Transcript CCNA1_v3_Module04_Cable_testing_Presentation

CCNA 1 Module 4
Cable Testing
Waves
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Frequency:
– the number of waves per second
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Period:
– the amount of time that it takes to
complete 1 cycle, in seconds
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Amplitude:
– the height of the wave, measured in volts
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Analog signal – sine wave
Varies continuously with time
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Analog signal – sine wave
Phase = 0
Phase = 1
Phase = .5
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Digital signal – square
wave
Maintains one value and then
suddenly changes to a different value
maintains one value and then suddenly changes to a different value
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Signals
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Bits (0s and 1s) need to be transmitted from
one host to another
Each bit is placed on the cable as a signal or
pulse
On copper cable the pulses are electrical
signals of different voltage levels
The simplest signalling schemes have only
two voltage levels
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A pulse
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Fourier Synthesis of a
Square Wave
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Noise
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Noise is unwanted
additional electrical
signals on a cable
The extra signals
are added to the
data signals and
distort them
Original signal
Noise
Resulting signal
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Noise
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Possible sources of noise:
– 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
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Bandwidth
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Two ways of considering bandwidth in
communications systems are analog
and digital.
– Analog bandwidth describes the range of
frequencies transmitted (hertz; Hz)
– Digital bandwidth measures how much
information can flow from one place to
another in a given amount of time (bps)
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Signals and Noise
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Bits are represented by either no voltage (0)
or +3 to +6 Volts (1).
A Signal Reference Ground attached close to
a computer’s digital circuits establishes the
baseline for no voltage.
The reference level is called a signal ground
Bits must arrive at the destination
undistorted in order to be properly
interpreted.
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Shielded vs Unshielded
Cable
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Shielding material protects the data signal from
external sources of noise and from noise generated
by electrical signals within the cable
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Benefits of shielding
– protects the inner conductor from external electrical noise
– keeps the transmitted signal confined to the cable
– protects each wire pair from noise generated by the other
pairs
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Shielded cable examples: coaxial, STP, ScTP
Unshielded cable: UTP
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Shielded vs Unshielded
Cable
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Shielded cable helps insulate the
signal from external noise.
Shielding makes the cable thicker and
therefore more difficult to install.
Twisted pair cable uses twisting of
pairs of wires to cancel signals from
adjacent wires.
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Optical Cable
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Fiber optic cable:
– transmits data signals by increasing and
decreasing the intensity of light.
– needs no insulation as it is not affected
by electrical noise.
– does not need to be grounded.
– is often used between buildings and
between floors within the building.
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What six things can
distort a bit?
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Propagation
Attenuation
Reflection
Noise
Timing problems
Collisions
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Propagation
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The travelling of bits down a wire
– Devices (eg printer) at receiving end
cannot handle the speed, and data is lost
– Solve with buffers on the device or a
message to slow down the transmission
rate
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Bits are distorted by:
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Propagation
Attenuation
Reflection
Noise
Timing problems
Collisions
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Attenuation
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Loss of signal strength
as bits move down a
wire (dB)
– 1s look like 0s, and the
message is unreadable
– Test by using the
highest frequencies
possible with the cable.
– Solved by using
repeaters, hubs to
amplify the signal
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Impedance
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Impedance is caused:
– Exceeding the maximum recommended cable
length
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Use repeaters or don’t exceed the 100m
– by defective connectors
– when signal energy is lost, when it leaks through
the insulation of the cable
– by the resistance of the copper cable converting
some of the electrical energy of the signal to
heat
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Bits are distorted by:
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Propagation
Attenuation
Reflection
Noise
Timing problems
Collisions
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Reflection
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Reflected energy moves back towards signal, distorting
it as bits run into each other.
– Caused by impedance mismatch (NIC and media)
– Jitter is created as the reflected signal bounces back and forth.
– Solve by correct wiring
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Bits are distorted by:
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Propagation
Attenuation
Reflection
Noise
Timing problems
Collisions
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Noise
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Unwanted additions to a signal
Results in 1s turned into 0s and 0s
into 1s. Too much noise can destroy
the message.
It is important to keep the signal to
noise ratio as high as possible (little
noise)
It is not possible to avoid noise
altogether
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Noise
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Four kinds
– Thermal
– Reference ground noise
– EMI/RFI
– Crosstalk
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External Noise
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Thermal noise is everywhere but isn’t a
problem.
Reference ground noise is AC line noise,
fixed by rewiring.
Electromagnetic interference (EMI)
Radio Frequency interference (RFI)
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EMI/RFI
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EMI and RFI attack the quality of
electrical signals on the cable.
Sources of EMI/RFI include:
– EMI – fluorescent lights; electric motors
– RFI – Radio systems
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EMI/RFI noise
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Source computer sends
out a digital signal.
EMI noise occurs along
the path.
The extra signal is
added to the data
signals and distorts it.
