Computer Communication Networks Lecture 4: Networks, The Physical Layer Notes taken by Sowmyashree Malur August 2007 Summary: This document contains the scribal version of the lecture delivered by Dr. Stephen Bush to the CSI416/516 class at University of Albany, on 2007 September 27, containing information about the Physical Network Layer. 1 Signal Processing Signal is a varying quantity that can carry information. Analysis, interpretation and manipulation of signals is signal processing. Processing includes storage and reconstruction, separation of information from noise, compression and extraction of information.Signals can be classi ed as: Analog signal: is time continuous signal. Processing of analog signal requires accurate reconstruction of signal with all data of the original signal and reconstruction is very harder. Digital signal: is discrete and quantized signal. Processing of digital signal requires discrete levels of data of the original signal and reconstruction is easier. 2 How do we get the signal across the line? The waveform pattern of voltage or current used to represent the 1s and 0s of a digital signal on a transmission link is called line encoding. Line coding technique is used in transporting digital data across the line. Line coding consists of representing the digital signal to be transported, by an amplitude and time- discrete signal that is optimally tuned for the speci c properties of the physical channel. There are common types of line coding: Unipolar encoding: A positive voltage represents a binary 1, and zero volts indicate a binary 0. Polar encoding: Voltage is +V for 1 and -V for 0. Differential encoding: Voltage does not change during a 0, toggles between +V and -V for a 1 Bipolar encoding: uses three voltage levels: positive, negative, and zero. The zero level in bipolar encoding is used to represent binary 0. The 1s are represented by alternating positive and negative voltages. If the rst 1 bit is represented by the positive amplitude, the second will be represented by the negative amplitude, the third by the positive amplitude, and so on. Manchester encoding: A 0 is expressed by a low-to-high transition, a 1 by high-to-low transition Lecture Notes for a course given by Stephen F. Bush at SUNY. 1 Differential Manchester encoding: A 1 is represented by one transition, and then a 0 is represented by two transitions and vice versa. Each encoding techniques have their own tradeoffs. 3 PSTN: Public switched telephone networks. This network refers to the international telephone system based on copper wires carrying analog voice data. Voice signals are analog in nature and are incapable of representing as discrete signals. This problem motivated the idea of turning continuous signal to discrete signals and hence the use of Fourier series and transforms. 4 Fourier series It is a mathematical tool used for analyzing periodic functions by decomposing such a function into a weighted sum of much simpler sinusoidal component functions. The Fourier series of a signal g(t) is given int lecture slide: where c represents constant displacement of the entire wave pattern, ak bk represents harmonics of the wave, n takes integer values. 5 Why Fourier series useful? Fourier series serve many useful purposes, as manipulation and conceptualization of the coef cients are often easier than with the original function. In telecommunication system, design can be optimized by using information about the spectral components of the data signal that the system will carry. Different encoding techniques mentioned before uses Fourier series for encoding. The Fourier series is explained with the example worked in the lecture slide. 6 What is Root mean square value? Root mean square (abbreviated RMS), also known as the quadratic mean, is a statistical measure of the magnitude of a varying quantity. It is especially useful when variants are positive and negative, e.g. waves. It can be calculated for a series of discrete values or for a continuously varying function. The name comes from the fact that it is the square root of the mean of the squares of the values. It is a power mean with the power p = 2. RMS gives information about the energy transmitted at certain frequency. Comparing the results for different values of n solved for an example in the lecture slides: As the value of n increases the signal obtained is very similar to the original signal, however at higher values of n signal attenuates more. 7 Fourier transforms: The Fourier transform of a signal x(t) can be thought of as a representation of a signal in the frequency domain ; i.e. how much each frequency contributes to the signal. Fourier transform decomposes a function into a continuous spectrum of its frequency components, and the inverse transform synthesizes a function from its spectrum of frequency components. The function is de ned in the lecture slides. The complex-valued function X, is said to represent in the frequency domain. I.e., if is a continuous function, then it can be reconstructed from by the inverse transform which is de ned in the lecture slides. 2 8 Fourier transforms usefulness over Fourier series: Dif cult operations like convolution in the time domain often have corresponding simpler operations like multiplication in the frequency domain (and vice versa) giving rise to the trick: Convert the operands to functions in the appropriate domain Do the (simpler) corresponding operation Convert the operands back to functions in the original domain which is often used in digital processing. 