Transcript for Semester 1 Chapter 4

Cisco CCNA
Semester 1
Chapter 4 V3.0
By: Terren L. Bichard
Overview
• Networking media is literally and
physically the backbone of the network.
– Poor quality results in network failures and
unreliability.
• All media requires testing to determine
quality.
Goals
• Part One
– Provide basic definitions
• Part Two
– Describes issues relating to the testing of
media used for connectivity.
Definitions
• Wave = energy traveling from one place
to another
– Caused by disturbances.
• Amplitude
– Height measured in meters
• Period
– Length of time between cycles
• Frequency
– number of cycles per second measured in Hertz
Types of Waves
• Copper wire
– Voltage waves
• Optical fiber
– Light waves
• Pulse
– A disturbance which is deliberately caused,
and involves a fixed, predictable duration
Sine Waves
• Sine waves have certain characteristics.
• Sine waves are periodic
– They repeat the same pattern at regular
intervals.
• Sine waves are continuously varying
– No two adjacent points on the graph have
the same value.
Square Waves
• Like sine waves are periodic.
– However, square wave graphs do not continuously
vary with time.
• The wave holds one value for some time, and
then suddenly changes to a different value.
• This value is held for some time, and then
quickly changes back to the original value.
• Square waves represent digital signals, or
pulses.
– Like all waves, square waves can be described in
terms of amplitude, period, and frequency.
Exponents and
Logarithms
• In networking, there are three important
number systems:
– Base 2 – binary
– Base 10 – decimal
– Base 16 – hexadecimal
• The base of a number system refers to
the number of different symbols that can
occupy one position
Example
• Binary numbers
– 2 positions
•0&1
• Decimal numbers
– 10 positions
• 0-9
• Hexadecimal numbers
– 16 positions
• 0-9 and A-F
Decibels
• A measurement unit important in
describing networking signals. (dB)
• There are two formulas for calculating
decibels:
• dB = 10 log10 (Pfinal / Pref)
• dB = 20 log10 (Vfinal / Vreference)
• The variables represent the following values.
– dB measures the loss or gain of the power of a
wave.
• Decibels are usually negative numbers representing a
loss in power as the wave travels, but can also be positive
values representing a gain in power if the signal is
amplified
– log10 implies that the number in parenthesis will be
transformed using the base 10 logarithm rule
– Pfinal is the delivered power measured in Watts
– Pref is the original power measured in Watts
– Vfinal is the delivered voltage measured in Volts
– Vreference is the original voltage measured in Volts
Formulas
• Db = 10 log10 (Pfinal / Pref)
– Describes decibels in terms of power (P)
• Db = 20 log10 (Vfinal / Vreference)
– Describes decibels in terms of voltage (V)
• Typically, light waves on optical fiber and radio
waves in the air are measured using the
power formula.
• Electromagnetic waves on copper cables are
measured using the voltage formula.
Signals and Frequency
• One of the most important facts of the
information age is that data symbolizing
characters, words, pictures, video, or music
can be represented electrically by voltage
patterns on wires and in electronic devices.
• Data represented by these voltage patterns
can be converted to light waves or radio
waves, and then back to voltage waves.
– The sound waves of the caller’s voice, on a
telephone, enter a microphone in the telephone.
– The microphone converts the patterns of sound
energy into voltage patterns of electrical energy
that represent the voice.
Viewing Signals
• An oscilloscope is an important
electronic device used to view electrical
signals such as voltage waves and
pulses.
Spectrum Analyzer
List Price: $24,000.00
Noise in Time and
Frequency
• Noise is an important concept in
communications systems, including LANS.
• Noise related to communications refers to
undesirable signals.
• Noise can originate from natural and
technological sources, and is added to the
data signals in communications systems.
Noise
• All communications systems have some amount of
noise.
• Noise cannot be eliminated
– Effects can be minimized if the sources of the noise are
understood.
• Many possible sources of noise:
– Nearby cables which carry data signals
– Radio frequency interference (RFI)
• Noise from other signals being transmitted nearby
– Electromagnetic interference (EMI)
• Noise from nearby sources such as motors and lights
• Laser noise at the transmitter or receiver of an optical
signal
Different Types of Noise
• White noise
– Noise that affects all transmission frequencies
• When detected on a radio receiver, white noise would
interfere with all radio stations.
• When detected on a LAN, white noise would affect all
data transmissions
• Narrowband interference
– Noise that only affects small ranges of frequencies
• Narrowband interference would affect only a few stations
whose frequencies are close together.
