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Wire Propagation Effects
•Propagation Effects
–Signal changes as it travels
–If change is too great, receiver may not beable to recognize it
Distance
Original
Signal
Final
Signal
2
Wire Propagation Effects:Attenuation
•Attenuation: Signal Gets Weaker as itPropagates
–May become too weak for receiver torecognize
Signal
Strength
Distance
3
Wire Propagation Effects:Distortion
•Distortion: Signal changes shape as itpropagates
–Adjacent bits may overlap
–May make recognition impossible for receiver
Distance
4
Wire Propagation Effects: Noise
•Noise: Thermal Energy in Wire Adds to Signal
–Noise floor is average noise energy
–Random energy, so noise spikes sometimes occur
Signal
Strength
Time
Signal
Noise
Spike
Noise Floor
Error
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Wire Propagation Effects
•Noise and Attenuation
–As signal attenuates, gets closer to noise floor
–So noise errors increase with distance, even ifthe average noise level is constant
Signal
Strength
Distance
Signal
Noise Floor
6
Wire Propagation Effects: SNR
•Want a high Signal-to-Noise Ratio (SNR)
–Signal strength divided by average noisestrength
–As SNR falls, errors increase
Signal
Strength
Distance
Signal
Noise Floor
SNR
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Wire Propagation Effects: Noise& Speed
•Noise and Speed
–As speed increases, each bit is briefer
–Noise fluctuations do not average out as much
–So noise errors increase as speed increases
One Bit
Noise
Spike
Average Noise
During Bit
Low Speed
(Long
Duration)
One Bit
Noise
Spike
Average Noise
During Bit
High Speed
(Short
Duration)
OK
Error
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Wire Propagation Effects:Interference
•Interference
–Energy from outside the wire (nearby motors, otherwires, etc.)
–Adds to signal, like noise
–Often intermittent (comes and goes), so hard todiagnose
Signal
Strength
Time
Signal
Interference
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Wire Propagation Effects: Cross-Talk Interference
•Cross-Talk Interference
–Often, multiple wires in a bundle
–Each radiates some of its signal
–Causes “cross-talk” interference in nearbywires
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Wire Propagation Effects:CrossTalk
•Wire Usually is Twisted
–Usually, several twists per inch
–Interference adds to signal over half twist,subtracts over other half
–Roughly cancels out
–Simple but effective
Single Twist
Interference
-
+
Signal
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Wire Propagation Effects:CrossTalk
•Terminal Cross-Talk Interference
–Wire must be untwisted at ends to fit into connectors
–So cross-talk interference is high at termination
–Problems severe if untwist more than about 1.25 cm (1/2inch)
–Usually the biggest propagation effect
Terminal
Cross Talk
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Practical Issues in PropagationEffects
•Distance limits in standards preventserious propagation effects
–For instance, usually 100 meters (328 feet)maximum for ordinary copper wire
•Problems usually occur at connectors
–Crossed wires
–Poor connections
–Cross-talk interference
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Wire Media: UTP to the Desktop
•UTP
–Dominant for line from desktop to first switch
–Inexpensive to buy and install
–Rugged: can take punishment of office work
–Easily 100 Mbps, 1 Gbps with careful insulation
UTP
First Hub or Switch
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Categories of UTP
•Cat 1 - traditional telephone cabling designed for voiceand not data (2 wire pairs RJ-11)
•Cat 2 - certified for transmissions of 4Mpbs (not used innetworks where typical transmissions are >=10Mbps
•Cat 3 - certified for transmission up to 10Mbps
–in 8 wire pair with RJ-45
–not recommended for new installations becausetransmission speeds are > 10 Mbps ~ 100 Mbps
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Categories of UTP
•Cat 4 - certified for transmissions up to 16-20Mbps
•Cat 5 - certified for transmission up to 100 Mbps
–typical cable in use today
•Enhanced Cat 5 - more twists transmission up to 200Mbps
•Cat 6 - extra foil insulation
–supports up to 600Mbps
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Wire Media: Optical Fiber
•Limited by Distortion
–Light entering at different angles travels differentdistances (different number of reflections)
–Different ways of traveling are called modes
–Light modes from successive bits will begin to overlapgiven enough distance, making the bits unreadable