9 Modulation: It is the process of varying a periodic waveform, i.e. a tone, in order to use that signal to convey a message. It is also called <a href="/keyword/phase-shift-keying/" >phase shift keying</a> . In other words, it encodes discrete data using sine waves by simultaneously altering one or more of the fundamental wave components [3, 6, 7] of a higher frequency carrier signal. Normally a high-frequency sinusoid waveform is used as carrier signal. The three key parameters of a sine wave are its amplitude ( volume ), its phase ( timing ) and its frequency ( pitch ), all of which can be modi ed in accordance with a low frequency information signal to obtain the modulated signal. A device that performs modulation is known as a modulator and a device that performs the inverse operation of modulation is known as a demodulator. A device that can do both operations is a modem Different modulation forms: Amplitude-shift keying (ASK) is a form of modulation that represents digital data as variations in the amplitude of a carrier wave. The amplitude of an analog carrier signal varies in accordance with the bit stream (modulating signal), keeping frequency and phase constant. The level of amplitude can be used to represent binary logic 0s and 1s. In the modulated signal, logic 0 is represented by the absence of a carrier, thus giving OFF/ON keying operation. Frequency-shift keying (FSK) is a form of frequency modulation in which the modulating signal shifts the output frequency between predetermined values. Usually, the instantaneous frequency is shifted between two discrete values Phase-shift keying (PSK) is a digital modulation scheme that conveys data by changing, or modulating, the phase of a reference signal (the carrier wave). Amplitude modulation encoding and decoding is performed as follows: A base band signal is sinusoidal with a value between 0 and some upper limit, say B. (A binary base band signal is unipolar). The transmitted signal is converted to a base band envelope and is interpolated by the crests of a high frequency carrier signal. The received signal is converted to a base band by a recti er and reconstructed by interpolation Sidebands in AM: The modulating data appears to have signal frequency components slightly higher and lower than the carrier. These components are called sidebands. Amplitude modulation is inef cient in terms of power usage and much of it is wasted. At least two-thirds of the power is concentrated in the carrier signal, which carries no useful information (beyond the fact that a signal is present); the remaining power is split between two identical sidebands, though only one of these is needed since they contain identical information. 10 <a href="/keyword/quadrature-amplitude-modulation/" >quadrature amplitude modulation</a> : It is a modulation scheme which conveys data by changing (modulating) the amplitude of two carrier waves. These two waves, usually sinusoids, are out of phase with each other by 90 and are thus called quadrature. <a href="/keyword/quadrature-amplitude-modulation/" >quadrature amplitude modulation</a> (QAM) combines <a href="/keyword/phase-shift-keying/" >phase shift keying</a> and amplitude shift keying to encode multiple bits per state change. The set of graphs given in the lecture slides represents different combinations of different bit levels and different bandwidth at the receiver. Ak represents the change of amplitude and Bk represents the change of frequency. 3 11 Pulse Code modulation It is a digital representation of an analog signal where the magnitude of the signal is sampled regularly at uniform intervals, then quantized to a series of symbols in a digital (usually binary) code. Sampling is the reduction of a continuous signal to a discrete signal. Quantization is the process of approximating a continuous range of values (or a very large set of possible discrete values) by a relatively-small set of discrete symbols or integer values. Quantization allows for (limited) error correction on the receiver side. 12 Corrupting In uences on Signals: Signal distortion is the alteration of the original shape (or other characteristic) of signal. Signal distortion can result due to: Attenuation: is the reduction in amplitude and intensity of a signal. Signals may be attenuated exponentially by transmission through a medium, in which case attenuation is usually reported in dB with respect to distance traveled through the medium Limited bandwidth: Causes the drop in information Noise: A disturbance that affects a signal and that may distort the information carried by the signal. Delay or Skew: is due to shifting the phase of the signal. Results in loss of power. In radio waves, distortion can be due to bouncing of things in environment and also adding of coherent signals. Signal-to-noise ratio: It is the ratio of a signal power to the noise power corrupting the signal. The higher the ratio, the less obtrusive the background noise is. This is shown in the lecture slide. Attenuation is a function of frequency:Signals are distorted at different frequency. Attenuation increases as frequency increases and as the gauge (thickness of wall of ber through which signal ows) gets thinner. Distortion and Equalizer: Equalization is the process of changing the frequency envelope of a sound. These are general all-purpose lters, which can be arranged to produce the effect of low pass, high pass, band pass and band stop lters. Th...