Physical Layer Propagation

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Physical Layer Propagation

• Sri Sharma
• Oakland University
• Winter 2004

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Topics

• Analog, Binary, and Digital
• Digital transmission on an analog medium
• Analog transmission on a digital medium
• UTP media and propagation effects
• UTP
• Noise and attenuation
• EMI, Crosstalk EMI, and Terminal crosstalk EMI
• Serial and parallel transmission

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Topics

• Optical fiber and propagation effects
• Optical fiber
• WDM and DWDM
• Half-duplex and full-duplex
• Single and multi-mode
• Radio transmission and propagation effects
• Antennas
• Frequency spectrum, service bands, and channels
• Wireless propagation problems
• Topology
• Point-to-point, start, mesh, bus, ring

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Signal and Propagation
Received Signal (Attenuated Distorted) Because
of Propagation Effects
Transmitted Signal
Propagation
Transmission Medium
Sender
Receiver
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Analog, Binary, and Digital

• Analog and binary data and analog, binary, and
  digital signals

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Analog and Binary Data
Binary Data
Analog Data
1101011000011100101
Smoothly changing among an infinite number of
states (loudness levels, etc.)
Two statesOne state represents 1The other
state represents 0
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Quiz

• Which is Analog? Which is Binary?

Gender
Thermometer
On/Off Switch
Clock
8
Binary Data and Binary Signal
15 Volts (0)
There are two states (in this case, voltage
levels). One, (high) represents a 0. The
other (low) represents a 1.
0
0
0 Volts
1
Transmitted Signal
-15 Volts (1)
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Binary Data and Binary Signal
Time is divided into clock cycles The 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.
15 Volts (0)
Clock Cycle
0
0
0 Volts
1
Transmitted Signal
-15 Volts (1)
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Binary Data and Digital Signal
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11
10
10
01
01
01
00
Client PC
00
Server
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Quiz

• Which is Analog? Which is Digital?

