Data and Signal h9z19zr
Data entities that convey meaning (computer file, music on a CD, voice message, phone call)
Signals the electric, optical or electromagnetic encoding of data (telephone conversation, web page download)
Transmission
Communication of data by propagation and processing of signals
Functions of computer networks and data / voice communication systems transmit signals
Both Data and Signal can be Analog vs. Digital
Data Codes
Data code
The set of all textual characters or symbols and their corresponding binary patterns.
Coding schemes
Using bits to represent characters
United States of America Standard Code for Information Interchange (USASCII or ASCII)
128 or 256
Extended Binary Coded Decimal Interchange Code (EBCDIC)
256, IBM’s standard

Data and Signal
Analog
Continuous values within some interval vary continuously within a range of values (1-100 /sound waves) e.g. sound, video
Digital
Discrete, non-continuous values specific discrete values (on/off) (traffic light) e.g. text, integers

Data and Signal
Components of Signal
Basic components
Amplitude: the height of the wave above or below a given reference point.
Frequency: the number of times a signal makes a complete cycle within a given time frame
Phase: the starting angle of the wave.
Derived components
Period: the length or time interval of one cycle.
Spectrum: the range of frequencies that a signal spans from minimum to maximum.
Bandwidth: the absolute value of the difference between the lowest and highest frequencies of a signal.
The average human voice <=300HZ and <=3400

Bandwidth
In digital systems, data speed a modem that works at 57,600 bps has twice the bandwidth of a modem that works at 28,800 bps.
In analog systems, the difference between the highest-frequency signal component and the lowest-frequency signal component.
The higher the frequency of the additional waveforms—the higher the bandwidth of the composite signal, the more the composite will look like the digital signal.
Noise and attenuation
Noise: unwanted electrical or electromagnetic energy that degrades the quality of signals and data.
Examples of noise in analog and digital signals.
Digital system has higher ability of separating noise from signals than does analog system
Attenuation: the loss of power or loss of signal strength.
A function of distance and friction within the medium.

Noise and attenuation
Signal strength: decibel
A relative measure of signal loss or gain
A 3 dB loss indicates the signal lost half of its power. dB = 10 log10 (P2 / P1)
P2: the power level at the starting point
P1: the power level at the ending point

Transmission
Transmission Media
Some can transmit both types of signals
For example,

Transmission Equipment
Determine what signal is really transmitted.
Digital Data? Digital Signal
Digital encoding schemes: converting Digital Data into Digital Signals
NRZ-L: 0 as positive voltage, 1 as 0 voltage;
Simple and inexpensive
NRZ-I: Non-Return to Zero Inverted
1 as voltage change at the beginning of a interval; 0 as no voltage change at the beginning of a interval;
Problem with NRZ: How about no data?
Manchester
Differential Manchester
Digital Data? Digital Signal
Manchester:
0 as voltage change from high to low in the middle of the interval; 1 as voltage change low to high in the middle of the interval
Differential Manchester: LAN
Similar to Manchester, a transition in the middle of the interval.
But, the transition does not differentiate the direction of the transition. 0 as a transition at the beginning of the interval; 1 as no transition at the beginning of the interval.
Does not differentiate high or low, just a jump.
Digital Data? Digital Signal
Baud or baud rate: the number of times a signal changes value per second
(Symbol rate)
Bit rate: bits transmitted per second.
Data rate: bit rate, or baud rate * bits per baud.
How to improve transmission speed?
Digital system?
Analog system?

Data rate
What does it mean? 56Kbps?
56,000 bits per second
Send 1 bit 1/56,000 second
Receive 1 bit 1/56,000 second
Voltage changes every 1/56,000 second?

