Introduction Modulation Signals at Different perspectives – Frequency and Time AM, FM, PM

Computer Data Communications Chapter 2

1

Introduction

• Modulation

• Signals at Different perspectives – Frequency and Time

• AM, FM, PM

• Signal conversion – Analog to Digital and Digital to Digital.


2

Signal Modulation The process of varying the electrical energy is modulation. The high amplitude, high frequency, and fixed frequency

level of energy that flows through the channel is the signal carrier.

An electronic device varies this carrier to reflect the information contained in a weaker voice or data signal.

Original computer and telephone signals often lack the strength to go far enough to be useful.

A modulation device can generate a combined signal strong enough to make it to its destination

Another device called a demodulator has to separate the signal that arrives at its destination


3

The Time Perspective of analog signal


4

Time perspective of digital signal.


5

The Frequency Perspective A signal has either a constant frequency over a

period or a frequency that varies over time. Whether the frequency remains constant or changes, it has a maximum and a minimum.

If the frequency is constant, the maximum and minimum frequency are the same.

This frequency range is the frequency perspective of a signal.


6

Frequency perspective of analog signals


7

Analog Signal Modulation The three primary analog techniques are amplitude modulation, frequency modulation, and

phase modulation. The fourth technique applies to digital

communications channel only and is called digital modulation.

A matching demodulation technique requires on the receiving end of the link


8

Amplitude Modulation First used this technique for radio broadcast. Easy to implement on the transmitting end, and the

demodulation was easy and Inexpensive on the receiving end.

Constant frequency but varies in amplitude over time to convey information.

Lower-frequency information signal is imposed on the carrier

The amplitude of the resulting compound signal varies within an envelope that matches the form of the signal.


9

Disadvantages: Any noise can adversely affect the

demodulation of the AM signal. Electrical noise can result in misinterpretation

of the signal Amplifiers amplify both the signal and the

noise that has contaminated the signal.


10Amplitude modulation signal generation


11

Frequency Modulation Has a constant amplitude but varies in frequency over

time to convey information. Imposing the lower-frequency information signal on the

carrier The frequency of the resulting compound signal varies to

match the form of the information signal. The disadvantage of FM is the bandwidth it takes to

transmit a signal has a wider spectrum Being wider in bandwidth the number of FM signals transmitted through a medium is smaller than the number of AM signals.

Low-speed modems use FM technique to convert digital signals into FM signals.

This process of frequency shift keying (FSK)


12

Frequency modulation signal generation and FM signal bandwidth


13

Phase Modulation This technique does not apply widely to radio

communications, Vendors use the technique to convey color information in

color television broadcasts. PM a better alternative to AM and FM techniques. More difficult to understand than AM and FM Allows computers to communicate at high data rates through

the telephone system. Requires at least two analog signals. The first signal is the carrier; all other signals modify the

carrier signal to convey information.


14

Imposes information on the carrier by changing the shape of the carrier’s signal curve at a given points in time.

Signal cross the amplitude reference line at different times and are therefore different in phase.

The different in phase between two sine waves is phase angle, from 0 to 360

The break point is the phase change that conveys the information for the compound signal.

A reference pattern allows designers to design and build PM circuits.

Can build a circuit that locks onto a carrier’s frequency to eliminates it to leave the information signal.


15

Carrier and information signals 180 degrees different in phase, and a phase modulated signal.


16

Very accurate and efficiency Multiple phase shift angles can be introduced. Quadrature phase modulation incorporates four different

phase angles, Each represents two bits: 45 degree phase shift = 11; 135 degree phase shift = 10; 225 degree phase shift = 01; 315 degree phase shift = 00. Quadrature phase modulation has double the efficiency of

simple phase shift modulation.


17Four phase angles of 45, 135, 225, and 315 degrees.


18Pulse code modulation (PCM) and pulse amplitude modulation (PAM).


19

Analog-to Digital Conversion (CODEC)

Codec (short for coder/decoder) translates analog voice signals into digital signals.

The codec samples the signal Process is called pulse amplitude modulation (PAM). Final modulation is pulse code modulation (PCM). PCM converts the stream of continuous varying PAM

signals into a stream of binary signals.


20

Digital to Digital Conversion

Three common coding techniques for Ethernet data:

1. Non Return to Zero (NRZ)

2. Manchester Coding, Differential Manchester

Coding, and

3. 4B/5B.


21

Non-Return to Zero Digital Encoding Schemes Non-Return to Zero-Level (NRZ-L) transmits 1s as zero

voltages and 0s as positive voltages. It is also simple and inexpensive to implement. Non-Return to Zero-Inverted (NRZ-I has a voltage change

at the beginning of a 1 and no voltage change at the beginning of a 0.

Problem: long sequences of 0s in the data produce a signal that never changes.

The receiver requires signal changes to produce a synchronization.


22

Manchester Digital Encoding Schemes Ensures that each bit has some type of signal change Solves the synchronization problem. There is always a transition in the middle of the interval. Manchester code signal: Changes from high to low in the middle of the interval

to transmit a 0, and Changes from low to high in the middle of the interval.


23

Differential Manchester Used in most local area networks No transmission in the beginning interval = 1 Transmission in the beginning interval = 0Receiver can synchronize itself with the incoming

bit stream.Disadvantage: Nearly half of the time there will be

two transitions during each bit. Hardware has to work twice as hard of that of NRZ encode signal.

Therefore it can be inefficient.


24

Examples of four digital encoding schemes


25

4B/5B Digital Encoding Scheme Solve the synchronization problem Relatively inefficient. Satisfy the synchronization problem and avoid the

inefficiency problem. The 4B/5B takes 4 bits of data, converts the four bits into a

unique five bit sequence, and encodes the five bits using NRZ-I.

Looking at next four bits in a data stream to be transmitted and replaced the four bits with a new 5-bit code.

Notice that there are no more than two consecutive zeros in all the new 5-bit code.


26

4B/5B Conversion Table

Valid Data Symbol

Original 4-bit data

New 5-bit code

0000000100100011010001010110011110001001101010111100110111101111

11110010011010010101010100101101110011111001010011101101011111010110111110011101


27

0000 11110 Original becomes 5-bit encoded 1 1 1 1 0

data data NRZ-I Encoded

signal

Introduction Modulation Signals at Different perspectives – Frequency and Time AM, FM, PM

Documents