Light
Source
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Wire Media: Optical Fiber
•Multimode Fiber
–Wide core makes easy to splice (50 or 62 microns)
–Many angles for rays (modes)
–Short propagation distance (usually 200 m to 500 m)
Light
Source
Md B
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Wire Media: Optical Fiber
•Single Mode Fiber
–Narrow core difficult to splice (5 or 8 microns)
–Only one angle for rays (one mode), so (almost) nodistortion
–Longer propagation distance (usually up to 2 km forLAN fiber, longer for long-distance fiber)
–Narrow core makes fiber fragile and difficult to splice
Mod B
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Single Mode Fiber
•Single Mode Fiber is very thin
–Only one mode will propagate even over fairly longdistances
–Expensive to produce
–Expensive to install (fragile, precise alignmentsneeded)
–Used by carriers to link distant switches
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Multimode Fiber
•Core is thick
–Modes will appear even over fairly short distances
–Must limit distances to a few hundred meters
–Inexpensive to purchase and install
–Dominates LANs
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Graded Index Multimode Fiber
•Index of fraction is not constant in core
–Varies from center to edge
–Reduces time delays between different modes
–Can go farther than if core has only a single index offraction (step index multimode fiber)
–Dominates multimode fiber today
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Multimode Optical Fiber andFrequency
•Signal Frequency Determines the PropagationDistance before Mode Problems Become Serious
•Short Wavelength (high frequency)
–Signals do not travel as far before mode problemsoccur
–Uses the least expensive light sources
–Good for LAN use within buildings
•Long Wavelength (low frequency)
–Signals travel farther but light sources cost more
–Within large buildings and between buildings
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Wave Division Multiplexing
•Use multiple light sources of differentfrequencies
–Place a separate signal on each
–Increases the capacity of the optical fiber
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Wire Media: Optical Fiber
•Optical Fiber
–High speeds over long distances
•200 m to 2 km
–Costs more than UTP, but worth it on long runs
–Good for all links between hubs and switches withinand between buildings in a site network
Optical
Fiber
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Wire Media: UTP and OpticalFiber
•The emerging pattern: UTP from first hub orswitch to desk, Fiber everywhere else on site
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Wire Media: Coax
•Coaxial Cable
–Used in cable TV, VCRs
–Central wire, external concentric cylinder
–Outer conductor wrapped in PVC
Screw-On Connector
Inner
Wire
Outer Conductor Wrapped in PVC
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Wire Media: Coaxial Cable
•Coaxial Cable
–Installed widely today in old 10 Mbps EthernetLANs
•ThinNet and ThickNet
–Not being used in new installations
•Optical fiber more cost-effective for long links
•UTP more cost-effective for desktop links
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Duplex
•Full-duplex transmission: both sides cantransmit simultaneously
–Even if only one sends, still full-duplex line
–Even if neither is sending, still full-duplex line
A
B
Time 1
Both can send
Both do
A
B
Time 1
Both can send
Only A does
A
B
Time 1
Both can send
Neither does
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Duplex
•Half-duplex transmission: only one cantransmit at a time; must take turns
–Still half duplex if neither transmits
A
B
A
B
Time 1
Only one side
Can send
A does
Time 2
Only one side
Can send
Neither does
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Duplex
•Duplex is a Characteristic of theTransmission System, Not of Use at aGiven Moment
–In full duplex, both sides can transmit atonce; in half duplex, only one side cantransmit at a time
–Still full duplex system if only one side orneither side actually is transmitting at amoment
–Still half duplex if neither side actually istransmitting at a moment
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Wireless Transmission
•Infrared -uses light beams to send signalsbetween pairs of devices
–Direct - transmitter and receiver are withinline-of-sight of each other
•ex. Laptop and printer in same room
–Indirect - signal bounces off walls , ceilingsetc.
