Ș.l. dr. ing. Lucian-Florentin Bărbulesculuci/DC/course_3.pdf · Non-Return to Zero – Inverted...
Transcript of Ș.l. dr. ing. Lucian-Florentin Bărbulesculuci/DC/course_3.pdf · Non-Return to Zero – Inverted...
Data: entities that convey meaning within a computer system
Signals: are the electric or electromagnetic impulses used to encode and transmit data
Characteristics ◦ Both exists in either analog or digital form
2
Represented as continuous waveforms
Can be at an infinite number of points between some given minimum and maximum
3
The most important shortcoming: Noise ◦ unwanted electrical or electromagnetic energy that degrades the
quality of signals and data
◦ found in every type of data and transmission system
◦ effects range from a slight hiss in the background to a complete loss of data or signal
◦ Is also analog - extremely difficult to separate noise from an analog waveform that represents data
4
Composed of a discrete or fixed number of values ◦ Digital Data - binary 1s and 0s
◦ Digital Signals – more complex
Most simple form is “square wave”
5
All signals have three characteristics: ◦ Amplitude
◦ Frequency
◦ Phase
Amplitude: the height of the wave above (or below) a given reference point
7
Frequency: the number of times a signal makes a complete cycle within a given time frame ◦ Measured in Hertz (Hz)
◦ Period: The time interval of one cycle (1 / frequency)
8
Signals are usually composed of more frequencies ◦ Spectrum: The range of frequencies that a signal spans from
minimum to maximum
Eg.: The spectrum of a simple telephone line must be between 300Hz and 3400Hz
◦ Bandwidth: the absolute value of the difference between the lowest and highest frequencies
Eg.: The bandwidth of a simple telephone line is 3100Hz (3400 – 300)
◦ Effective bandwidth: the real-life bandwidth
Less than the theoretical bandwidth
Value influenced by interferences and noise
9
Four possible data-to-signal conversions
◦ Analog data-to-analog signal
◦ Digital data-to-digital signal
◦ Digital data-to-(a discrete) analog signal
◦ Analog data-to-digital signal
11
An analog waveform is converted in another analog waveform
The operation performed is modulation ◦ the process of sending data over a signal by varying
either its amplitude(AM), frequency(FM), or phase(PM)
12
Digital data is converted to a signal which have a finite set of possible values
The operation is called digital encoding
Several encoding schemes: ◦ NRZ-L
◦ NRZ-I
◦ Manchester
◦ Differential Manchester
◦ Bipolar-AMI (alternate mark inversion)
◦ 4B/5B
14
Non-Return to Zero ◦ Non-Return to Zero – Level (NRZ-L)
1 -> 0 V
0 -> Positive V
◦ Non-Return to Zero – Inverted (NRZ-I)
1 -> Voltage change
0 -> Voltage keep
15
Non-Return to Zero ◦ Advantages: easy to implement, baud rate equal to bit rate
Baud rate: the number of times a signal changes value per second.
Bit rate: the number of data bits sent in one second.
◦ Disadvantages: no signal change for long streams of 0 or 1 (only for NRZ-L) – problem with receiver synchronization
16
Manchester ◦ Manchester
1 -> low to high transition
0 -> high to low transition
◦ Differential Manchester
1 -> One transition: at the middle
0 -> Two transitions: one at the beginning, one at the middle
17
Manchester ◦ Advantage: guaranteed transitions for each bit
◦ Disadvantage: Large baud rate With Manchester – baud rate = 2 * bit rate
Eg.: for 5 zeros the bit rate is 5 and the baud rate is 10
18
Bipolar-AMI (alternate mark inversion) ◦ 0 -> 0 V
◦ 1 -> Either positive or negative voltage, depending on previous 1
◦ Advantages: zero voltage sum - useful in certain types of electronic systems
◦ Disadvantages: no signal change for long streams of 0
19
4B/5B ◦ 4 bits are encoded in 5 bits and sent using NRZ-I
◦ The 5 bits never contain more than two consecutive zeros
20
Digital data is converted to an analog wave
A modulator is used
The analog signal takes on a discrete number of signal levels
Three simple techniques (plus other complex) ◦ Amplitude Shift Keying ◦ Frequency Shift Keying ◦ Phase Shift Keying
22
Amplitude Shift Keying ◦ 1 and 0 are represented by different levels of the signal
◦ More than two levels can be used
23
Amplitude Shift Keying
◦ Advantages: the most simple form of modulation
◦ Disadvantages:
susceptible to sudden noise impulses
not very efficient – very few levels can be used
not used for high data rates
24
Frequency Shift Keying
◦ Advantages: resistant to sudden noise impulses
◦ Disadvantages:
subject to intermodulation distortion (the frequencies of two or more signals mix together and create new frequencies)
not used for high data rates
26
Analog wave is converted to a signal which have a discrete number of values
The equipment used is called a codec
Different encoding techniques: ◦ Pulse Code Modulation (PCM)
◦ Delta Modulation
30
Pulse Code Modulation ◦ The analog value is converted at specific time moments
(sampling points) to the closest level
◦ Approximations are made (quantization errors)
31
Pulse Code Modulation ◦ Correct reconstruction depends on the sampling interval
and quantization errors.
32
Pulse Code Modulation ◦ Better results are obtained with larger sampling rate and
more output levels.
33