Chapter Two- Physical Layer PDF

Physical CHAPTER 2 Layer and Media 11/19/2024 1 OBJECTIVES discusses the relationship between data, which are created by a device, and electromagnetic signals, which are transmitted over a medium. digital transmission. We discuss how we...

Physical CHAPTER 2 Layer and Media 11/19/2024 1 OBJECTIVES discusses the relationship between data, which are created by a device, and electromagnetic signals, which are transmitted over a medium. digital transmission. We discuss how we can covert digital or analog data to digital signals. analog transmission. We discuss how we can covert digital or analog data to analog signals. how we can use the available bandwidth efficiently. We discuss two separate, but related topics, multiplexing and spreading. characteristics of transmission media, both guided and unguided, in this chapter. Although transmission media operates under the physical layer, they are controlled by the physical layer. 11/19/2024 2 PHYSICAL LAYER interacts with the transmission media, the physical part of the network that connects network components together. One of the services provided by the physical layer is to create a signal that represents this stream of bits. The physical layer must also take care of the physical network, the transmission medium.  The transmission medium must be controlled by the physical layer. The physical layer decides on the directions of data flow. The physical layer decides on the number of logical channels for transporting data coming from different sources. 11/19/2024 3 Note To be transmitted, data must be transformed to electromagnetic signals. 11/19/2024 3.4  3-1 ANALOG AND DIGITAL Data can be analog or digital. The term analog data refers to information that is continuous; digital data refers to information that has discrete states. Analog data take on continuous values. Digital data take on Topics discrete values. discussed in this section:  Analog and Digital Data  Analog and Digital Signals  Periodic and Nonperiodic Signals 11/19/2024 3.5  ANALOG AND DIGITAL DATA Data can be analog or digital. Analog data are continuous and take continuous values. Digital data have discrete states and take discrete values. 11/19/2024 3.6  ANALOG AND DIGITAL SIGNALS Signals can be analog or digital. Analog signals can have an infinite number of values in a range. Digital signals can have only a limited number of values. 11/19/2024 3.7 Figure 3.1 Comparison of analog and digital signals 11/19/2024 3.8 eriodic and Nonperiodic Signals Both analog and digital signals can take one of two forms: periodic or nonperiodic A periodic signal completes a pattern within a measurable time frame, 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. 11/19/2024 3.9  3-2 PERIODIC ANALOG SIGNALS In data communications, we commonly use periodic analog signals and nonperiodic digital 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 Topics discussed in this section: waves.  Sine Wave  Wavelength  Time and Frequency Domain  Composite Signals  Bandwidth 11/19/2024 3.10 Figure 3.2 A sine wave simple oscillating curve, its change over the course of a cycle is smooth and consistent, a continuous, rolling flow. A sine wave can be represented by three parameters: the peak amplitude, the frequency, and the phase. These three parameters fully describe a sine wave. periodic analog signal. 11/19/2024 3.11 Signal characteristics The peak amplitude The peak amplitude of a signal is the absolute value of its highest intensity, proportional to the energy it carries. For electric signals, peak amplitude is normally measured in volts. 11/19/2024 12 gure 3.3 Two signals with the same phase and frequency, but different amplitudes 11/19/2024 3.13  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 I s. 11/19/2024 14 Note Frequency and period are the inverse of each other. 11/19/2024 3.15 gure 3.4 Two signals with the same amplitude and phase, but different frequencies 11/19/2024 3.16  Period is formally expressed in seconds. Frequency is formally expressed in Hertz (Hz), which is cycle per second. Table 3.1 Units of period and frequency 11/19/2024 3.17  FREQUENCY Frequency is the rate of change with respect to time. Change in a short span of time means high frequency. Change over a long span of time means low frequency. 11/19/2024 3.18 Note If a signal does not change at all, its frequency is zero. If a signal changes instantaneously, its frequency is infinite. 11/19/2024 3.19 Note Phase describes the position of the waveform relative to time 0. 