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Transcript of 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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222
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/
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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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Straight-through Cable
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Straight-through Cable
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.
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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.
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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)
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.
Near-End Crosstalk (NEXT)
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Near-End Crosstalk (NEXT)
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)
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.
Power Sum NEXT (PSNEXT)
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Power Sum NEXT (PSNEXT)
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.
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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.