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Wire Propagation Effects

• Propagation Effects– Signal changes as it travels– If change is too great, receiver may not be able

to recognize it

Distance

OriginalSignal

FinalSignal

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Wire Propagation Effects: Attenuation

• Attenuation: Signal Gets Weaker as it Propagates– May become too weak for receiver to

recognizeSignal

Strength

Distance

3

Wire Propagation Effects: Distortion

• Distortion: Signal changes shape as it propagates– 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

SignalStrength

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 if

the average noise level is constant

SignalStrength

Distance

Signal

Noise Floor

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Wire Propagation Effects: SNR

• Want a high Signal-to-Noise Ratio (SNR)– Signal strength divided by average noise

strength– As SNR falls, errors increase

SignalStrength

Distance

Signal

Noise FloorSNR

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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 BitNoiseSpike

Average NoiseDuring Bit

Low Speed(Long

Duration)

One BitNoiseSpike

Average NoiseDuring Bit

High Speed(Short

Duration)

OK Error

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Wire Propagation Effects: Interference

• Interference– Energy from outside the wire (nearby motors, other

wires, etc.)– Adds to signal, like noise– Often intermittent (comes and goes), so hard to

diagnose

SignalStrength

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 nearby

wires

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Wire Propagation Effects:Cross Talk

• 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:Cross Talk

• 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 (1/2

inch)– Usually the biggest propagation effect

TerminalCross Talk

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Practical Issues in Propagation Effects

• Distance limits in standards prevent serious 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 voice and not data (2 wire pairs RJ-11)

• Cat 2 - certified for transmissions of 4Mpbs (not used in networks 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 because

transmission 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 200

Mbps• 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 different distances (different number of reflections)

– Different ways of traveling are called modes

– Light modes from successive bits will begin to overlap given enough distance, making the bits unreadable

LightSource

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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)

LightSource

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) no distortion

– Longer propagation distance (usually up to 2 km for LAN 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 long distances

– Expensive to produce

– Expensive to install (fragile, precise alignments needed)

– 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 of fraction (step index multimode fiber)

– Dominates multimode fiber today

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Multimode Optical Fiber and Frequency

• Signal Frequency Determines the Propagation Distance before Mode Problems Become Serious

• Short Wavelength (high frequency) – Signals do not travel as far before mode problems

occur– 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 different frequencies– 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 within and between buildings in a site network

OpticalFiber

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Wire Media: UTP and Optical Fiber

• The emerging pattern: UTP from first hub or switch 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

InnerWire

Outer Conductor Wrapped in PVC

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Wire Media: Coaxial Cable

• Coaxial Cable

– Installed widely today in old 10 Mbps Ethernet LANs

• 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 can transmit simultaneously– Even if only one sends, still full-duplex line– Even if neither is sending, still full-duplex line

A B

Time 1Both can send

Both do

A B

Time 1Both can sendOnly A does

A B

Time 1Both can sendNeither does

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Duplex

• Half-duplex transmission: only one can transmit at a time; must take turns– Still half duplex if neither transmits

A B A B

Time 1Only one side

Can sendA does

Time 2Only one side

Can sendNeither does

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Duplex

• Duplex is a Characteristic of the Transmission System, Not of Use at a Given Moment

– In full duplex, both sides can transmit at once; in half duplex, only one side can transmit at a time

– Still full duplex system if only one side or neither side actually is transmitting at a moment

– Still half duplex if neither side actually is transmitting at a moment

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Wireless Transmission

• Infrared -uses light beams to send signals between pairs of devices– Direct - transmitter and receiver are within

line-of-sight of each other• ex. Laptop and printer in same room

– Indirect - signal bounces off walls , ceilings etc.

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Radio Propagation

• Broadcast signal– Not confined to a wire

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Radio Waves

• When Electron Oscillates, Gives Off Radio Waves– Single electron gives a very weak signal– Many electrons in an antenna are forced to

oscillate 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 interfere with 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 Second7 Cycles

1 Hz = 1 cycle per second

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Radio Propagation: Frequency Spectrum

• Frequency Spectrum– Frequencies vary (like strings in a harp)

– Frequencies measured in hertz (Hz)

– Frequency spectrum: all possible frequencies from 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: Service Bands

• Service Bands– Divide spectrum into bands for services– A band is a contiguous range of frequencies– FM radio, cellular telephone service bands etc.

0 Hz

Cellular Telephone

FM Radio

AM Radio

ServiceBands

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Radio Propagation: Channels and Bandwidth

• Service Bands are Further Divided into Channels– Like television channels

– Bandwidth of a channel is highest frequency minus lowest frequency

0 Hz

Channel 3

Channel 2

Channel 1

ServiceBand

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Radio Propagation: Service Bands

• Service Bands– Divide spectrum into bands for services– A band is a contiguous range of frequencies– FM radio, cellular telephone service bands etc.

0 Hz

Cellular Telephone

FM Radio

AM Radio

ServiceBands

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Radio Propagation: Channels and 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

ServiceBand

ChannelBandwidth

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Radio Propagation: Channels and Bandwidth

• Shannon’s Equation– W is maximum possible (not actual)

transmission speed in a channel

– B is bandwidth of the channel: highest frequency minus lowest frequency

– S/N is the signal-to-noise ratioW = 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), the

faster the maximum possible transmission speed (W)

– W = B Log2 (1+S/N)

MaximumPossible

Speed

Bandwidth

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Telephony is Narrowband

• Bandwidth in Telephone Channels is Narrow

– Sounds below about 300 Hz cut off to reduce equipment hum within telephone system

– Sounds above about 3,400 Hz cut off to reduce the bandwidth needed to send a telephone signal

20 kHz300 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 transmission speed of about 35 kbps

20 kHz300 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 single channel AND the bandwidth of the channel is large

– Therefore, maximum possible transmission speed is high

• Popularly, if the signal is fast, the system is called “broadband” whether it uses channels at all