11/19/2024 3.20 ure 3.5 Three sine waves with the same amplitude and frequency, but different phases 11/19/2024 3.21  Example 3.3 A sine wave is offset 1/6 cycle with respect to time 0. What is its phase in degrees and radians? Solution We know that 1 complete cycle is 360°. Therefore, 1/6 cycle is 11/19/2024 3.22  Wavelength Wavelength is a property of any type of signal. The wavelength is the distance a simple signal can travel in one period. Wavelength binds the period or the frequency of a simple sine wave to the propagation speed of the medium. frequency of a signal is independent of the medium, the wavelength depends on both the frequency and the medium. In data communications, we often use wavelength to describe the transmission of light in an optical fiber. 11/19/2024 3.23 Figure 3.6 Wavelength and period 11/19/2024 3.24 Wavelength can be calculated if one is given the propagation speed (the speed of light) and the period of the signal. However, since period and frequency are related to each other, if we represent wavelength by , propagation speed by c (speed of light), and frequency by f, we get. Wavelength =propagation speed x period of a signal = propagation speed /frequency The propagation speed of electromagnetic signals depends on the medium and on the frequency of the signal. For example, in a vacuum, light is propagated with a speed of 3 x 10 8 mls. That speed is lower in air and even lower in cable. The wavelength is normally measured in micrometers (microns) instead of meters. For example, the wavelength of red light (frequency =4 x 1014) in air is In a11/19/2024 coaxial or fiber-optic cable, however, the wavelength is shorter25 gure 3.7 The time-domain and frequency-domain plots of a sine wave 11/19/2024 3.26 Note A complete sine wave in the time domain can be represented by one single spike in the frequency domain. 11/19/2024 3.27 ure 3.8 The time domain and frequency domain of three sine waves 11/19/2024 3.28 SIGNALS AND COMMUNICATION A single-frequency sine wave is not useful in data communications We need to send a composite signal, a signal made of many simple sine waves. According to Fourier analysis, any composite signal is a combination of simple sine waves with different frequencies, amplitudes, and phases. 11/19/2024 3.29  COMPOSITE SIGNALS AND PERIODICITY If the composite signal is periodic, the decomposition gives a series of signals with discrete frequencies. If the composite signal is nonperiodic, the decomposition gives a combination of sine waves with continuous frequencies. 11/19/2024 3.30 ure 3.10 Decomposition of a composite periodic signal in the time and frequency domains 11/19/2024 3.31  BANDWIDTH AND SIGNAL FREQUENCY The bandwidth of a composite signal is the difference between the highest and the lowest frequencies contained in that signal. 11/19/2024 3.32 ure 3.12 The bandwidth of periodic and nonperiodic composite signal 11/19/2024 3.33  Group Discussion 1. How a dtat in the physical medium will travel? 2. What is the relationship between period and frequency? 3. What does the amplitude of a signal measure?What does the frequency of a signal measure? What does the phase of a signal measure? 4. How can a composite signal be decomposed into its individual frequencies? 5. Can we say whether a signal is periodic or nonperiodic by just looking at its frequency domain plot? How? 6. Is the frequency domain plot of a voice 11/19/2024 signal discrete or continuous? 34  3-3 DIGITAL SIGNALS In addition to being represented by an analog signal, information can also be represented by a digital signal. For example, a 1 can be encoded as a positive voltage and a 0 as zero voltage. A digital signal can have more than two levels. In this case, we can send more than 1 bit for each level. Topics discussed in this section:  Bit Rate  Bit Length  Digital Signal as a Composite Analog Signal  Application 11/19/2024 Layer 3.35 gure 3.16 Two digital signals: one with two signal levels and the other with four signal levels 11/19/2024 3.36  Bit Rate Most digital signals are nonperiodic, and thus period and frequency are not appropriate characteristics. Another term-bit rate (instead of frequency)-is used to describe digital signals. The bit rate is the number of bits sent in 1s, expressed in bits per second (bps). Figure 3.16 shows the bit rate for two signals. 