Introduction Physical Layer Basic Concepts of Data Transmissionyzhu/courses/comnet/slides/Lec07.pdf · 2015. 3. 27. · Transmission media How to use (analog) signals for digital

1

Introduction

Home network

Institutional network

Mobile network

Global ISP

Regional ISP

Basic Concepts of Data Transmission

Yanmin Zhu

Department of Computer Science and Engineering

CSE Department 1

Introduction

Physical Layer

Provides the means to transmit bits from sender to receiver,

Defines the mechanical, electrical, and timing interfaces to the network,

Involves Transmission media How to use (analog) signals for digital

information Techniques such as modulation techniques

(the actual encoding), multiplexing, and switching

CSE Department 2

Introduction


Fourier Analysis

Data Transmission

Bandwidth & the Maximum Data Rate of a Channel

Asynchronous and Synchronous Communication

Serial and Parallel Communication

CSE Department 3

Introduction

Fourier Analysis

Any reasonably behaved periodic function, g(t) with period T can be constructed as the sum of a (possibly infinite) number of sines and cosines:

where f = 1/T is the fundamental frequency, an and bn are the sine and cosine amplitudes of the nth harmonics , and c is a constant

1 1

1( ) sin(2 ) cos(2 )

2n n

n n

g t c a nft b nft

CSE Department 4

Introduction

Fourier Analysis

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Introduction


Fourier Analysis

Data Transmission




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2

Introduction

Communication System

source：analog signal, digital signal

transformation：modulation, multiplexing, encoding

channel：bandwidth, bit-rate

transformsource destinationtransform

noise

channel

CSE Department 7

Introduction

Baseband

an adjective that describes signals and systems whose range of frequencies is measured from close to 0 hertz to a cut-off frequency, a maximum bandwidth or highest signal frequency;

a band of frequencies starting close to zero.

CSE Department 8

Introduction

Passband

A passband is the range of frequencies or wavelengths that can pass through a filter without being attenuated.

CSE Department 9

Introduction

Modulation

Baseband (DC) signaling is not suitable for long distance transmission because of its wide frequency spectrum

a continuous tone called sine wave carrier (AC signaling) is introduced

its amplitude, frequency, or phase can be modulated to transmit information

Three basic forms of modulation: amplitude modulation, frequency modulation and phase modulation.

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Introduction

Analog signals transmission for analog data

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Let’s start with

Introduction

Carrier Signal: Three Parameters

Carrier signal

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3

Introduction

Illustration of AM

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Introduction

AM

CSE Department 14

Introduction

FM

CSE Department 15

Introduction

PM

CSE Department 16

Introduction

Data Transmission

Analog signals transmission for digital data:

ASK(Amplitude Shift Keying)

FSK(Frequency Shift Keying)

PSK(Phase Shift Keying)

QPSK(Quadrature Phase Shift Keying)

Digital signals transmission for digital data Non-return-to zero encoding Return-to zero encoding Manchester encoding 4B/5B

Digital signals transmission for analog data Pulse Code Modulation

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Introduction

Analog signals transmission for digital data

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4

Introduction

Three Types of Modulations

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Introduction

QPSK (Quadrature Phase Shift Keying)

QAM-646bit/signal

QAM-164bit/signal

QPSK2bit/signal

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Introduction

Digital signals transmission for digital data

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Introduction

NRZ-L ( Non-Return-to-Zero-Level)

Uses two different voltage levels (one positive and one negative) as the signal elements for the two binary digits.

Time

Amplitude

CSE Department 22

Introduction

NRZ-I ( Non-Return-to-Zero-Invert)NRZI is a differential encoding (i.e., the signal is decoded

by comparing the polarity of adjacent signal elements.)

1 existence of a signal transition at the beginning of the bit time

(either a low-to-high or a high-to-low transition)

0 no signal transition at the beginning of the bit time

USB

Time

Amplitude

CSE Department 23

Introduction

RZ (Return-to-Zero)

Uses two different voltage levels (one positive and one negative)

existence of a signal transition at the middle of the bit time (high-to-zero or low-to-zero)

Low duty cycleTime

Voltage Level

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5

Introduction

Manchester encoding

Straight binary encoding with 0 volts for a 0 bit and 5 volts for a 1 bit because it leads to ambiguities. Different clock speeds can cause the receiver

and sender to get out of synchronization about where the bit boundaries are

Manchester encoding is introduced for receivers to unambiguously determine the start, end, or middle of each bit without reference to an external clock.

CSE Department 25

Introduction

Manchester Encoding Example

bit流

二进制编码

曼切斯特编码

差分曼切斯特编码

bit与bit之间有跳变，下一个bit

为0

1 0 0 0 0 1 0 1 1 1 1 0

bit与bit之间无跳变，下一个bit

为1

Ethernet

CSE Department 26

Introduction

Digital signals transmission for analog data

CSE Department 27

Introduction

Pulse Code Modulation

AD Transformation: sampling、quantifying、

encoding

PCM forms the heart of the modern telephone

system：Sampling period :125us，256 level

quantifying，Bit Rate：8*8000=64Kbps

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Introduction


Fourier Analysis

Data Transmission




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Introduction

Bandwidth and Bandwidth-Limited Signals

The range of frequencies transmitted without being strongly attenuated is called the bandwidth.

The bandwidth is a physical property of the transmission medium usually depends on the construction, thickness,

and length of the medium.

A wide band signal will be distorted when transmitted thru relatively narrower band channel with the higher harmonics cut off or hold back.

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6

Introduction

Symbol, Baud Rate and Bit Rate

The baud rate is the number of samples/sec made.Each sample sends one piece of information, that is, one symbol. The baud rate and symbol rate are thus the same.The modulation technique (e.g., QPSK) determines the number of bits/symbol

The bit rate is the amount of information sent over the channel

equal to the number of symbols/sec times the number of bits/symbol.

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Introduction

Maximum Data Rate

For the maximum data rate for a finite bandwidth noiseless channel, Nyquist's theorem states:

Maximum data rate=2Hlog2V (b/s)Where H is the Bandwidth and V is the discrete

levels of the signal.

Shannon's major result is that the maximum data rate of a noisy channel whose bandwidth is H Hz, and whose signal-to-noise ratio is S/N, is given by

Maximum data rate = Hlog2(1+S/N)

S/N（dB）= 10log10S/NCSE Department 32

Introduction

Impact of Bandwidth on Digital Transmission

hz

hz

hz

hz

hz

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Introduction


Fourier Analysis

Data Transmission




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Introduction

Asynchronous and Synchronous CommunicationSynchronous Communication:

sender and receiver should synchronize before each transmission. The data transmitted contain information that the receiver can use to determine where individual bits begin and end. It is useful for system which always have data ready to send, e.g. the PSTN.

Asynchronous Communication: sender and receiver don’t need to coordinate before data can be transmitted. Thus a sender can wait arbitrarily long between transmissions, and can transmit whenever data becomes ready. Useful for devices such as keyboards which don’t always have data ready to send.

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Introduction


Fourier Analysis

Data Transmission




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7

Introduction


Parallel: send multiple bits at a time over multiple lines

Serial: sent over a single channel one bit at a time

CSE Department 37

Introduction

Home network

Institutional network

Mobile network

Global ISP

Regional ISP

Advanced

CSE Department 38

Documents

Introduction Physical Layer Basic Concepts of Data Transmissionyzhu/courses/comnet/slides/Lec07.pdf · 2015. 3. 27. · Transmission media How to use (analog) signals for digital