Chapter 2: Fundamentals of Data and Signals

Introduction


Data and signals are two of the basic building blocks of any computer network. In order for a computer network to transmit data, the data must first be converted into appropriate signals. One thing data and signals have in common is that both can be either analog or digital form. The conversion is performed by modulation techniques and is found in systems such as telephones, AM and FM radio. Converting digital data to square-wave digital signals is relatively straightforward and involves numerous digital encoding techniques. Converting digital data to analog signals requires some form of a modem. Finally, converting analog data to digital signals is called digitalization; telephone system and music system are two common examples .


Data And Signals

• Data is entities that convey meaning within a computer or computer system.
• Signals are the electronic or electromagnetic impulses used to encode and transmit data

1. Analog Vs Digital

• Data and signals can exist in either analog or digital form
• Analog Data and Analog signals : are continuos waveforms that can be at an infinite number of points between some given minimum and maximum. The most common example is human voice or music and video in their natural states.

• It is difficult to remove noise from original waveform
• Noise: is unwanted electrical or electromagnetic energy that degrades the quality of signals and data

• Digital data and Digital signals: are composed of a discrete or fixed number of values, rather than a continuous or infinite number of values
• First type of digital signal is a "square wave" with simple patterns of high and low voltages

• It is fairly easy to separate original digital waveform from the Noise if the amount of noise is small enough

• If noise becomes so great that it is no longer possible to distinguish a high from low, then the noise has taken over the signals and can no longer understand the original waveform

2. Fundamentals of signals

There are three basic components: Amplitude, frequency, and phase

• Amplitude: is the height of the wave above or below a given reference point
• the height often denotes the voltage level of signal (measured in volts), but it also denote the current level of the signal (measured in amps) or the power level of the signal (measured in watts)

• Frequency: is the number of times a signal makes a complete cycle within a given time fram
  - Period: is the length, or time interval of one cycle
  -  Frequency is measured in hertz (Hz), or cycles per second (period = 1/frequency)
  - Spectrum: the range of frequencies that a signal spans from minimum to maximum
  - Bandwidth: the absolute value of the difference between the lowest and the highest frequencies
  - Extra noise degrades original signals, an electronic device usually has an effective bandwidth that is less than its bandwidth.
  

• Phase: is the position of the waveform relative to a given moment of time, or relative time to 0
  - The wave never makes an abrupt change but is a continuos sine wave
  - A phase change (or shift) involves jumping forward (or backward) in the waveform at a given moment of time.

• Attenuation is the loss of power of signal strength when signal travels through any type of medium.
• Decibel (dB) is a relative measure of signal loss or gain and is used to measure the logarithmic loss or gain of a signal
• Amplification: is the opposite of attenuation, the signal gains in decibels when a signal is amplified by an amplifier

• The formula to measure signal loss or gain is dB = 10 x log10(P2/P1) where P2 and P1 are the ending and beginning power levels, respectively and is expressed in watts. 

Converting Data into Signals

There are four main combinations of data and signals:

- Analog data transmitted using analog signals
- Digital data transmitted using square-wave digital signals
- Digital data transmitted using discrete analog signals
- Analog data transmitted using digital signals

1. Transmitting analog data with analog signals

• Data is analog waveform is simply being transformed to another analog waveform, the signal. The basic operation performed is modulation
• Modulation: is the process of sending data over a signal by varying its amplitude, frequency or phase 
• Landline telephones, AM radio, FM radio and older broadcast television are the common examples

