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Physical Layer Propagation
Chapter 3 (Revised August 2002)
Copyright 2003 Prentice-HallPanko’s Business Data Networks and Telecommunications, 4th edition
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The Physical Layer
Chapter 2 – StandardsStandards above the physical layer
Chapter 3 – The Physical Layer
Real connections between machines
No messages
Propagation effects that change the signal as it propagates over the transmission medium
Perspective
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Figure 3.1: Signal and Propagation
Sender
Transmitted Signal
Transmission Medium
Received Signal(Attenuated &
Distorted)Because of
Propagation Effects
Receiver
Propagation
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Analog, Binary, and Digital
Analog and binary data and analog, binary, and digital signals
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Figure 3.2: Analog and Binary Data
Binary Data
1101011000011100101
Analog Data
Smoothly changing among an infinite number of states (loudness levels,
etc.)
Two states:One state represents 1
The other state represents 0
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Quiz
Which is Analog? Which is Binary?
Gender
Clock
On/OffSwitch
Thermometer
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Figure 3.3: Binary Data and Binary Signal
15 Volts(0)
0 Volts
-15 Volts(1)
TransmittedSignal
0 0
1
There are two states (in this case,voltage levels).
One, (high) represents a 0. The other (low) represents a 1.
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Figure 3.3: Binary Data and Binary Signal
15 Volts(0)
Clock Cycle
0 Volts
-15 Volts(1)
TransmittedSignal
0 0
1
Time is divided into clock cyclesThe State is held constant
within each clock cycle.It can jump abruptly at the end of
each cycle. One bit is sent per clock cycle.
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Figure 3.4: Binary Data and Digital Signal
10
11
00
01 01
11
10
01
00Client PC Server
In binary transmission, there are two states.In digital transmission, there are few states (in this case, four).
With four states, two information bits can be sent per clock cycle.
00, 01, 10, and 11 Binary transmission is a special case of digital transmission.
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Quiz
Which is Analog? Which is Digital?
Calendar Clock
NumberOf
Fingers
Audio CD
On/OffSwitch
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Figure 3.4: Baud Rates for Digital Signals
10
11
00
01 01
11
10
01
00Client PCServer
Suppose that the clock cycle is 1/10,000 second.Then the baud rate is 10,000 baud (10 kbaud).
The bit rate will be 20 kbps (two bits/clock cycle times 10,000 clock cycles per second).
(The bit rate gives the number of information bits per second.)
Baud Rate =# of Clock Cycles/Second
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Bit Rate versus Baud Rate
Number of Possible States
Bits per ClockCycle
2 (Binary)
4
8
16
1
2
3
4
If a Baud Rate is 1,200 Baud,Bit Rate is
1,200 bps
2,400 bps
3,600 bps
4,800 bps
Each Doubling of States Gives One More Bit per Clock Cycle
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Perspective
Analog Data Smooth changes among an infinite number of
states—like hands going around an analog clock
Digital Data Few states
In a digital clock, each position can be in one of ten states (the digits 0 through 9)
Binary Data Two states (a special case of digital)
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Figure 3.5: Using a Modem to Send Binary Data Over an Analog Transmission Line
Computer
Modem
Telephone
PSTN
Modulated AnalogSignal
1 0 1 1
Amplitude (Loudness or Intensity) Modulation
1010010101
Binary Data
1011 becomes loud-soft-loud-loud
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Figure 3.5: Using a Modem to Send Binary Data Over an Analog Transmission Line
Computer
Modem
Telephone
PSTN
Modulated AnalogSignal
1 0 1 1
Amplitude (Loudness or Intensity) Modulation
1010010101
DemodulatedBinary Data
Loud-soft-loud-loud becomes 1011
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Figure 3.6: Sending Analog Data Over a Digital Line
AnalogData
Source
DigitalTransmission
Line
Encoding
Decoding
Digital Signal110010101
(Binary Example)
Digital Signal100001101
(Binary Example)
Many Time PeriodsSo Fairly Smooth
Codec
Analog Data
Analog Data
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Data and Signals: Modems Vs. Codecs
Analog Line Signal Digital Line Signal
Analog data Codec
Digital data (including binary
data)Modem
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UTP Media and Propagation Effects
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Figure 3.7: 4-Pair Unshielded Twisted Pair Cable with RJ-45 Connector
Single Twisted Pair
Jacket
Four pairs (each pair is twisted)
There is insulation around each wire.
