03 Introduction to Physical layer
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This document provides an overview of the physical layer of communication systems, focusing on the different types of data and signals, including analog and digital signals. It explains key concepts such as periodic and non-periodic signals, signal parameters like amplitude, frequency, and phase, and discusses transmission impairments like attenuation, distortion, and noise. Additionally, it covers the implications of bandwidth and its relationship with data rate in communication channels.
03 Introduction to Physical layer
- 2. OUTLINE 3.1 DATA AND SIGNALS 3.1.1 Analog and Digital Signals 3.1.2 Periodic and Non- periodic 3.2 PERIODIC ANALOG SIGNALS 3.2.1 Sine Wave 3.2.2 Phase 3.2.3 Wavelength 3.2.4 Time and Frequency Domains 3.2.5 Composite Signals 3.3 DIGITAL SIGNALS 3.3.1 Bit Rate 3.3.2 Bit Length 3.4 TRANSMISSION IMPAIRMENT 3.4.1 Attenuation 3.4.2 Distortion 3.4.3 Noise 2
- 3. 3.1 DATA AND SIGNALS 3
- 4. 3.1.1 ANALOG AND DIGITAL DATA • Analog data • Data take on continuous values • E.g., human voice, temperature reading • Digital data • Data take on discrete values • E.g., text, integers 4
- 5. 3.1.1 ANALOG AND DIGITAL SIGNAL • Analog signals • have an infinite number of values in a range • Digital signals • Have a limited number of values • Both signals can take one of two forms: periodic or nonperiodic 5 value time value time
- 6. DATA AND SIGNALS 6 Telephone Analog Data Analog Signal Modem Digital Data Analog Signal Codec Analog Data Digital Signal Digital transmitter Digital Data Digital Signal
- 7. Analog Digital Analog signal is a continuous signal which represents physical measurements. Digital signals are discrete time signals generated by digital modulation. Denoted by sine waves Denoted by square waves Uses continuous range of values to represent information Uses discrete or discontinuous values to represent information Human voice in air, analog electronic devices. Computers, CDs, DVDs, and other digital electronic devices. Analog technology records waveforms as they are. Samples analog waveforms into a limited set of numbers and records them. Can be used in analog devices only. Best suited for audio and video transmission. Best suited for Computing and digital electronics. 7
- 8. 3.1.2 PERIODIC AND NONPERIODIC • A periodic signal completes a pattern within a timeframe, called a period and repeats that pattern over subsequent identical periods. • The completion of one full pattern is called a cycle. • A nonperiodic signal changes without exhibiting a pattern or cycle that repeats over time. 8
- 9. 3.2 PERIODIC ANALOG SIGNALS 9
- 10. 3.2 PERIODIC ANALOG SIGNALS • Periodic analog signals can be classified as simple or composite. • A simple periodic analog signal, a sine wave, cannot be decomposed into simpler signals. • A composite periodic analog signal is composed of multiple sine waves. 10
- 11. 3.2.1 SINE WAVES • It is most simplest form of periodic signal. • A sine wave can be represented by three parameters: • Peak amplitude • Frequency • Phase 11 period T = 1/f peak amplitude time signal strength
- 12. 3.2.1.1 PEAK AMPLITUDE • Its highest intensity, proportional to the energy it carries. • For electric signals, peak amplitude is normally measured in volts. 12
- 13. 3.2.1.2 PERIOD AND FREQUENCY • Period refers to the amount of time, in seconds, a signal needs to complete 1 cycle. • Frequency refers to the number of periods in one second. • Frequency and period are the inverse of each other. 13
- 14. 3.2.1.3 PHASE • Phase describes the position of the waveform relative to time 0. • Phase is measured in degrees or radians 14
- 15. VARYING SINE WAVES 15 -3 -2 -1 0 1 2 3 0 0.5 1 1.5 2 2.5 3 A = 1, f = 1, = 0 -3 -2 -1 0 1 2 3 0 0.5 1 1.5 2 2.5 3 A = 2, f = 1, = 0 -3 -2 -1 0 1 2 3 0 0.5 1 1.5 2 2.5 3 A = 1, f = 2, = 0 -3 -2 -1 0 1 2 3 0 0.5 1 1.5 2 2.5 3 A = 1, f = 1, = /4
- 16. 3.2.3 WAVELENGTH 16 • The wavelength of a signal refers to the relationship between frequency (or period) and propagation speed of the wave through a medium. • The distance between successive crests of a wave, especially points in a sound wave or electromagnetic wave. • It is represented by the symbol: Lambda
- 17. 3.2.4 TIME AND FREQUENCY DOMAINS 17
- 18. THE TIME-DOMAIN AND FREQUENCY-DOMAIN PLOTS OF A SINE WAVE 18
- 19. 19 THE TIME DOMAIN AND FREQUENCY DOMAIN OF THREE SINE WAVES
- 20. 3.2.5 COMPOSITE SIGNALS 20 • A composite signal is a combination of two or more simple sines waves with different frequency, phase and amplitude. • If the composite signal is periodic, the decomposition gives a series of signals with discrete frequencies. • If the composite signal is non-periodic, the decomposition gives a combination of sine waves with continuous frequencies.