• Narrowband interference might disrupt only certain
signals. If the band of frequencies affected by the
narrowband interference included all frequencies
transmitted on the LAN, then the performance of the
Bandwidth
• Two types of bandwidth on LANs are analog
bandwidth and digital bandwidth.
• Analog bandwidth
– radio station frequencies
– an electronic amplifier
– The units of measurement for analog bandwidth is
Hertz
•
•
•
•
analog bandwidth values are 3 kHz for telephony
20 kHz for audible signals
5 kHz for AM radio stations
200 MHz for FM radio stations.
Bandwidth (cont.)
• Digital bandwidth measures how much
information can flow from one place to
another in a given amount of time.
– The unit of measurement for digital bandwidth is
bits per second (bps).
– Since LANs are capable of speeds of millions of
bits per second, measurement is expressed in
kilobits per second (Kbps) or megabits per second
(Mbps).
– Physical media, current technologies, and the laws
of physics limit bandwidth.
Bandwidth (cont.)
• During cable testing, analog bandwidth is used to
determine the digital bandwidth of a copper
cable.
• Analog frequencies are transmitted from one end
and received on the opposite end.
• The two signals are then compared, and the
amount of attenuation of the signal is calculated.
• In general, media that will support higher analog
bandwidths without high degrees of attenuation
will also support higher digital bandwidths.
Signaling Over Copper
and Fiber Cable
• On copper cable, data signals are represented by
voltage levels that represent binary ones and
zeros.
• Voltage levels are measured with respect to a
reference level of zero volts at both the
transmitter and the receiver.
• This reference level is called the signal ground.
• It is important that both transmitting and receiving
devices refer to the same zero volt reference
point.
– When they do, they are said to be properly
grounded.
Signaling Over Copper
and Fiber Cables
• In order for the LAN to operate properly,
the receiving device must be able to
accurately interpret the binary ones and
zeros transmitted as voltage levels.
• As technology progresses it is critical
that all devices and media meet highest
standards of excellence.
Copper Cable
• Two basic types
– Unshielded
• UTP
– Shielded
• STP
• Coax
Fiber Optic Cable
• Transmits data signals by increasing
and decreasing the intensity of light to
represent binary ones and zeros.
• Not affected by electrical noise
• Does not need to be grounded
• Used between buildings and between
floors within the building
Attenuation on Copper
Media
• Attenuation
– The decrease in signal amplitude over the
length of a link
– Expressed in decibels (db) using negative
numbers
– The larger the number the better the link.
Sources of Noise on
Copper
• Noise is any electrical energy on the
transmission cable that makes it difficult
for a receiver to interpret the data sent
from the transmitter.
– Crosstalk
• The transmission of signals from one wire to a
nearby wire.
• Twisting one pair of wires in a cable also helps
to reduce crosstalk of data or noise signals from
an adjacent wire pair.
Three Types of
Crosstalk
• Near-end Crosstalk (NEXT)
• Far-end Crosstalk (FEXT)
• Power Sum Near-end Crosstalk
(PSNEXT)
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.
• This difference is expressed in a negative
value of decibels (dB).
– Low negative numbers indicate more noise, just as
low negative temperatures indicate more heat.
FEXT
• Far-End crosstalk, or FEXT.
• Crosstalk occurring further away from the
transmitter creates less noise on a cable than
NEXT.
• Noise caused by FEXT still travels back to the
source, but it is attenuated as it returns.
• FEXT is not as significant a problem as 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 the other 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.
Cable Testing Standards
• The TIA/EIA-568-B standard specifies
ten tests that a copper cable must pass
if it will be used for modern, high-speed
Ethernet LANs.
• The ten primary test parameters that
must be verified for a cable link to meet
TIA/EIA standards are:
Cable Testing Standards
•
•
•
•
•
•
•
•
•
•
Wire map
Insertion loss
Near-end crosstalk (NEXT)
Power sum near-end crosstalk (PSNEXT)
Equal-level far-end crosstalk (ELFEXT)
Power sum equal-level far-end crosstalk
(PSELFEXT)
Return loss
Propagation delay
Cable length
Delay skew
Cable Testing Standards
• Ethernet standard specifies that each of
the pins on an RJ-45 connector have a
particular purpose.
• A NIC transmits signals on pins 1 and 2,
and it receives signals on pins 3 and 6.
Wiring Tests
• Wire map test insures that no open or short
circuits exist
• Reversed-pair fault occurs when a wire pair is
correctly installed on one connector, but
reversed on the other connector.