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Figure 3.7: 4-Pair Unshielded Twisted Pair Cable with RJ-45 Connector
A length of UTP is called a cord.
There is no metal shielding aroundThe individual pairs or around the entire Cord. Hence the name unshielded UTP
UTP Cord
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Figure 3.7: 4-Pair Unshielded Twisted Pair Cable with RJ-45 Connector
The cord terminates in an 8-pinRJ-45 connector, which plugsinto an RJ-45 jack in the NIC, switchOr wall jack.
Pin 1 on left of Jack
RJ-45Jack
8-PinRJ-45
Connector
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Figure 3.7: 4-Pair Unshielded TwistedPair Cable with RJ-45 Connector
RJ-45Connector
UTP Cord
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Figure 3.7: 4-Pair Unshielded TwistedPair Cable with RJ-45 Connector
WithRJ-45
Connector
4 PairsSeparated
Pen
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Figure 3.8: Noise and Attenuation
Distance
Signal Noise Floor(average)
Noise
Power
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Figure 3.8: Noise and Attenuation
Distance
Signal Noise Spike
Noise
Power
DamageNoise Floor(average)
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Figure 3.8: Noise and Attenuation
Distance
Signal
Noise Floor(average)
Noise
Signal-to-Noise
Ratio(SNR)
PowerSNR = Signal Power / Noise PowerIf SNR is high, noise errors are rare
As signal travels, it attenuates, and noise errors increase
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Noise and Attenuation
The TIA/EIA-568 standard recommends that UTP runs be kept to 100 meters
If this distance limit is observed, problems with noise and attenuation usually are minor
Low-tech solution, but it works well.
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Figure 3.9: Twisting Wire Paris to ReduceElectromagnetic Interference (EMI)
Interference On the Two Halves of a Twist Cancels Out
Interference
TwistedWire
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Figure 3.10: Crosstalk Electromagnetic Interference (EMI) and Terminal Crosstalk Interference
Signal
Crosstalk Interference
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Figure 3.10: Crosstalk Electromagnetic Interference (EMI) and Terminal Crosstalk Interference
Untwistedat Ends Signal
Terminal CrosstalkInterference
Crosstalk Interference
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Figure 3.10: Crosstalk Electromagnetic Interference (EMI) and Terminal Crosstalk Interference
EMI is any interference Signals in adjacent pairs interfere with one another
(crosstalk interference) is a specific type of EMI.
Crosstalk interference is worst at the ends, where the wires are untwisted. This is terminal crosstalk interference—a specific type of crosstalk EMI.
Solution: untwist wires for connector no more than 1.25 cm (0.5 in). Does not eliminate terminal cross-talk interference
but makes it negligible
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Limiting UTP Propagation Problems
Two simple things can limit UTP propagation problems Limit cord distances to 100 meters to control
attenuation and noise effects
Limit the untwisting of wires at the connectors to 1.25 cm (0.5 inch) to control terminal crosstalk interference.
If these rules are followed strictly, propagation problems should be negligible
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Figure 3.11: Serial versus Parallel Transmission
SerialTransmission
(1 bit per clock cycle)
Parallel Transmission(1 bit per clock cycle
per wire pair)4 bits per clock cycle
on 4 pairs
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Figure 3.11: Serial versus Parallel Transmission
Serial Transmission: one bit per clock cycle if binary transmission
Parallel Transmission with N wire pairs: N bits per clock cycle if binary transmission Not limited to four wire pairs (can be 2, 8, 100, etc.)
The advantage of parallel transmission is that it is faster than serial transmission
Only works over very short distances.
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Optical Fiber Media and Propagation Effects
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Figure 3.12: Optical Fiber Cabling
LightSource(LED orLaser)
Cladding
Core
LightRay
Reflection at Core/Cladding Boundary
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Figure 3.13: Wavelength Division Multiplexing (WDM) in Optical Fiber
LightSource 2
LightSource 1
Optical Fiber Core
Multiple Light Sources Transmit on Different WavelengthsEach Carries a Separate Signal
More Capacity Per Fiber
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Figure 3.14:Full-Duplex Optical Fiber Cord
Switch Router
Fiber Cord
Fiber Cord
A pair of fibers is needed for full-duplex (simultaneous 2-way) transmission.Each fiber carries a signal in only one direction.
