Transcript IST 201

IST 201
Chapter 4
Waves
• energy traveling from one place to another frequency
• period – amount of time between each wave.
measured in seconds.
• frequency – number of waves(cycles) in a given
amount of time (hertz).
• amplitude – height of the wave. Measured in volts.
• pulse – signal that is deliberately caused and involves
a fixed, predictable duration.
Types of Waves - Networking
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Voltage waves – copper
Light waves – optical fiber
Electromagnetic waves – wireless
An electric motor completes 60 cycles every second. What is
the frequency? What is the period?
a.
b.
Frequency = 60 hertz
Period = 1/60
Analog signals (sine waves)
• Periodic (repeat same pattern at intervals)
• continuously varying
Square Waves
• periodic
• remain the same for some time; suddenly
change; remain the same for some time;
suddenly change back to original value.
Decibels
• A unit for expressing the ratio of two amounts of electric or acoustic signal
power equal to 10 times the common logarithm of this ratio.
• Two formulas for determining.
Decibel Formula
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Power formula: dB = 10 log10 (Pfinal/Pref)
radio & light waves
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Pfinal = delivered power (watts)
Pref = original power (watts)
OR
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Voltage formula: dB = 20 log10 (Vfinal/Vref)
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Vfinal = delivered voltage (volts)
Vref = original voltage (volts)
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Decibels are usually negative representing loss of power as the wave travels, but can
be positive representing gain if signal is amplified.
Oscilloscope
• Device used to view electrical signals such as
voltage waves & pulses.
• Used as part of time-domain analysis
• x-axis represents time
• y-axis represents whatever is being measured
Spectrum Analyzer
• Device that creates graphs for signal analysis
• x-axis represents frequency
• y-axis represents whatever is being measured
Noise
• undesirable signals
• can originate from natural or technological
sources
• added to data signals
Sources of Noise
• Nearby cables carrying data signals
• Thermal noise – generated by thermal agitation
of electrons in a conductor
• RFI – from other signals being transmitted
nearby
• EMI – from nearby motors and lights
• Laser noise at transmitter or receiver
White Noise
• Noise that affects all frequencies equally
• All data transmissions affected on a LAN
Narrowband Interference
• Small range of frequencies affected
• May affect only certain signals on a LAN
Bandwidth
• Analog – frequency range of a radio station or
electronic amplifier
• Units of measure
• frequency
• cycles per second
• hertz
Digital Bandwidth
• Measures how much information can flow from
one place to another in a given amount of time.
• Unit of measure
• bits per second
• kbps
• Mbps
Electronic Signal
• < 5 volts
• Transmitting/receiving devices must be properly
grounded so that the reference point for the
signal is zero volts.
Attenuation
• Decrease in signal amplitude over the length of
a link
• Contributors to attenuation
• Long cable runs
• High signal frequencies
• Resistance of copper cable converts some of the
electrical signal into heat
• Leaks through insulation
• Defective connectors
Jitter
• Slight movement of a signal in time or phase
that can introduce errors and loss of
synchronization
• Caused by:
• long cable runs
• high data rates
Crosstalk
• Transmission of signals from one wire pair to
nearby pairs
• Can be within the same cable or from a nearby
cable (alien crosstalk)
Types of Crosstalk
• Near-end crosstalk (NEXT)
• Far-end crosstalk (FEXT)
• Power sum near-end crosstalk (PSNEXT)
NEXT
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Closest to the transmitter
Check both ends of the cable
#1 reason: Too much untwisting
#2 reason: Split pairs
Measured in decibels
Larger the negative number, the lower the NEXT
Displays on the tester as positive
FEXT
• Further away from the transmitter
• Less noise than NEXT because of attenuation
PSNEXT
• Cumulative effects of NEXT on each wire pair
by the other three wire pairs
• Very important in Gigabit Ethernet where all
pairs are used.
ELFEXT
• Equal level far end crosstalk measures FEXT
between pairs
• FEXT = 45 dB + Attenuation = 11 dB
• ELFEXT = 45 - 11 = 34 dB
This is the effect on
the disturbing pair.
This how
much it affects
the adjacent
pair
PSELFEXT
• Power sum equal level far end crosstalk
• Combined effect of ELFEXT of the three other
wire pairs on each pair
Insertion Loss
• Combination of the effects of attenuation and
resistance on the cable segment
• Amount of energy lost as the signal reaches the end of
the cable.
• High frequency = greater resistance
• Long cable runs
• Poorly terminated connectors
• Temperature absorption by some types of cable (usu.
those with PVC)
• Measured in decibels
Propagation Delay
• Measure of how long it takes a signal to travel
from one end of the cable & back
• Tested on a TDR
• Time domain reflectometer
• Length the signal travels is further than cable
length (wires are twisted).