cn ch2 1

The Physical Layer

Chapter 2

cn ch2 2

The Theoretical Basis for Data Communication

• Fourier Analysis

• Bandwidth-Limited Signals

• Maximum Data Rate of a Channel

cn ch2 3

Bandwidth-Limited Signals

A binary signal and its root-mean-square Fourier amplitudes.

(b) – (c) Successive approximations to the original signal.

cn ch2 4

Bandwidth-Limited Signals (2)

(d) – (e) Successive approximations to the original signal.

cn ch2 5

Bandwidth-Limited Signals (3)

Relation between data rate and harmonics.

cn ch2 6

Guided Transmission Data

• Magnetic Media

• Twisted Pair

• Coaxial Cable

• Fiber Optics

cn ch2 7

Twisted Pair

(a) Category 3 UTP.(b) Category 5 UTP.

cn ch2 8

Coaxial Cable

A coaxial cable.

cn ch2 9

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.

cn ch2 10

Transmission of Light through Fiber

Attenuation of light through fiber in the infrared region.

cn ch2 11

Fiber Cables

(a) Side view of a single fiber.(b) End view of a sheath with three fibers.

cn ch2 12

Fiber Cables (2)

A comparison of semiconductor diodes and LEDs as light sources.

cn ch2 13

Fiber Optic Networks

A fiber optic ring with active repeaters.

cn ch2 14

Fiber Optic Networks (2)

A passive star connection in a fiber optics network.

cn ch2 15

Wireless Transmission

• The Electromagnetic Spectrum

• Radio Transmission

• Microwave Transmission

• Infrared and Millimeter Waves

• Lightwave Transmission

cn ch2 16

The Electromagnetic Spectrum

The electromagnetic spectrum and its uses for communication.

cn ch2 17

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.

cn ch2 18

Politics of the Electromagnetic Spectrum

The ISM bands in the United States.

cn ch2 19

Lightwave Transmission

Convection currents can interfere with laser communication systems.

A bidirectional system with two lasers is pictured here.

cn ch2 20

Communication Satellites

• Geostationary Satellites

• Medium-Earth Orbit Satellites

• Low-Earth Orbit Satellites

• Satellites versus Fiber

cn ch2 21

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.

cn ch2 22

Communication Satellites (2)

The principal satellite bands.

cn ch2 23

Communication Satellites (3)

VSATs using a hub.

cn ch2 24

Low-Earth Orbit SatellitesIridium

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

(b) 1628 moving cells cover the earth.

cn ch2 25

Globalstar

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

cn ch2 26

Public Switched Telephone System

• Structure of the Telephone System

• The Politics of Telephones

• The Local Loop: Modems, ADSL and Wireless

• Trunks and Multiplexing

• Switching

cn ch2 27

Structure of the Telephone System

(a) Fully-interconnected network.

(b) Centralized switch.

(c) Two-level hierarchy.

cn ch2 28

Structure of the Telephone System (2)

A typical circuit route for a medium-distance call.

cn ch2 29

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

cn ch2 30

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.

cn ch2 31

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.

cn ch2 32

Modems

(a) A binary signal

(b) Amplitude modulation(c) Frequency modulation

(d) Phase modulation

cn ch2 33

Modems (2)

(a) QPSK.

(b) QAM-16.

(c) QAM-64.

cn ch2 34

Modems (3)

(a) V.32 for 9600 bps.

(b) V32 bis for 14,400 bps.

(a) (b)

cn ch2 35

Digital Subscriber Lines

Bandwidth versus distanced over category 3 UTP for DSL.

cn ch2 36

Digital Subscriber Lines (2)

Operation of ADSL using discrete multitone modulation.

cn ch2 37

Digital Subscriber Lines (3)

A typical ADSL equipment configuration.

cn ch2 38

Wireless Local Loops

Architecture of an LMDS system.

cn ch2 39

Frequency Division Multiplexing

(a) The original bandwidths.

(b) The bandwidths raised in frequency.

(b) The multiplexed channel.

cn ch2 40

Wavelength Division Multiplexing

Wavelength division multiplexing.

cn ch2 41

Time Division Multiplexing

The T1 carrier (1.544 Mbps).

cn ch2 42

Time Division Multiplexing (2)

Delta modulation.

cn ch2 43

Time Division Multiplexing (3)

Multiplexing T1 streams into higher carriers.

cn ch2 44

Time Division Multiplexing (4)

Two back-to-back SONET frames.

cn ch2 45

Time Division Multiplexing (5)

SONET and SDH multiplex rates.

cn ch2 46

Circuit Switching

(a) Circuit switching.

(b) Packet switching.

cn ch2 47

Message Switching

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

cn ch2 48

Packet Switching

A comparison of circuit switched and packet-switched networks.

cn ch2 49

The Mobile Telephone System

• First-Generation Mobile Phones: Analog Voice

• Second-Generation Mobile Phones: Digital Voice

• Third-Generation Mobile Phones:Digital Voice and Data

cn ch2 50

Advanced Mobile Phone System

(a) Frequencies are not reused in adjacent cells.

(b) To add more users, smaller cells can be used.

cn ch2 51

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

cn ch2 52

D-AMPS Digital Advanced Mobile Phone System

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

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

cn ch2 53

GSMGlobal System for Mobile Communications

GSM uses 124 frequency channels, each of which uses an eight-slot TDM system

cn ch2 54

GSM (2)

A portion of the GSM framing structure.

cn ch2 55

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.

cn ch2 56

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.

cn ch2 57

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

cn ch2 58

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

cn ch2 59

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.

cn ch2 60

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)

cn ch2 61

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

cn ch2 62

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.

cn ch2 63

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.)

cn ch2 64

Cable Television

• Community Antenna Television

• Internet over Cable

• Spectrum Allocation

• Cable Modems

• ADSL versus Cable

cn ch2 65

Community Antenna Television

An early cable television system.

cn ch2 66

Internet over Cable

Cable television

cn ch2 67

Internet over Cable (2)

The fixed telephone system.

cn ch2 68

Spectrum Allocation

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

cn ch2 69

Cable Modems

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