

Common Core/Cladding Sizes


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The much smaller and more refined fiber core in single-mode fiber is

the reason single-mode has a higher bandwidth and cable run distance

than multimode fiber. However, it entails more manufacturing costs.

3.2.8 Other Optical Components


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Connector: a mechanical device attached to the fiber ends so that the fibers can be connected to

the ports on the transmitter and receiver. The type of connector most commonly used withmultimode fiber is the Subscriber Connector (SC). On single-mode fiber, the Straight Tip (ST)

connector is frequently used.

Transmitter: an electronic package that converts an electrical signal to an optical signal. The

transmitter receives data to be transmitted from switches and routers. This data is in the form of

electrical signals. The transmitter converts the electronic signals into their equivalent light pulses.

Receiver: an electronic package that converts optical signals to electrical signals. The first job ofthe receiver is to detect a light pulse that arrives from the fiber. Then the receiver converts the light

pulse back into the original electrical signal that first entered the transmitter at the far end of the fiber.

Light Sources


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Light Emitting Diode (LED): a light source producing infrared light with wavelengths ofeither 850 nm or 1310 nm. These are used with multimode fiber in LANs. Lenses areused to focus the infrared light on the end of the fiber.

Light Amplification by Stimulated Emission Radiation (LASER): a light sourceproducing a thin beam of intense infrared light usually with wavelengths of 1310nm or1550 nm. Lasers are used with single-mode fiber over the longer distances involved in

WANs or campus backbones. Extra care should be exercised to prevent eye injury.

Types of Connectors


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Subscriber Connector (SC):the type of connector most commonlyused with multimode fiber, made frommolded plastic, using push-pullmechanics.

Straight Tip (ST) connector:

the type of connector most commonly

used with single-mode fiber, featuringa bayonet-style nut.

Parts of a Connector


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Optical Fiber: Pros


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Fiber-optic cable is not affected by the sources ofexternal noise that cause problems on copper mediabecause external light cannot enter the fiber except atthe transmitter end.

The transmission of light on one fiber in a cable doesnot generate interference that disturbs transmission onany other fiber. This means that fiber does not havethe problem with crosstalk that copper media does.

Fiber is the best of all the transmission media atcarrying large amounts of data over long distances.

Optical Fiber: Cons


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Absorption: When a light ray strikes some types of chemical impurities in a fiber, theimpurities absorb part of the energy. This light energy is converted to a small amountof heat energy. Absorption makes the light signal a little dimmer.

Dispersion: Another factor that causes attenuation of the light signal ismanufacturing irregularities or roughness in the core-to-cladding boundary. Power islost from the light signal because of the less than perfect total internal reflection in thatrough area of the fiber.

Scattering: Thescattering of light in a fiberis caused by microscopicnon-uniformity (distortions)in the fiber that reflectsand scatters some of thelight energy.

Optical Fiber: Cons


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Improper Installation: A majorcause of too much attenuation infiber-optic cable is improperinstallation. If the fiber is stretched orcurved too tightly, it can cause tinycracks in the core that will scatter thelight rays. Bending the fiber in tootight a curve can change the incidentangle of light ray.

Dirty Ends: Once the fiber-optic cable and connectors have been installed,the connectors and the ends of the fibers must be kept spotlessly clean.

Fiber End Face Finishes


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Fiber End Face Polishing Techniques


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Splicing


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Calibrated Light Sources and Light Meter


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4.2.8 Testing Optical Fiber


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A fiber link consists of two separate glass fibers functioning as independent datapathways. One fiber carries transmitted signals in one direction, while the second

carries signals in the opposite direction.

Each glass fiber is surrounded by a sheath that light cannot pass through, so there

are no crosstalk problems on fiber optic cable.

External electromagnetic interference or noise has no affect on fiber cabling.

Attenuation does occur on fiber links, but to a lesser extent than on copper cabling.

Optical Fiber Impedance


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Fiber links are subject to the

optical equivalent of UTP

impedance discontinuities.

When light encounters an

optical discontinuity, like an

impurity in the glass or a

micro-fracture, some of the

light signal is reflected back in

the opposite direction.