SC, ST, or otherconnector
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Optical Fiber Cabling
STConnectors(Popular)
SCConnectors
(Recommended)
Two fiber cords for full-duplex (two-
way) transmission
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Figure 3.15: Multimode & Single-Mode Fiber
LightSource
Core
Cladding
Multimode Fiber
Modes
Light only travels in one of several allowed modesLight travels faster at the edges to speed modes going the farthest
Multimode fiber must keep its distance short or limit modal distortionMultimode fiber goes a few hundred meters and is inexpensive to lay
It is dominant in LANs
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Figure 3.15: Multimode & Single-Mode Fiber
LightSource
Core
Cladding
Graded Index of Refraction(Decreasing from Center)
Graded Index Multimode Fiber
Modes
Signals Travel FastestOn Outside of Core
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Figure 3.15: Multimode & Single-Mode Fiber
LightSource
Single Mode Fiber
Cladding
Core
Single Mode
Core is so thin that only one mode can propagate.
No modal dispersion, so can span long distances without distortion.
Expensive, so not widely used in LANs. Popular in WANs
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Multimode and Single-Mode Fiber
Multimode Limited distance (a few hundred meters)
Inexpensive to install
Dominates fiber use in LANs
Single-Mode Fiber Longer distances: tens of kilometers
Expensive to install
Commonly used by WANs and telecoms carriers
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Radio Transmission and Propagation Effects
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Figure 3.16: Omnidirectional and Dish Antennas
Dish Antenna
Concentrates incomingand outgoing signalsSignals can travel far
Omnidirectional Antenna
No need to pointto sender or receiverRapid attenuation
with distance
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Figure 3.17: Radio Wave
Amplitude
Wavelength
FrequencyMeasured in Hertz (Cycles per Second)
2 Cycles in one Second, so 2 Hz
Wavelength * Frequency = Speed of Propagation
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Figure 3.18: The Frequency Spectrum, Service Bands, and Channels
Channel 4
Channel 3
Channel 5
Channel 2
Channel 1
FrequencySpectrum
(0 Hzto infinity)
ServiceBand
0 Hz
A service band has a specificpurpose, such as FM radio orcellular telephony.
Service bands are divided intochannels. Signals sent in different channels do not interfere with oneanother.
Channels with wider bandwidthscan carry signals faster.
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Shannon’s Law
Here W = maximum possible speed in channel
B = bandwidth (highest frequency minus lowest frequency)
S/N = signal to noise ratio
Wide bandwidth (broadband) gives high speed
Small bandwidth (narrowband) gives low speeds
W = B * Log2 (1 + S/N)
Figure 3.18: The Frequency Spectrum, Service Bands, and Channels
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LaptopComm. Tower
Figure 3.19: Wireless Propagation Problems
Inverse SquareLaw Attenuation
Very Rapid Attenuation with DistanceCompared to Wires and Fiber
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LaptopComm. Tower
ShadowZone:
No Signal
Figure 3.19: Wireless Propagation Problems
MultipathInterference
Signals Arriving at SlightlyDifferent Times Can Interfere
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Golden Zone
At lower frequencies, there is little total bandwidth.
At very high frequencies, propagation is poor.
Mobile devices tend to work in the “golden zone” from the high megahertz to the low gigahertz range.
Frequencies in the golden zone are limited and in high demand.
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Topology
Transmit
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Figure 3.20: Major Topologies
A network technology’s topology is the order in which stations are connected to one another via media.
Point-to-Point
The Simplest Topology
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Figure 3.20: Major Topologies
Star (Modern Ethernet) Extended Star or Hierarchy(Modern Ethernet)
RootSwitch
Only one possible path between two
stations
Switch
Switch
Switch
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Figure 3.20: Major Topologies
Mesh (Routers, Frame Relay, ATM)
Multiple alternative paths between two
stations
AB
CD
PathABD
PathACD
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Figure 3.20: Major Topologies
Ring (802.5, FDDI, SONET/SDH)
Only one possible path between two
stations
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Figure 3.20: Major Topologies
Daisy Chain Bus(Ethernet 10Base2)
Multidrop Line Bus(Ethernet 10Base5)
All stations hear each transmissionOnly one possible path between two stations
Transmit Transmit
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Recap
Analog, Binary, and Digital: Data and Signals
Transmission Media UTP (limit distance and wire untwisting) Optical Fiber (multimode for most LAN use) Radio (freedom but weird propagation and limited
spectrum)
General Concepts Propagation effects Full duplex Serial versus parallel transmission