2. Transmitting digital data with square-wave digital signals: digital encoding schemes

• Nonreturn to zero digital encoding schemes:
  - Nonreturn to zero-level (NRZ-L) transmits 1s as zero voltage and 0s as positive voltages
  - Nonreturn to zero inverted (NRZI) has a voltage change at the beginning of a 1 and no voltage change at the beginning of a 0
  - The difference between these two schemes is that with NRZ-L, the receiver has to check the voltage level for each bit to determine whether the bit is a 0 or 1 and with NRZI, the receiver has to check whether there is a change at the beginning of the bit to determine if it is a 0 or a 1
• Manchester digital encoding schemes
  - The manchester class of digital encoding schemes ensures that each bit has some type of signal change.
  - To transmit a 1, the signal changes from low to high in the middle of the interval
  - To transmit a 0, the signal changes from high to low in the middle of the interval
  - The manchester schemes have an advantage over the NRZ schemes is that there is always a transition in the middle of a bit. So, the receiver can expect a signal change at regular intervals and can synchronize itself with the incoming bit stream
  - One big disadvantage of Manchester schemes is that roughly haft of the time there will be 2 transitions during each bit.
  - Baud Rate: is the number of times a signal changes value per second.
  - Data rate is measured in bits per second (bps)
• Bipolar-AMI encoding scheme
  - Bipolar-AMI is unique because it uses three voltage levels
  - When a device transmits a binary 0, a zero voltage is transmitted
  - when the device transmits a binary 1, either a positive voltage or a negative voltage is transmitted
• 4B/5B digital encoding scheme
  - 4B/5B: convert the 4 bits into a unique 5-bit sequence and encode the 5-bits using NRZI

3. Transmitting digital data with discrete analog signals

• Amplitude shift keying
  - A data value of 1 and data value of 0 are represented by two different amplitudes of a signal
  - During each bit period, the amplitude of the signal is constant
  - Can have two or more possible amplitude levels
  - Weakness:  it is susceptible to sudden noise impulses such as the static charges by lighting storm
  - The least efficient encoding techniques
• Frequency Shift keying
  - Uses two different frequency ranges to represent data values of 0 and 1
  - Frequency shift keying doesnt have problem with sudden noise spikes that can cause data loss
  - However, it is subject to Intermodulation distortion- a phenomenon that occurs when the frequencies of two or more signals mix together and create new frequencies
• Phase Sift Keying
  - One phase change encodes a 0 while another phase change encodes a 1
  - Less susceptible to noise and can be used at higher frequencies
  - It is so accurate that the signal transmitter can increase efficiency by introducing multiple phase shift angles
  - Quadrature phase shift keying: incorporates four different phase angles, each of which represent 2 bits
  - Quadrature amplitude modulation: a signaling method in which a combination of 12 different phase shift angles with two different amplitudes

4. Transmitting analog data with digital signals

• Pulse Mode Modulation (PCM)
  - Hardware, specially a codec converts the analog data to a digital by tracking the analog waveform and taking "snapshots" of the analog data at fixed intervals
  - An analog value is converted ti ab equivalent fixed-sized binary value
  - Binary value can then be transmitted by means' of a digital encoding format
  - The closer the snapshots are taken to one another, the more accurate the reconstructed waveform will be
  - Sampling rate: the frequency at which the snapshots are taken
• Delta Modulation
  - A codec tracks the incoming analog data by assessing up and down "steps"
  - Uses a binary 1 to represent a rise in voltage and a 0 to represent a drop
  - Two problems with delta modulation are (1) if the analog waveform rises or drops too quickly, the codec may not be able to keep up with the change and slop overload noise results (2) when analog waveform doesnt change at all, the waveform generates a pattern of 1010101010... because the codec outputs a 1 or 0 only for a rise or fall, thus generating quantizing noise.

Data Code

• Data Code: is the set of all textual characters or symbols and their correspoding binary patterns 
• Three important data codes are
  1. EBCDIC
  2. ASCII
  3. Unicode

1. EBCDIC or the extended binary coded decimal interchange code

• An 8 bit code allowing 256 possible combinations of textual symbols

2. ASCII - the American Standard Code ode Information Interchange

• Is government standard in the US and one of the most widely used codes in the world.
• Character set exists in a few different forms including a 7-bit version that allows for 128 possible combinations of textual symbols

3. Unicode

• Is an encoding technique that provides a unique coding value for every character in every language, no matter what platform
• Currently supports more than 110 different code charts