11/19/2024 3.37  Bit Length We discussed the concept of the wavelength for an analog signal: the distance one cycle occupies on the transmission medium. We can define something similar for a digital signal: the bit length. The bit length is the distance one bit occupies on the transmission medium. Bit length =propagation speed x bit duration 11/19/2024 3.38  Bit Length Data Rate Limits: one of the most 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: a.The bandwidth available b.The level of the signals we use c.The quality of the channel (the level of noise) 11/19/2024 3.39  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 are attenuation, distortion, and noise. Topics discussed in this section:  Attenuation  Distortion  Noise 11/19/2024 3.40 Figure 3.25 Causes of impairment 11/19/2024 3.41 ATTENUATION where signal strength falls off with distance depends on medium received signal strength must be:  strong enough to be detected  sufficiently higher than noise to receive without error so increase strength using amplifiers/repeaters is also an increasing function of frequency so equalize attenuation across band of frequencies used  eg. using loading coils or amplifiers 11/19/2024 3.42 MEASUREMENT OF ATTENUATION Attenuation is measured in terms of Decibels. The decibel (dB) measures the relative strengths of two signals or one signal at two different points. Note that the decibel is negative if a signal is attenuated and positive if a signal is amplified. dB = 10log10P2/P1 P1 - input signal P2 - output signal 11/19/2024 3.43 Figure 3.26 Attenuation 11/19/2024 3.44 DISTORTION Means that the signal changes its form or shape Distortion can occur in a composite signal made of different frequencies. Each frequency component has its own propagation speed traveling through a medium. The different components therefore arrive with different delays at the receiver. That means that the signals have different phases at the receiver than they did at the source. 11/19/2024 3.45 Figure 3.28 Distortion 11/19/2024 3.46 NOISE Noise is any unwanted signal that is mixed or combined with the original signal during transmission. Due to noise the original signal is altered and signal received is not same as the one sent. 11/19/2024 3.47 NOISE There are different types of noise  Thermal - is the random motion of electrons in a wire which creates an extra signal not originally sent by the transmitter.  Induced – Induced noise comes from sources such as motors and appliances. These devices act as a sending antenna, and the transmission medium acts as the receiving antenna..  Crosstalk - Crosstalk is the effect of on the other. One wire acts as a sending antenna and the other as the receiving antenna.  Impulse - Impulse noise is a spike (a signal with high energy in a very short time) that comes from power lines, lightning, and so on. 11/19/2024 3.48 Figure 3.29 Noise 11/19/2024 3.49 SIGNAL TO NOISE RATIO (SNR) To measure the quality of a system the SNR is often used. It indicates the strength of the signal wrt the noise power in the system. SNR= Average Signal power / Average Noise Power It is the ratio between two powers. It is usually given in dB and referred to as SNRdB. defined as SNRdB = l0logl0 SNR 11/19/2024 3.50 Figure 3.30 Two cases of SNR: a high SNR and a low SNR 11/19/2024 3.51  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) Topics discussed in this section:  Noiseless Channel: Nyquist Bit Rate  Noisy Channel: Shannon Capacity  Using Both Limits 11/19/2024 3.52 Note Increasing the levels of a signal increases the probability of an error occurring, in other words it reduces the reliability of the system. 11/19/2024 3.53 CAPACITY OF A SYSTEM The bit rate of a system increases with an increase in the number of signal levels we use to denote a symbol. A symbol can consist of a single bit or “n” bits. The number of signal levels = 2n. As the number of levels goes up, the spacing between level decreases -> increasing the probability of an error occurring in the presence of transmission impairments. 11/19/2024 3.54 NYQUIST THEOREM Nyquist gives the upper bound for the bit rate of a transmission system by calculating the bit rate directly from the number of bits in a symbol (or signal levels) and the bandwidth of the system (assuming 2 symbols/per cycle and first harmonic). Nyquist theorem states that for a noiseless channel: C = 2 B log22n C= capacity in bps B = bandwidth in Hz 11/19/2024 3.55 SHANNON’S THEOREM Shannon’s theorem gives the capacity of a system in the presence of noise. C = B log2(1 + SNR) In this formula, bandwidth is the bandwidth of the channel, SNR is the signal-to-noise ratio, and capacity is the capacity of the channel in bits per second. 11/19/2024 3.56 Note The Shannon capacity gives us the upper limit; the Nyquist formula tells us how many signal levels we need. 11/19/2024 3.57