- 21. 3.2.6 BANDWIDTH • The range of frequencies contained in a composite signal is its bandwidth. • The band-width is normally a difference between two numbers. • For example, if a composite signal contains frequencies between 1000 and 5000, its bandwidth is 5000 − 1000, or 4000. 21
- 23. 3.3 DIGITAL SIGNALS • A digital is a signal that has discrete values. • The signal will have value that is not continuous. • Information in digital signal can be represented in the form of voltage levels. • A 1 can be encoded as a positive voltage and a 0 as zero negative voltage. • A digital signal can have more than two levels 23
- 24. FIGURE: TWO DIGITAL SIGNALS: ONE WITH TWO SIGNAL LEVELS AND THE OTHER WITH FOUR SIGNAL LEVELS 24
- 25. 3.3.1 BIT RATE • It is number of bits sent in per second. • It is expressed in bits per second (bps). • Bit interval – duration of 1 bit • 3.3.2 Bit Length • The distance one bit occupies on the transmission medium. • Bit length = propagation speed x bit duration 25
- 26. 3.3.3 DIGITAL SIGNAL AS A COMPOSITE ANALOG SIGNAL • A digital signal, in the time domain, comprises connected vertical and horizontal line segments. • A vertical line in the time domain means a frequency of infinity. • a horizontal line in the time domain means a frequency of zero 26
- 27. 27 The time and frequency domains of periodic and nonperiodic digital signals
- 29. 3.3.4 TRANSMISSION OF DIGITAL SIGNALS • A digital signal by using one of two different approaches • Baseband transmission • Broadband transmission • Baseband Transmission: • Sending a digital signal over a channel without changing it to an analog signal. • Baseband transmission requires a low-pass channel • LP is a channel with a bandwidth that starts from zero. • Broadband Transmission: • A digital signal cannot be transmitted directly through it. • In broadband transmission we use modulation, i.e. we change the signal to analog signal before transmitting it. • Broadband transmission requires bandpass channel. 29
- 30. BANDWIDTH • A property of a medium • Indicates the difference between the highest and the lowest frequencies allowed to pass • <highest freq allowed> – <lowest freq allowed> • Also a property of a single spectrum 30 Cutoff frequency (half of power is lost)
- 32. 3.4 TRANSMISSION IMPAIRMENT • Signals travel through transmission media, which are not perfect. • The imperfection causes signal impairment. • This means that the signal at the beginning of the medium is not the same as the signal at the end of the medium. • What is sent is not what is received. • Three causes of impairment • Attenuation • Distortion • Noise 32
- 34. 3.4.1 SIGNAL ATTENUATION • Attenuation Loss of energy • Signal strength falls off with distance • Attenuation depends on medium • Attenuation is an increasing function of frequency 34 Transmission medium
- 35. 3.4.2 SIGNAL DISTORTION • Distortion Change in signal shape • Only happens in guided media • Propagation velocity varies with frequency 35
- 36. 3.4.3 NOISE • Noise Undesirable signals added between the transmitter and the receiver • Types of noise • Thermal • Due to random motion of electrons in a wire 36
- 37. 3.4.3 NOISE (CONT..) • Types of noise (cont’d) • Crosstalk • Signal from one line picked up by another • Impulse • Irregular pulses or spikes • E.g., lightning • Short duration • High amplitude 37 Wire 1 Wire 2
- 38. 3.5 DATA RATE LIMITS • A very important consideration in data communications is how fast we can send data, in bits per second, over a channel. Data rate depends on three factors: 1. The bandwidth available 2. The level of the signals we use 3. The quality of the channel (the level of noise) • Two theoretical formulas were developed to calculate the data rate • Noiseless Channel: Nyquist Bit Rate • Noisy Channel: Shannon Capacity 38
- 39. NOISELESS CHANNEL: NYQUIST BIT RATE 3.39
- 41. SUMMARY • Data need to take form of signal to be transmitted • Frequency domain representation of signal allows easier analysis • Fourier analysis • Medium's bandwidth limits certain frequencies to pass • Bit rate is proportional to bandwidth • Signals get impaired by attenuation, distortion, and noise 41