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Radio Propagation
•Broadcast signal
–Not confined to a wire
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Radio Waves
•When Electron Oscillates, Gives OffRadio Waves
–Single electron gives a very weak signal
–Many electrons in an antenna are forced tooscillate in unison to give a practical signal
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Radio Propagation Problems
•Radio Propagation is Difficult
–Signals are reflected
–May arrive at a destination via multiple paths
–Signals arriving by different paths can interferewith one another
–This is called multipath interference
Nw
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Radio Propagation Problems
•Wires Propagation is Predictable
–Signals go through a fixed path: the wire
–Propagation problems can be easily anticipated
–Problems can be addressed easily
•Radio Propagation is Difficult
–Signals begin propagating as a simple sphere
–Inverse square law attenuation
–If double distance, only ¼ signal strength
–If triple distance only 1/9 signal strength
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Radio Propagation: Waves
•Waves
Amplitude
(strength)
Wavelength
(meters)
Frequency in hertz (Hz)
Cycles per Second
One Second
7 Cycles
1 Hz = 1 cycle per second
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Radio Propagation: FrequencySpectrum
•Frequency Spectrum
–Frequencies vary (like strings in a harp)
–Frequencies measured in hertz (Hz)
–Frequency spectrum: all possible frequenciesfrom 0 Hz to infinity
0 Hz
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Frequencies
•Metric system
–kHz (1,000 Hz) kilohertz; note lower-case k
–MHz (1,000 kHz) megahertz
–GHz (1,000 MHz) gigahertz
–THz (1,000 GHz) terahertz
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Radio Propagation: ServiceBands
•Service Bands
–Divide spectrum into bands for services
–A band is a contiguous range of frequencies
–FM radio, cellular telephone service bandsetc.
0 Hz
Cellular Telephone
FM Radio
AM Radio
Service
Bands
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Radio Propagation: Channelsand Bandwidth
•Service Bands are Further Divided intoChannels
–Like television channels
–Bandwidth of a channel is highest frequencyminus lowest frequency
0 Hz
Channel 3
Channel 2
Channel 1
Service
Band
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Radio Propagation: ServiceBands
•Service Bands
–Divide spectrum into bands for services
–A band is a contiguous range of frequencies
–FM radio, cellular telephone service bandsetc.
0 Hz
Cellular Telephone
FM Radio
AM Radio
Service
Bands
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Radio Propagation: Channelsand Bandwidth
•Example
–Highest frequency of a radio channel is 43 kHz
–Lowest frequency of the radio channel is 38 kHz
–Bandwidth of radio channel is 5 kHz (43-38 kHz)
0 Hz
Channel 3
Channel 2
Channel 1
Service
Band
Channel
Bandwidth
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Radio Propagation: Channelsand Bandwidth
•Shannon’s Equation
–W is maximum possible (not actual)transmission speed in a channel
–B is bandwidth of the channel: highestfrequency minus lowest frequency
–S/N is the signal-to-noise ratio
W = B Log2 (1 + S/N)
D = 2 B Log2 K (Nyquist’s Theorem)
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Radio Transmission: Broadband
•Speed and Bandwidth
–The wider the channel bandwidth (B), thefaster the maximum possible transmissionspeed (W)
–W = B Log2 (1+S/N)
Maximum
Possible
Speed
Bandwidth
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Telephony is Narrowband
•Bandwidth in Telephone Channels isNarrow
–Sounds below about 300 Hz cut off to reduceequipment hum within telephone system
–Sounds above about 3,400 Hz cut off toreduce the bandwidth needed to send atelephone signal
20 kHz
300 Hz
3.4 kHz
3.1 kHz
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Telephony is Narrowband
•Bandwidth in Telephone Channels is Narrow
–A radio channel would have to be from 0 to 3.4 kHz(3.4 kHz)
–This would mean a maximum possible transmissionspeed of about 35 kbps
20 kHz
300 Hz
3.4 kHz
3.1 kHz
Required Radio Channel
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Broadband
•Two Uses of the Term “Broadband”
•Technically, the signal is transmitted in a singlechannel AND the bandwidth of the channel is large
–Therefore, maximum possible transmission speedis high
•Popularly, if the signal is fast, the system is called“broadband” whether it uses channels at all