• Split-pair wiring fault occurs when two wires
from different wire pairs are connected to the
wrong pins on both ends of the cable.
• Transposed-pair wiring faults occur when a
wire pair is connected to completely different
pins at both ends.
Other Test Parameters
• Insertion loss = the combination of the effects
of signal attenuation and impedance
discontinuities on a communications link.
– Insertion loss is measured in decibels at the far
end of the cable.
– The TIA/EIA standard requires that a cable and its
connectors pass an insertion loss test before the
cable can be used as a communications link in a
LAN.
Other Test Parameters
• Crosstalk is measured in four
separate tests.
– PSNEXT
• Power Sum equal-level Far-End Crosstalk
– FEXT
• Far-End crosstalk
– ELFEXT
• Equal-Level Far-End Crosstalk
– PSELFEXT
• is the combined effect of ELFEXT from all wire
pairs.
Other Test Parameters
• Return loss
– measure in decibels of reflections that are
caused by the impedance discontinuities at
all locations along the link.
Time-Based Parameters
• Propagation delay
– How long it takes for a signal to travel (propagate)
along the cable being tested.
• delay in a wire pair depends on its length, twist rate, and
electrical properties.
• Cable Length
– TDR test is used not only to determine length, but
also to identify the distance to wiring faults such as
shorts and opens.
• Delay Skew
– The delay difference between pairs is called.
Testing Optical Fiber
• A fiber link consists of two separate
glass fibers functioning as independent
data pathways.
• Testing fiber optic cable primarily
involves shining a light down the fiber
and measuring whether a sufficient
amount of light reaches the receiver.
Testing Optical Fiber
• Optical link loss budget.
– The acceptable amount of signal power
loss that can occur without dropping below
the requirements of the receiver.
New Cat 6 Standard
• On June 20, 2002, the Category 6 (or Cat 6)
addition to the TIA-568 standard was published.
– The official title of the standard is ANSI/TIA/EIA568-B.2-1
• Cat 6 tests are essentially the same as those
specified by the Cat 5 standard.
– Cat 6 cable must pass the tests with higher scores
to be certified.
– Cat6 cable must be capable of carrying
frequencies up to 250 MHz and must have lower
levels of crosstalk and return loss.
Summary
• Waves are energy traveling from one place to
another, and are created by disturbances.
– All waves have similar attributes such as
amplitude, period, and frequency.
• Sine waves are periodic, continuously varying
functions.
– Analog signals look like sine waves.
• Square waves are periodic functions whose
values remain constant for a period of time
and then change abruptly.
– Digital signals look like square waves.
Summary (cont)
• Exponents are used to represent very large or
very small numbers.
– The base of a number raised to a positive
exponent is equal to the base multiplied by itself
exponent times.
• For example, 103 = 10x10x10 = 1000.
• Logarithms are similar to exponents.
– A logarithm to the base of 10 of a number equals
the exponent to which 10 would have to be raised
in order to equal the number.
• For example, log10 1000 = 3 because 103 = 1000.
Summary (cont)
• Decibels are measurements of a gain or loss
in the power of a signal.
– Negative values represent losses and positive
values represent gains.
• Time-domain analysis is the graphing of
voltage or current with respect to time using
an oscilloscope.
• Frequency-domain analysis is the graphing of
voltage or power with respect to frequency
using a spectrum analyzer.
Summary (Cont)
• Undesirable signals in a communications
system are called noise.
– Noise originates from other cables, RFI, and EMI.
• White noise affects all frequencies, while narrowband
interference affects only a certain subset of frequencies.
• Analog bandwidth is the frequency range that
is associated with certain analog
transmission, such as television or FM radio.
• Digital bandwidth measures how much
information can flow from one place to
another in a given amount of time.
– Its units are in various multiples of bits per second.
Summary (Cont)
• Most LAN problems occur at the physical
layer.
– The only way to prevent or troubleshoot many of
these problems is through the use of cable testers.
• Proper cable installation according to
standards increases LAN reliability and
performance.
• Copper media is available in shielded and
unshielded forms.
– Unshielded cable is more susceptible to noise.
Summary (Cont)
• Signal degradation is due to various factors such as
noise, attenuation, impedance mismatch, and several
types of crosstalk.
– These factors cause decreased network performance.
• The TIA/EIA-568-B standard specifies ten tests that a
copper cable must pass if it will be used for modern,
high-speed Ethernet LANs.
• Optical fiber must also be tested according to
networking standards.
• Category 6 cable must meet more rigorous frequency
testing standards than Category 5 cable.