Download CCNA1 3.0-04 Cable Testing

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Transcript
Measuring Frequency
The amplitude of an electrical signal represents height and is
measured in volts. The period is the amount of time to
complete one cycle, measured in seconds. The frequency is
the # of complete cycles per second, measured in Hertz.
Exponents & Logarithms
In networking, there are three important number systems:
Base 2 – binary
Base 10 – decimal
Base 16 – hexadecimal
One way to work with the very large and very small numbers
that occur in networking is to use logarithms.
While the study of logarithms is beyond the scope of this
course, the terminology is used commonly in calculating
decibels, a way of measuring signals on copper, optical, and
wireless media.
Exponents & Logarithms
In networking, there are three important number systems:
Base 2 – binary
Base 10 – decimal
Base 16 – hexadecimal
One way to work with the very large and very small numbers
that occur in networking is to use logarithms.
While the study of logarithms is beyond the scope of this
course, the terminology is used commonly in calculating
decibels, a way of measuring signals on copper, optical, and
wireless media.
Decibels
The decibel (dB) is a measurement unit important in describing networking
signals. 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
Noise
Noise is an important concept in communications systems, including
LANS. While noise usually refers to undesirable sounds, noise related to
communications refers to undesirable signals. All communications systems
have some amount of noise. Even though noise cannot be eliminated, its
effects can be minimized if the sources of the noise are understood. There
are many possible sources of noise:
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Nearby cables which carry data signals
Radio frequency interference (RFI)
which is noise from other signals being transmitted nearby
Electromagnetic interference (EMI),
which is noise from nearby sources such as motors and lights
Laser noise at the transmitter or receiver of an optical signal
Noise that affects all transmission frequencies equally is called white
noise. Noise that only affects small ranges of frequencies is called
narrowband interference.
Bandwidth
Bandwidth is an extremely important concept in communications systems.
Two ways of considering bandwidth that are important for the study of
LANs are analog bandwidth and digital bandwidth.
Analog Bandwidth typically refers to the frequency range of an analog
electronic system. Analog bandwidth could be used to describe the range
of frequencies transmitted by a radio station. Its units of measurement is
Hertz, the same as the unit of frequency.
Digital Bandwidth measures how much information can flow from one
place to another in a given amount of time. The fundamental unit of
measurement for digital bandwidth is bits per second (bps). Physical
media, current technologies, and the laws of physics limit bandwidth.
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.
Signals Over
Copper Cabling
On copper cable, data signals are represented by voltage levels that represent binary
ones and zeros. The 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 Fiber Optic Cabling
Fiber optic cable is used to transmit data signals by increasing and decreasing the
intensity of light to represent binary ones and zeros. The strength of a light signal
does not diminish like 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. Therefore, optical fiber is often used between buildings
and between floors within the building. As costs decrease and demand for speed
increases, optical fiber may become a more commonly used LAN media.
Attenuation & Insertion Loss
on Copper Media
Attenuation is the decrease in signal amplitude over the length of a link.
Attenuation is expressed in decibels (dB) using negative numbers. Smaller
negative dB values are an indication of better link performance.
Impedance is a measurement of the resistance of the cable to alternating
current (AC) and is measured in ohms. The normal, or characteristic,
impedance of a Cat5 cable is 100 ohms. If a connector is improperly
installed on Cat5, it will have a different impedance value than the cable.
This is called an impedance discontinuity or an impedance mismatch.
Impedance discontinuities cause attenuation because a portion of a
transmitted signal will be reflected back to the transmitting device rather
than continuing to the receiver, much like an echo.
The combination of the effects of signal attenuation and impedance
discontinuities on a communications link is called insertion loss.
Sources of Noise on Copper Media
Noise is any electrical energy on the transmission
cable that makes it difficult for a receiver to
interpret the data sent from the transmitter.
TIA/EIA-568-B certification of a cable now
requires testing for a variety of types of noise.
Crosstalk involves the transmission of signals
from one wire to a nearby wire.
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 and filtered at the receiver. Twisting one
pair of wires in a cable also helps to reduce
crosstalk of data or noise signals from adjacent
wires.
Types of Crosstalk
There are three distinct types of crosstalk:
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Near-end Crosstalk (NEXT)
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Far-end Crosstalk (FEXT)
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Power Sum Near-end Crosstalk (PSNEXT)
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.
Far-end Crosstalk (FEXT)
Due to attenuation, crosstalk occurring further away from the
transmitter creates less noise on a cable than NEXT. This is
called far-end crosstalk, or FEXT. 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.
Power Sum Near-end Crosstalk
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.
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:
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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
Ethernet Standards
The Ethernet standard specifies that each of the pins on an
RJ-45 connector have a particular purpose. A NIC transmits
signals on pins 1 & 2, and it receives signals on pins 3 & 6.
Remember…
A straight-thru cable has T568B on both ends. A crossover (or
cross-connect) cable has T568B on one end and T568A on the
other. A console cable had T568B on one end and reverse T568B
on the other, which is why it is also called a rollover cable.
Wiring Faults
Time Based Parameters
Propagation delay is a simple measurement of how long it
takes for a signal to travel along the cable being tested.
The delay in a wire pair depends on its length, twist rate, and
electrical properties.
Delays are measured in hundredths of nanoseconds.
One nanosecond is one-billionth (0.000000001) of a second.
The TIA/EIA-568-B standard sets a limit for propagation delay
for the various categories of UTP.
The delay difference between pairs is called the delay skew.
A New 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/EIA-568-B.2-1.
Although the tests for certifying Cat 6 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.