Download The Physical Layer

Chapter 2
The Physical Layer
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The Theoretical Basis for Data
Communication
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Fourier Analysis
Bandwidth-Limited Signals
Maximum Data Rate of a Channel
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Bandwidth-Limited Signals
A binary signal and its root-mean-square Fourier amplitudes.
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(c) Successive approximations to the original signal.
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Bandwidth-Limited Signals (2)
(d) – (e) Successive approximations to the original signal.
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Bandwidth-Limited Signals (3)
Relation between data rate and harmonics.
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Guided Transmission Data
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Magnetic Media
Twisted Pair
Coaxial Cable
Fiber Optics
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Twisted Pair
(a) Category 3 UTP.
(b) Category 5 UTP.
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Coaxial Cable
A coaxial cable.
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Fiber Optics
(a) Three examples of a light ray from inside a silica fiber impinging
on the air/silica boundary at different angles.
(b) Light trapped by total internal reflection.
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Transmission of Light through Fiber
Attenuation of light through fiber in the infrared region.
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Fiber Cables
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(a) Side view of a single fiber.
(b) End view of a sheath with three fibers.
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Fiber Cables (2)
A comparison of semiconductor diodes and LEDs as light sources.
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Fiber Optic Networks
A fiber optic ring with active repeaters.
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Fiber Optic Networks (2)
A passive star connection in a fiber optics network.
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Wireless Transmission
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The Electromagnetic Spectrum
Radio Transmission
Microwave Transmission
Infrared and Millimeter Waves
Lightwave Transmission
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The Electromagnetic Spectrum
The electromagnetic spectrum and its uses for communication.
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Radio Transmission
(a) In the VLF, LF, and MF bands, radio waves follow the
curvature of the earth.
(b) In the HF band, they bounce off the ionosphere.
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Politics of the Electromagnetic Spectrum
The ISM bands in the United States.
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Lightwave Transmission
Convection currents can interfere with laser communication systems.
cn ch2A bidirectional system with two lasers is pictured here.
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Communication Satellites
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Geostationary Satellites
Medium-Earth Orbit Satellites
Low-Earth Orbit Satellites
Satellites versus Fiber
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Communication Satellites
Communication satellites and some of their properties,
including altitude above the earth, round-trip delay time
and number of satellites needed for global coverage. 21
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Communication Satellites (2)
The principal satellite bands.
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Communication Satellites (3)
VSATs using a hub.
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Low-Earth Orbit Satellites
Iridium
(a) The Iridium satellites from six necklaces around the earth.
(b)
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Globalstar
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(a) Relaying in space.
(b) Relaying on the ground.
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Public Switched Telephone System
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Structure of the Telephone System
The Politics of Telephones
The Local Loop: Modems, ADSL and Wireless
Trunks and Multiplexing
Switching
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Structure of the Telephone System
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(a) Fully-interconnected network.
(b) Centralized switch.
(c) Two-level hierarchy.
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Structure of the Telephone System (2)
A typical circuit route for a medium-distance call.
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Major Components of the
Telephone System
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Local loops
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Trunks
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Digital fiber optics connecting the switching
offices
Switching offices
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Analog twisted pairs going to houses and
businesses
Where calls are moved from one trunk to another
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The Politics of Telephones
The relationship of LATAs, LECs, and IXCs. All the
circles are LEC switching offices. Each hexagon
belongs to the IXC whose number is on it.
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The Local Loop: Modems,
ADSL, and Wireless
The use of both analog and digital transmissions for a computer to
computer call. Conversion is done by the modems and codecs.
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Modems
(a) A binary signal
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(b)cnAmplitude
modulation
(c) Frequency modulation
(d) Phase modulation
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Modems (2)
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(a) QPSK.
(b) QAM-16.
(c) QAM-64.
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Modems (3)
(a)
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(a) V.32 for 9600 bps.
(b) V32 bis for 14,400 bps.
(b)
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Digital Subscriber Lines
Bandwidth versus distanced over category 3 UTP for DSL.
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Digital Subscriber Lines (2)
Operation of ADSL using discrete multitone modulation.
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Digital Subscriber Lines (3)
A typical ADSL equipment configuration.
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Wireless Local Loops
Architecture of an LMDS system.
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Frequency Division Multiplexing
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(a) The original bandwidths.
(b) The bandwidths raised in frequency.
(b) The multiplexed channel.
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Wavelength Division Multiplexing
Wavelength division multiplexing.
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Time Division Multiplexing
The T1 carrier (1.544 Mbps).
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Time Division Multiplexing (2)
Delta modulation.
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Time Division Multiplexing (3)
Multiplexing T1 streams into higher carriers.
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Time Division Multiplexing (4)
Two back-to-back SONET frames.
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Time Division Multiplexing (5)
SONET and SDH multiplex rates.
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Circuit Switching
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(a) Circuit switching.
(b) Packet switching.
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Message Switching
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(a) Circuit
switching (b) Message switching (c) Packet switching
Packet Switching
Acncomparison
of circuit switched and packet-switched networks.
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The Mobile Telephone System
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First-Generation Mobile Phones:
Analog Voice
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Second-Generation Mobile Phones:
Digital Voice
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Third-Generation Mobile Phones:
Digital Voice and Data
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Advanced Mobile Phone System
(a) Frequencies are not reused in adjacent cells.