Digital Transmission Modes
Parallel Mode the internal transfer inside a computer
Byte-by-byte, simultaneously
Serial Mode
The predominant method in data communications
Bit-by-bit, sequentially

Digital Transmission
Digital Data?Analog Signals
Modulation: the technique of converting digital data to analog signal.
Amplitude modulation
One amplitude encodes a 0 while another amplitude encodes a 1
Susceptible to noise
Frequency modulation
One frequency encodes a 0 while another frequency encodes a 1
Susceptible to intermodulation distortion
Phase modulation
One phase encodes a 0 while another phase encodes a 1
Less susceptible:
Quadrature phase modulation: using 4 different phase angles.

Amplitude Modulation
Frequency Modulation
Phase Modulation
Inclass Exercise 2
Is digital TV true digital?
Composite signals
Sending Multiple Bits

2 bits using 2^2 states AM,FM,PM
3 bits using 2^3 states
4 bits using 2^4 states
5 bits using 2^5 states
Maximum, hard to tell 32 different amplitude
Composite signals
Quadrature Amplitude Modulation
Most often seen in modems
A method to combine modulation techniques
8 different phases
2 different amplitude
16 different possible states
Each signal represents 4 bits

Data Transfer Rates
How do you calculate a maximum data rate?
Use Shannon’s equation:
S(f) = f log2 (1 + W/N)
Where f = signal frequency, W is signal power, and N is noise power
For example, what is the data rate of a 3400 Hz signal with 0.2 watts of power and 0.0002 watts of noise?
S(f) = 3400 x log2 (1 + 0.2/0.0002)
= 3400 x log2 (1001)
= 3400 x 9.97
= 33898 bps

Analog Data? Digital Signal
Music recording; telephone,
Pulse Code Modulation: The amplitude of the sound wave is sampled at regular intervals and translated into a binary number.

Codec vs. Modem
Codec coding analog data into digital form and decoding it back.
The digital data coded are samples of analog waves.
Modem modulating digital data into analog form and demodulating it back analog symbols carry digital data
Pulse Code Modulation
Pulse Code Modulation
Pulse Code Modulation
Pulse Code Modulation
Quantizing error: The difference between the original analog signal and the translated digital signal two ways to reduce quantizing error and improve the quality of the PCM signal.
Increase the number of amplitude levels
Sample more frequently (oversampling).
Analog Data?Analog Signals
Many times it is necessary to modulate analog data onto a different set of analog frequencies.
Broadcast radio (AM, FM) and television (broadcast and cable) are common examples

Spread Spectrum Technology
Spread spectrum is an RF communications system in which the baseband signal bandwidth is intentionally spread over a larger bandwidth by injecting a higher-frequency signal.
Wireless communication (cordless telephone)
A secure encoding technique that uses multiple frequencies or codes, rather than transmitting the signal on one fixed frequency
Analog transmission of both digital and analog signal
Very secure: eavesdropping
Spread Spectrum Technology
Frequency hopping spread spectrum
A pseudorandom number generator to indirectly control the synthesis of frequencies onto which the original data is modulated.

Spread Spectrum Technology
Direct sequence spread spectrum
Spreads the transmission of a signal over a wide range of frequencies using mathematical values
This technology replaces each binary 0 and binary 1 with a unique pattern, or sequence, of 1s and 0s.
For example, one transmitter may transmit the sequence 10010100 for each binary 1, and 11001010 for each binary 0.
Another transmitter may transmit the sequence 11110000 for each binary 1, and 10101010 for each binary 0.

CDMA
CDMA (Code-Division Multiple Access) any of several protocols used in so-called second-generation (2G) and third-generation (3G) wireless communications. a form of multiplexing, which allows numerous signals to occupy a single transmission channel, optimizing the use of available bandwidth. ultra-high-frequency (UHF) cellular telephone systems in the 800-MHz and 1.9-GHz bands.

CDMA employs analog-to-digital conversion (ADC) in combination with spread spectrum technology.
Audio input is first digitized into binary elements. The frequency of the transmitted signal is then made to vary according to a defined pattern (code), intercepted only by a receiver whose frequency response is programmed with the same code. trillions of possible frequency-sequencing codes; this enhances privacy and makes cloning difficult.