Cn ch21 The Physical Layer Chapter 2. cn ch22 The Theoretical Basis for Data Communication Fourier Analysis Bandwidth-Limited Signals Maximum Data Rate.

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The Physical Layer

Chapter 2

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The Theoretical Basis for Data Communication

• 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.

(b) – (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

• 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

(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

• 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.

A bidirectional system with two lasers is pictured here.

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Communication Satellites

• 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.

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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 SatellitesIridium

(a) The Iridium satellites from six necklaces around the earth.

(b) 1628 moving cells cover the earth.

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Globalstar

(a) Relaying in space.(b) Relaying on the ground.

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Public Switched Telephone System

• 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

(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

• Local loops Analog twisted pairs going to houses and

businesses

• Trunks Digital fiber optics connecting the switching

offices

• Switching offices 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

(b) Amplitude modulation(c) Frequency modulation

(d) Phase modulation

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Modems (2)

(a) QPSK.

(b) QAM-16.

(c) QAM-64.

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Modems (3)

(a) V.32 for 9600 bps.

(b) V32 bis for 14,400 bps.

(a) (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

(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

(a) Circuit switching.

(b) Packet switching.

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Message Switching

(a) Circuit switching (b) Message switching (c) Packet switching

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Packet Switching

A comparison of circuit switched and packet-switched networks.

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The Mobile Telephone System

• First-Generation Mobile Phones: Analog Voice

• Second-Generation Mobile Phones: Digital Voice

• 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:

• Control (base to mobile) to manage the system

• Paging (base to mobile) to alert users to calls for them

• Access (bidirectional) for call setup and channel assignment

• Data (bidirectional) for voice, fax, or data

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D-AMPS Digital Advanced Mobile Phone System

(a) A D-AMPS channel with three users.

(b) A D-AMPS channel with six users.

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GSMGlobal System for Mobile Communications

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• 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• 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 for its negation.•Two chip sequence S and T are orthogonal if S•T=0.

• if S•T=0 then S• =0

• • All chip sequences must be pariwise orthogonal.• S• =-1• CDMA Example• Let S=A+ +C, S•C=(A+ +C)•C=A•C+ •C+C•C=0+0+1=1

m

iiim tsTS

1

1

S

T

S

B B B

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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(d) Recovery of station C’s signal

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Walsh-Hadamard Matrix forOrthogonal 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 SendingChannel 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 Sequencec1: (-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/DecodingSum 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, -4Binary Output: 0, 1, 0

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Power Control/Assignment• 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

• 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

• 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

Frequency allocation in a typical cable TV system used for Internet access

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Cable Modems

Typical details of the upstream and downstream channels in North America.