(b) To add more users, smaller cells can be used.
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Channel Categories
The 832 channels are divided into four categories:
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Control (base to mobile) to manage the system
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Paging (base to mobile) to alert users to calls
for them
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Access (bidirectional) for call setup and
channel assignment
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Data (bidirectional) for voice, fax, or data
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D-AMPS
Digital Advanced Mobile Phone System
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(a) A D-AMPS channel with three users.
(b) A D-AMPS channel with six users.
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GSM
Global System for Mobile Communications
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GSM uses 124 frequency channels, each of which
uses an eight-slot TDM system
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GSM (2)
A portion of the GSM framing structure.
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CDMA: Code Division Multiple Access
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Allow each station to transmit over entire frequency spectrum all the time.
Multiple simultaneous transmissions are separated using coding theory.
Colliding frames may not be totally garbled.
There are techniques to separate signals sent by different senders.
Similar to a party where different conversations use different language.
Extract desired signal and reject others as random noises.
Each bit time is subdivided into m short intervals called chips,
typically 64-128 chips per bit.
Each station is assigned a unique m-bit code or chip sequence.
To send a bit 1, a station sends its chip sequence.
To send a bit 0, a station sends the complement of its chip sequence.
For m=8, A is assigned 00011011.
A sends 00011011 as bit 1, and 11100100 as bit 0.
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Simple Analysis of CDMA
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Assume 1 MHz band for 100 stations
Use FDM, one station has 10kHz and 10 kbps
(assume 1 bit per Hz)
Use CDMA, one station has 1MHz, and 1Mchips per
seconds.
If CDMA uses less than 100 chips per bit
then CDMA will be more efficient.
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CDMA Coding Theory
•Let’s use bipolar notation +1 for binary 1 (chip signal), -1 for binary 0.
•A bit1, A send 00011011 or (-1 -1 -1 +1 +1 -1 +1 +1).
•Let S be the m-chip vector for station s and S for its negation.
m
•Two chip sequence S and T are orthogonal if S•T=0.
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S T 
• if S•T=0 then S•T =0
m
s t
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• All chip sequences must be pariwise orthogonal.
• S• S =-1
• CDMA Example
• Let S=A+B +C, S•C=(A+B+C)•C=A•C+B•C+C•C=0+0+1=1
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CDMA – Code Division Multiple Access
(a) Binary chip sequences for four stations
(b) Bipolar chip sequences
(c) Six examples of transmissions
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(d) Recovery of station C’s signal
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Walsh-Hadamard Matrix for
Orthogonal Spreading Sequence
a) The Walsh-Hadamard Matrix provides the Orthogonal Chip Sequences of
length n=2m.
Replacing 0 with signal -1 and 1 with signal +1.
b) It can be recursively constructed. W1=[0].
W2n=
where
contains complement elements of Wn.
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CDMA Sending
Channel 1: 110 -> +1+1+1 -> (-1,-1,-1,-1),(-1,-1,-1,-1),(+1,+1,+1,+1)
Channel 2: 010 -> -1+1-1 -> (+1,-1,+1,-1),(-1,+1,-1,+1),(+1,-1,+1,-1)
Channel 3: 001 -> -1-1+1 -> (+1,+1,-1,-1),(+1,+1,-1,-1),(-1,-1,+1,+1)
Sum Signal:
(+1,-1,-1,-3),(-1,+1,-3,-1),(+1,-1,+3,+1)
Chip Sequence
c1: (-1,-1,-1,-1)
c2: (-1,+1,-1,+1)
c3: (-1,-1,+1,+1)
c4: (-1,+1,+1,-1)
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CDMA Receiving/Decoding
Sum Signal:
(+1,-1,-1,-3),(-1,+1,-3,-1),(+1,-1,+3,+1)
Channel 2 Sequence: (-1,+1,-1,+1),(-1,+1,-1,+1),(-1,+1,-1,+1)
Correlator Output: (-1,-1,+1,-3),(+1,+1,+3,-1),(-1,-1,-3,+1)
Integrated Output:
-4,
+4,
-4
Binary Output:
0,
1,
0
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Power Control/Assignment
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For the CDMA to work, the power levels of signals from all stations
should be the same (or within certain tolerance level) when
received by the receiver.
A good heuristic: Each mobile station sends signal with the power
level inverse of that received from the base station.
The base station can tell mobile station to increase/decrease its
power.
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Third-Generation Mobile Phones:
Digital Voice and Data
Basic services an IMT-2000 network should provide
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High-quality voice transmission
Messaging (replace e-mail, fax, SMS, chat, etc.)
Multimedia (music, videos, films, TV, etc.)
Internet access (web surfing, w/multimedia.)
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Cable Television
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Community Antenna Television
Internet over Cable
Spectrum Allocation
Cable Modems
ADSL versus Cable
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Community Antenna Television
An early cable television system.
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Internet over Cable
Cable television
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Internet over Cable (2)
The fixed telephone system.
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Spectrum Allocation
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Frequency allocation in a typical cable TV system
used for Internet access
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Cable Modems
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Typical details of the upstream and downstream
channels in North America.
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