Original signal
Noise
Resulting signal
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EMI/RFI noise
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Two ways to prevent EMI/RFI Noise:
– shielding the wires in the cable with a
metal braid or foil. (Increases cost and
diameter of the cable)
– cancellation – the wires are twisted
together in pairs providing self-shielding
within the network media.
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EMI/RFI noise
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UTP Cat 5 has eight
wires twisted into four
pairs.
In each pair, one wire
is sending data and
the other is receiving.
Electrons flowing down
the wire create a
small, circular
magnetic field around
the wire.
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EMI/RFI noise
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Since the two wires
are close together,
their opposing
magnetic fields cancel
each other.
They also cancel out
outside magnetic fields
(EMI/RFI).
Twisting of the wires
enhances cancellation
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Noise on Copper Media
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Internal Noise - Crosstalk
1.
2.
3.
4.
5.
6.
A voltage signal passes along a wire
It generates energy that radiates out, like
a radio signal
Other nearby wires can pick up the signal
This signal adds a small voltage to the
signal on the nearby wires
This effect is called crosstalk
Crosstalk is cancelled by the twisting of
pairs of wires.
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Testing for Crosstalk
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The cable testing instrument puts a signal
on one wire pair in a cable.
It then measures, in dB the amplitude of the
crosstalk signals.
The lower (smaller) the negative number,
the more noise there is.
– -30dB is a CLEARER signal than -10dB.
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Each pair should be measured from each
other pair in a UTP link, and from both ends
of the link.
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Types of Crosstalk - NEXT
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Near-end Crosstalk
NEXT is the test signal measured from the
same end of the link (cable).
Radiated EM energy
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Types of Crosstalk - FEXT
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Far-end Crosstalk
FEXT occurs further away from the source, like at
the other end of the cable.
Due to attenuation, FEXT creates less noise than
NEXT.
Generates weak
FEXT at other pairs
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Types of Crosstalk PSNEXT
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Power Sum Near-end Crosstalk
PSNEXT is the combined effect of NEXT on
one pair of wires from the other 3 pairs of
wires in the cable.
Cables need to be tested to measure the
amount of PSNEXT.
It is particularly important when all the
wires in a cable are being used for
transmission, as in Gigabit Ethernet.
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Bits are distorted by:
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Propagation
Attenuation
Reflection
Noise
Timing problems
Collisions
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Timing Problems
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Dispersion – similar to attenuation; is the
broadening of a signal as it travels down the media.
Jitter – the reflected signal strikes the first
discontinuity, and some of the signal rebounds in
the original direction, creating multiple echo effects.
The echoes strike the receiver at different intervals.
This is called jitter and can result in data errors.
Latency – is the delay of a network signal caused
by:
– The time it takes a bit to travel to its destination
– Devices the bit travels through
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Bits are distorted by:
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Propagation
Attenuation
Reflection
Noise
Timing problems
Collisions
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Collisions
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Collisions occur in broadcast topologies
where devices share access to the network
media.
A collision happens when two devices
attempt to communicate on the sharedmedium at the same time.
Collisions destroy data requiring the source
to retransmit.
The prevention of collisions will be discussed
in more detail later in the semester.
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Cable Testing Standards
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TIA/EIA-568-B standard applies to
copper cable in a LAN.
Each pair of pins on an RJ45
connector have a specific purpose.
– A NIC transmits signals on pins 1 and 2,
and receives signals on pins 3 and 6.
– The other four pins are not used except
in gigabit ethernet.
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TIA/EIA-568-B Standard
– the ten cable tests
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
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
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Wire Map
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ensures
‘Open’that
circuit
no open
occurs
orifshort
a
wire
circuits
does
not
exist on
an
‘Short’
two
wires
are
the
attach
cable
properly
(Good
at
Wire
the
Map)
connector.
connected
to each
other.
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Wire Map and Wiring
Faults
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The wire map verifies
that all eight wires are
connected to the
correct pins on both
ends of the cable
The reversed-pair fault
occurs when a wire
pair is correctly
installed on one
connector, but reversed
on the other connector.
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Wire Map and Wiring
Faults
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A split-pair wiring fault
occurs when one wire
from one pair is
switched with one wire
from a different pair at
both ends. Less
cancellation means
more crosstalk.
A transposed pair fault
occurs when two wire
pairs are correct at one
end but reversed at the
other end.
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Time Domain
Reflectometry-TDR
– Wires inside the cable are twisted
– Signals actually travel farther than the physical length of
the cable
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TDR measurement:
– 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
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TDR test
– determine length
– identify the distance to wiring faults such as shorts &opens
– When the pulse encounters an open, short, or poor
connection, all or part of the pulse energy is reflected back
to the tester
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A New Standard
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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
Cables certified as Cat 6 cable must pass all
10 tests
Cat 6 cable must pass the tests with higher
scores to be certified.
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Good Luck
on the
Exam!!
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