On/Off Switch
Number Of Fingers
Clock
Calendar
Audio CD
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Baud Rates for Digital Signals
Baud Rate of Clock Cycles/Second
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11
10
10
01
01
01
00
Client PC
00
Server
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.)
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Bit Rate versus Baud Rate
Number of Possible States
Bits per Clock Cycle
If a Baud Rate is 1,200 Baud, Bit Rate is
2 (Binary)
1
1,200 bps
4
2
2,400 bps
8
3
3,600 bps
16
4
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
  stateslike 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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Sending Digital Data Over an Analog Line
Modulated Analog Signal
Telephone
Binary Data
1010010101
PSTN
Modem
Computer
Amplitude (Loudness or Intensity) Modulation
1
0
1
1
1011 becomes loud-soft-loud-loud
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Sending Digital Data Over an Analog Line
Modulated Analog Signal
Demodulated Binary Data
Telephone
1010010101
PSTN
Modem
Computer
Amplitude (Loudness or Intensity) Modulation
1
0
1
1
Loud-soft-loud-loud becomes 1011
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Sending Analog Data Over a Digital Line
Analog Data
Digital Signal 110010101 (Binary Example)
Encoding
Analog Data Source
Digital Transmission Line
Codec
Analog Data
Digital Signal100001101 (Binary Example)
Decoding
Many Time Periods So Fairly Smooth
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Data and Signals Modems Vs. Codecs
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UTP Media and Propagation Effects
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4-Pair Unshielded Twisted Pair Cable with RJ-45
Connector
Four pairs (each pair is twisted)
Single Twisted Pair
There is insulation around each wire.
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4-Pair Unshielded Twisted Pair Cable with RJ-45
Connector
A length of UTP is called a cord. There is no
metal shielding around The individual pairs or
around the entire Cord. Hence the name
unshielded UTP
UTP Cord
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4-Pair Unshielded Twisted Pair Cable with RJ-45
Connector
The cord terminates in an 8-pin RJ-45 connector,
which plugs into an RJ-45 jack in the NIC,
switch Or wall jack.
Pin 1 on left of Jack
8-Pin RJ-45 Connector
RJ-45 Jack
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4-Pair Unshielded TwistedPair Cable with RJ-45
Connector
UTP Cord
RJ-45 Connector
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4-Pair Unshielded TwistedPair Cable with RJ-45
Connector
With RJ-45 Connector
Pen
4 Pairs Separated
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Noise and Attenuation
Power
Noise Floor (average)
Signal
Distance
Noise
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Noise and Attenuation
Power
Noise Spike
Signal
Noise Floor (average)
Damage
Distance
Noise
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Noise and Attenuation
SNR Signal Power / Noise Power If SNR is high,
noise errors are rare As signal travels, it
attenuates, and noise errors increase
Power
Signal
Signal- to-Noise Ratio (SNR)
Noise Floor (average)
Noise
Distance
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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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Twisting Wire Paris to ReduceElectromagnetic
Interference (EMI)
Interference
Interference On the Two Halves of a Twist Cancels
Out
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Crosstalk Electromagnetic Interference (EMI) and
Terminal Crosstalk Interference
Signal
Crosstalk Interference
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Crosstalk Electromagnetic Interference (EMI) and
Terminal Crosstalk Interference
Untwisted at Ends
Signal
Crosstalk Interference
Terminal Crosstalk Interference
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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 interferencea 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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Serial versus Parallel Transmission
Serial Transmission (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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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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Optical Fiber Cabling
Cladding
Core
Light Ray
Reflection at Core/Cladding Boundary
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Wavelength Division Multiplexing (WDM) in Optical
Fiber
Light Source 1
Optical Fiber Core
Light Source 2
Multiple Light Sources Transmit on Different
Wavelengths Each Carries a Separate Signal More
Capacity Per Fiber
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Full-Duplex Optical Fiber Cord
SC, ST, or other connector
Fiber Cord
Fiber Cord
Switch
Router
A pair of fibers is needed for full-duplex
(simultaneous 2-way) transmission. Each fiber
carries a signal in only one direction.
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Optical Fiber Cabling
Two fiber cords for full-duplex (two-way)
transmission
SC Connectors (Recommended)
ST Connectors (Popular)
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Multimode Single-Mode Fiber
Light Source
Modes
Multimode Fiber
Light only travels in one of several allowed
modesLight travels faster at the edges to speed
modes going the farthestMultimode fiber must
keep its distance short or limit modal
distortionMultimode fiber goes a few hundred
meters and is inexpensive to layIt is dominant
in LANs
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Multimode Single-Mode Fiber
Signals Travel Fastest On Outside of Core
Graded Index of Refraction (Decreasing from
Center)
Light Source
Modes
Graded Index Multimode Fiber
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Multimode Single-Mode Fiber
Single Mode
Light Source

Single Mode Fiber
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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Omnidirectional and Dish Antennas
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Radio Wave
Wavelength
Amplitude
Frequency Measured in Hertz (Cycles per Second) 2
Cycles in one Second, so 2 Hz
Wavelength Frequency Speed of Propagation
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The Frequency Spectrum, Service Bands, and
Channels
Frequency Spectrum (0 Hz to infinity)
A service band has a specific purpose, such as FM
radio or cellular telephony. Service bands are
divided into channels. Signals sent in different
channels do not interfere with one another. Chann
els with wider bandwidths can carry signals
faster.
Channel 5
Channel 4
Service Band
Channel 3
Channel 2
Channel 1
0 Hz
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The Frequency Spectrum, Service Bands, and
Channels

• Shannons 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)
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Wireless Propagation Problems
Inverse Square Law Attenuation
Laptop
Comm. Tower
Very Rapid Attenuation with Distance Compared to
Wires and Fiber
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Wireless Propagation Problems
Multipath Interference
Shadow ZoneNo Signal
Laptop
Comm. Tower
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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Major Topologies

• A network technologys topology is the order in
  which stations are connected to one another via
  media.

The Simplest Topology
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Major Topologies
Star (Modern Ethernet)
RootSwitch
Switch
Switch
Only one possible path between two stations
Switch
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Hierarchical Star
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Major Topologies
A
Path ABD
B
C
D
Multiple alternative paths between two stations
Path ACD
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Major Topologies
Ring (802.5, FDDI, SONET/SDH)
Only one possible path between two stations
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Major Topologies
Multidrop Line Bus (Ethernet 10Base5)
Daisy Chain Bus (Ethernet 10Base2)
Transmit
Transmit
All stations hear each transmissionOnly one
possible path between two stations
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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