This means only a fraction of the original light signal will continue down the fiber

towards the receiver. This results in a reduced amount of light energy arriving at the

receiver, making signal recognition difficult.Just as with UTP cable, improperly installed connectors are the main cause of light

reflection and signal strength loss in optical fiber.

Optical Fiber Light Signal


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Because noise is not an issue

when transmitting on optical

fiber, the main concern with a

fiber link is the strength of the

light signal that arrives at the

receiver.

If attenuation weakens the

light signal at the receiver,

then data errors will result.

Testing fiber optic cable

primarily involves shining a

light down the fiber and

measuring whether a

sufficient amount of light

reaches the receiver.

Optical Link Loss Budget


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On a fiber optic link, the

acceptable amount of signal

power loss that can occur

without dropping below the

requirements of the receiver

must be calculated.

This calculation is referred to

as the optical link loss budget.

A fiber test instrument, known

as a light source and power

meter, checks whether the

optical link loss budget hasbeen exceeded.

OTDR


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If the fiber fails the test, another cable test instrument can be used to indicatewhere the optical discontinuities occur along the length of the cable link.

An optical TDR known as an

OTDR is capable of locating

these discontinuities.

Usually, the problem is one

or more improperly attached

connectors.

The OTDR will indicate the

location of the faulty

connections that must be

replaced. When the faults

are corrected, the cable

must be retested.

Agilent E6000 Series Mini-OTDR

5.1.8 Wireless


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Main Benefits:

User mobility

No cables to end user

Infrared (IR) Pros and Cons


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Pros

An infrared-based network suits environments where all the digital devices that

require network connectivity are in one room

New IR technologies will be able to work out of sight

Technology can be installed quickly

InexpensiveCons

Current technology requires devices to be in the line of sight of the transmitter

Limited distance

Small ratio of receiver-to-devices

IR technology doesnt work very well in direct sunlight

Data signals can be weakened or obstructed by people who walk across the

room or by moisture in the air

Radio Frequency (RF) Pros and Cons


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Pros

It supports the ability to send data at a faster rate

It has a large broadcast range

It is low maintenance

It is omni directional. An omni directional system does not require line-of-sight,

so it can operate through wallsCons

RF spectrum is a limited and regulated resource; it is therefore an expensive

resource

Prone to congestion and interference.

Since RF signals are not restricted to well-defined boundaries, RF transmissioncan be picked up by anyone within range of the transmitter, making it difficult to

secure

Spread Spectrum Invention


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The idea of spread-spectrum radio transmission was proposed by the military who was

seeking ways to prevent radio signals from being monitored or blocked by hostile parties.

The two inventors came up with thenotion of changing the frequency of atransmission at regular intervals fasterthan the enemy could retune.

A special receiver that knew thefrequency-hopping pattern could follow itand pick up the entire transmission.

The hopping patterns were controlled bythe punched holes in piano rolls becameknown as frequency-hopping spreadspectrum (FHSS).

Later, as digital logic became popular, direct-sequencespread spectrum (DSSS) was developed. In this methodof transmission, the signal does not hop from onefrequency to another but is passed through a spreadingfunction and distributed over the entire band at once.

Spread Spectrum FHSS & DSSS


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DSSS usually provides slightly higher data rates and shorter delays than FHSS, because

the transmitter and receiver don't have to spend time retuning.

Both FHSS and DSSS are resistant to interference from conventional radio transmitters.

Because the signal doesn't stay inone place on the band, FHSS canelude a jammer (a transmitter

designed to block radiotransmissions on a given frequency).

DSSS avoids interference byconfiguring the spreading function inthe receiver to concentrate thedesired signal but spread out and

dilutes any interfering signal.

IEEE 802.11


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The IEEE 802.11 specifications are wireless standards that specify an "over-the-air"

interface between a wireless client and a base station or access point, as well as amongwireless clients. The 802.11 standards can be compared to the IEEE 802.3 standard forEthernet for wired LANs.

The IEEE 802.11 specifications address both the Physical (PHY) and Media AccessControl (MAC) layers and are tailored to resolve compatibility issues betweenmanufacturers of Wireless LAN equipment.

OSI Physical Layer Supplement

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