02_Handout_1(18).pdf

IT2006 Data and Signals (Ibe, 2017) Data must be transformed to electromagnetic signals. These signal-converted data are classified as either analog (or continuous time) or digital (or d...

IT2006 Data and Signals (Ibe, 2017) Data must be transformed to electromagnetic signals. These signal-converted data are classified as either analog (or continuous time) or digital (or discrete time). Analog signals, which include speech, audio, and video, have an infinite number of values. These are represented as a sine wave. Digital signals are predominantly binary in nature and thus are represented by two (2) values or bits: 0 and 1. These are represented as a square wave. Both analog and digital signals can take one of two (2) forms: Periodic signal – It completes a pattern within a measurable time frame, called a period, and repeats that pattern over subsequent identical periods. The completion of one (1) full pattern is called a cycle. Nonperiodic signal – It changes without exhibiting a pattern or cycle that repeats over time. Both analog and digital signals can be periodic or nonperiodic. Signal Terminologies Wavelength is directly related to the frequency of a given waveform. Formula for Wavelength 𝑐 𝜆= 𝑓 where: 𝜆 – wavelength (𝑚) 𝑓 – Frequency (𝐻𝑧) Figure 1. Parts of a Signal Source: Fundamentals of Data Communication Networks, 2017 𝑐 – wave speed or speed of light (𝑚/𝑠) Frequency refers to the number of waves that pass a given point in each time period and is often expressed in terms of hertz (𝑯𝒛) or cycles per second. Formula for Frequency 1 𝑓= 𝑇 where: 𝑓 – Frequency (𝐻𝑧) 𝑇 – Period (𝑠) Amplitude is a measure of how big the wave is, which is measured as follows: o The height from the equilibrium point to the highest point of a crest o The depth from the equilibrium point to the lowest point of a trough. Examples: 1) What is the wavelength of a radio wave with a 3) What would be the frequency of a signal with a given frequency of 900 𝑘𝐻𝑧? period of 0.00235? 𝑐 3 × 108 𝑚/𝑠 1 1 𝜆= = = 333.33 𝑚 𝑓= = = 425.53 𝐻𝑧 𝑓 900 𝑘𝐻𝑧 𝑇 0.00235 2) What is the frequency of violet light with a 4) If a sampling signal with frequency 50 𝐻𝑧 is to wavelength of 400 𝑛𝑚? be passed in a modulator, what will its period 𝑐 3 × 108 𝑚/𝑠 be over 1 second? 𝑓= = = 7.5 × 1014 𝐻𝑧 1 1 𝑠𝑒𝑐 𝜆 400 𝑛𝑚 𝑇= = = 0.02 𝑠𝑒𝑐/𝑐𝑦𝑐𝑙𝑒 𝑓 50 𝐻𝑧 02 Handout 1 *Property of STI  [email protected] Page 1 of 3  IT2006 Modems The term “modem” is a contraction of the words “modulator” and “demodulator.” The sending modem modulates the data into a signal that is compatible with the phone line, and the receiving modem demodulates the signal back into digital data. Modulation is the process of converting data into radio waves by adding information to an electronic or optical carrier signal. A carrier signal is one with a steady waveform—constant height, or amplitude, and frequency. Kinds of Modulation Amplitude modulation (AM) – It is the modulation technique in which carrier amplitude varies based on analog baseband information signal to be transmitted in a wireless medium. Frequency modulation (FM) – It is the modulation technique in which carrier frequency varies based on analog baseband information signal to be transmitted in a wireless medium. Phase modulation (PM) – It is the modulation technique in which the carrier phase varies based on analog baseband information signal to be transmitted in a wireless medium. Analog-to-Digital Conversion: From PAM to PCM This method is used to convert an analog signal, such as voice and video, into a digital signal. Each time a sample is taken, it is measured by the ADC (Analog-to-Digital Converter) that converts the analog value to a digital (binary) equivalent. Sampling lays out of the analog signal in a graph. The