1 Introduction Introduction Communication System Baseband an - - PDF document

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Introduction

Home network Institutional network Mobile network Global ISP Regional ISP

Basic Concepts of Data Transmission

Yanmin Zhu Department of Computer Science and Engineering

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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

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Introduction

Basic Concepts of Data Transmission

 Fourier Analysis  Data Transmission  Bandwidth & the Maximum Data Rate of

a Channel

 Asynchronous and Synchronous

Communication

 Serial and Parallel Communication

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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 ) 2

n n n n

g t c a nft b nft

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Introduction

Fourier Analysis

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Introduction

Basic Concepts of Data Transmission

 Fourier Analysis  Data Transmission  Bandwidth & the Maximum Data Rate of

a Channel

 Asynchronous and Synchronous

Communication

 Serial and Parallel Communication

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SLIDE 2

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Introduction

Communication System

 source：analog signal, digital signal  transformation：modulation, multiplexing,

encoding

 channel：bandwidth, bit-rate

transform source destination transform noise channel

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Introduction

Baseband

 an adjective that

describes signals and systems whose range

• f 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.

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Introduction

Passband

 A passband is the

range of frequencies

• r wavelengths that

can pass through a filter without being attenuated.

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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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SLIDE 3

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Introduction

Illustration of AM

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Introduction

AM

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Introduction

FM

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Introduction

PM

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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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SLIDE 4

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Introduction

Three Types of Modulations

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Introduction

QPSK (Quadrature Phase Shift Keying)

QAM-64

6bit/signal

QAM-16

4bit/signal

QPSK

2bit/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

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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

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Introduction

RZ (Return-to-Zero)

 Uses two different voltage levels (one positive

and one negative)

 existence of a signal transition at the middle

• f the bit time (high-to-zero or low-to-zero)

Low duty cycle

Time Voltage Level

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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.

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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 1 1 1 1 1 bit与bit之间无跳变，下一个bit 为1

Ethernet

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Introduction

Digital signals transmission for analog data

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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

Basic Concepts of Data Transmission

 Fourier Analysis  Data Transmission  Bandwidth & the Maximum Data Rate of

a Channel

 Asynchronous and Synchronous

Communication

 Serial and Parallel Communication

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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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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

• ver 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/N

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Introduction

Impact of Bandwidth on Digital Transmission

hz hz hz hz hz

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Introduction

Basic Concepts of Data Transmission

 Fourier Analysis  Data Transmission  Bandwidth & the Maximum Data Rate of

a Channel

 Asynchronous and Synchronous

Communication

 Serial and Parallel Communication

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Introduction

Asynchronous and Synchronous Communication

Synchronous 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

Basic Concepts of Data Transmission

 Fourier Analysis  Data Transmission  Bandwidth & the Maximum Data Rate of

a Channel

 Asynchronous and Synchronous

Communication

 Serial and Parallel Communication

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Introduction

Serial and Parallel Communication

Parallel: send multiple bits at a time over multiple lines Serial: sent

• ver a single

channel one bit at a time

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Introduction

Home network Institutional network Mobile network Global ISP Regional ISP

Advanced

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