sampling process essentially converts analog amplitudes to discrete levels and is a type of modulation called pulse amplitude modulation (PAM). The next step is to quantize the digital pulses, which means to approximate the amplitude value of a pulse to the nearest integer on a predefined set of permitted integers. Quantization layers the discrete signal in the analog signal with less margin of error. Encoding (pulse code modulation [PCM]) converts discrete signals into highs (1) and lows (0), making these the binary equivalent of a time-bound discrete signal. Line coding is the process of converting digital data into digital signals. It is the representation of the digital signal to be transmitted by a digital waveform. Channel Impairments As a signal propagates along a communication path from its source to its destination, it is subject to different types of impairments. Attenuation – The strength of a signal decreases as it travels along a transmission medium. The amount of attenuation depends on the medium, but in general, it increases with distance. Noise – It is usually defined as an unwanted signal that is superimposed on a desired signal. o Atmospheric noise is the noise that is caused by such natural atmospheric phenomena as lightning discharge in thunderstorms and other electrical disturbances that occur in nature. o Man-made noise is an electromagnetic (EM) noise that is caused by human activities, which are associated with the use of electrical equipment. High-voltage wires and fluorescent lamps also produce this type of noise. o Extraterrestrial noise is the noise that comes from outside the earth and includes solar noise and cosmic noise. Solar noise is the noise that originates from the sun, while cosmic noise is generated by distant stars. 02 Handout 1 *Property of STI  [email protected] Page 2 of 3  IT2006 o Thermal noise occurs in electrical conductors and is caused by the thermal agitation of the charges in the material. o Shot noise arises from the time-dependent fluctuations in electrical current. This is caused by the discrete nature of electron charges. It is particularly noticeable in semiconductor devices. Distortion – It refers to the change or alteration of an object. Thus, in terms of data transmission, distortion means that the signal changes its form or shape. Delay distortion is a phenomenon that is peculiar to guided transmission media. Signal-to-Noise Ratio It is often useful to have a quantitative method for describing the quality of a signal in terms of its corruption by noise. Signal-to-noise is the ratio of the magnitude of the signal to that of the noise. 𝑃𝑠 𝑆/𝑁 𝑟𝑎𝑡𝑖𝑜 = 10 log10 ( ) 𝑑𝐵 𝑃𝑠 – Signal Power 𝑃𝑛 𝑉𝑠 2 𝑃𝑛 – Noise Power 𝑆/𝑁 𝑟𝑎𝑡𝑖𝑜 = 10 log10 ( ) 𝑑𝐵 𝑉𝑛 𝑉𝑠 – Signal Voltage 𝑉𝑠 𝑆/𝑁 𝑟𝑎𝑡𝑖𝑜 = 20 log10 ( ) 𝑑𝐵 𝑉𝑛 – Noise Voltage 𝑉𝑛 Examples: 1) A signal of 2.5 𝑣𝑜𝑙𝑡𝑠 𝑟𝑚𝑠 is corrupted by 2) A signal with 350 watts is transmitted through a 10 𝑚𝑉 𝑟𝑚𝑠 of noise in a circuit. What is the cable medium with 60 watts of noise. What is 𝑆/𝑁 𝑟𝑎𝑡𝑖𝑜 at this point? its 𝑆/𝑁 𝑟𝑎𝑡𝑖𝑜? 𝑉𝑠 2.5 𝑃𝑠 350 𝑆𝑁𝑅 = 20 log10 ( ) = 20 log10 ( ) = 48 𝑑𝐵 𝑆𝑁𝑅 = 10 log10 ( ) = 10 log10 ( ) = 7.66 𝑑𝐵 𝑉𝑛 0.01 𝑃𝑛 60 References: Ibe, O. (2018). Fundamentals of data communication networks (1st ed.). Wiley & Sons, Inc. Kurose, F. & Ross, K. (2017). Computer networking: A top-down approach (7th ed.). Pearson. Patniak, S., Yang, X., Tavana, M., Popentiu-Vlădicescu, & F., Qiao, F. (2019). Digital business: Business algorithms, cloud computing and data engineering. Springer International. Sklar, B. (2017). Digital communications: Fundamentals and applications (2nd ed.). Prentice Hall. Speidel, J. (2019). Introduction to digital communications. Springer Nature. 02 Handout 1 *Property of STI  [email protected] Page 3 of 3