COTN 1220 Data Communication - PDF

COTN 1220 – Data Communication Data and Signals ANALOG AND DIGITAL DATA To be transmitted, data must be transformed to electromagnetic signals. Data can be analog or digital. Analog data – Analog data are continuous and take continuous values. Digital data...

COTN 1220 – Data Communication Data and Signals ANALOG AND DIGITAL DATA To be transmitted, data must be transformed to electromagnetic signals. Data can be analog or digital. Analog data – Analog data are continuous and take continuous values. Digital data – Digital data have discrete states and take discrete values. ANALOG AND DIGITAL SIGNALS Signals can be analog or digital. a. Analog signals can have an infinite number of values in a range. b. Digital signals can have only a limited number of values. PERIODIC AND NONPERIODIC SIGNALS Analog and Digital Signals Periodic Nonperiodic Simple Composite PERIODIC AND NONPERIODIC SIGNALS (cont.) Periodic signals – completes a pattern within a measurable time frame, called a period, and repeats that pattern over subsequent identical periods. Cycle – completion of one full pattern is called a cycle. Nonperiodic signals – changes without exhibiting a pattern or cycle that repeats over time. PERIODIC ANALOG SIGNALS Periodic analog signals Simple –cannot be decomposed into simpler signals (sine wave). Composite – is composed of multiple sine waves. PEAK AMPLITUDE Peak amplitude – the absolute value of its highest intensity, proportional to the energy it carries. PERIOD AND FREQUENCY Period – the amount of time, in seconds, a signal needs to complete 1 cycle. (T) Frequency – number of periods in 1s. (f) # 𝒄𝒚𝒄𝒍𝒆𝒔 # 𝒔𝒆𝒈 𝒇= 𝒂𝒏𝒅 𝑻 = # 𝒔𝒆𝒈 # 𝒄𝒚𝒄𝒍𝒆𝒔 PERIOD AND FREQUENCY (cont.) Period is formally expressed in seconds. Frequency is formally expressed in Hertz (Hz), which is cycle per second. PERIOD AND FREQUENCY (cont.) The power we use at home has a frequency of 60 Hz. The period of this sine wave can be determined as follows: 𝟏 𝑻= 𝒇 𝟏 𝟏 𝑻= = = 𝟎. 𝟎𝟏𝟔𝟔𝒔 = 𝟏𝟔. 𝟔 𝒎𝒔 𝒇 𝟔𝟎 PERIOD AND FREQUENCY (cont.) The period of a signal is 100 ms. What is its frequency in kilohertz? 𝟏 𝒇= 𝑻 𝟏 𝟏 𝒇= = = 𝟏𝟎𝑯𝒛 𝑻 𝟎. 𝟏𝒔 𝟎. 𝟎𝟎𝟏𝒌𝑯𝒛 𝟏𝟎𝑯𝒛 𝒙 = 𝟎. 𝟎𝟏𝒌𝑯𝒛 𝟏𝑯𝒛 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. FREQUENCY (cont.) What if a signal does not change at all? What if it maintains a constant voltage level for the entire time it is active? But what if a signal changes instantaneously? If a signal does not change at all, its frequency is zero. If a signal changes instantaneously, its frequency is infinite. TIME AND FREQUENCY DOMAINS The time-domain plot shows changes in signal amplitude with respect to time (it is an amplitude- versus-time plot). A frequency-domain plot is concerned with only the peak value and the frequency. TIME AND FREQUENCY DOMAINS (cont.) A complete sine wave in the time domain can be represented by one single spike in the frequency domain. TIME AND FREQUENCY DOMAINS (cont.) Composite periodic signal shown in a frequency- domain plot. WAVELENGHT Wavelength is another characteristic of a signal traveling through a transmission medium. Wavelength binds the period or the frequency of a simple sine wave to the propagation speed of the medium. In other words, the distance a simple signal can travel 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. 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. COMPOSITE SIGNALS The following figure shows a periodic composite signal with frequency f. This type of signal is not typical of those found in data communications. We can consider it to be three alarm systems, each with a different frequency. The analysis of this signal can give us a good understanding of how to decompose signals. COMPOSITE SIGNALS (cont.) Periodic Signal COMPOSITE SIGNALS The following figure shows a nonperiodic composite signal. It can be the signal created by a microphone or a telephone set when a word or two is pronounced. In this case, the composite signal cannot be periodic, because that implies that we are repeating the same word or words with exactly the same tone. COMPOSITE SIGNALS (cont.) Nonperiodic Signal BANDWIDTH The range of frequencies contained in a composite signal is its bandwidth. The bandwidth of a composite signal is the difference between the highest and the lowest frequencies contained in that signal BANDWIDTH (cont.) BANDWIDTH (cont.) If a periodic signal is decomposed into five sine waves with frequencies of 100, 300, 500, 700, and 900 Hz, what is its bandwidth? Draw the spectrum, assuming all components have a maximum amplitude of 10 V. 𝑩 = 𝒇𝒉 − 𝒇𝒍 B = 900 Hz − 100 Hz = 𝟖𝟎𝟎𝑯𝒛 BANDWIDTH (cont.) A periodic signal has a bandwidth of 20 Hz. The highest frequency is 60 Hz. What is the lowest frequency? Draw the spectrum if the signal contains all frequencies of the same amplitude. 𝑩 = 𝒇𝒉 − 𝒇𝒍 20Hz = 60 Hz − fl fl = 60Hz − 20 Hz = 40Hz DIGITAL SIGNALS 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. DIGITAL SIGNALS (cont.) DIGITAL SIGNALS (cont.) If a signal has L levels, each level needs log 2 𝐿 bits (2𝑏𝑖𝑡𝑠 ≥ 𝐿) Example: How many bits are needed per level if a digital signal has four levels? 2𝑏𝑖𝑡𝑠 ≥ 4 22 ≥ 4 bits = 2 DIGITAL SIGNALS (cont.) A digital signal has nine levels. How many bits are needed per level? Solution: 2𝑏𝑖𝑡𝑠 ≥ 9 24 ≥ 9 16 ≥ 9 bits = 4 BIT RATE (Frequency) Most digital signals are nonperiodic, and thus period and frequency are not appropriate characteristics. The term bit rate (instead of frequency) is used to describe digital signals. Bit rate is the number of bits sent in 1s, expressed in bits per second (bps). BIT RATE (cont.) Real life application 4K TV uses digital signals to broadcast high quality video signals. The 4K screen is normally a ratio of 16 : 9. There are 4096 by 2160 pixels (24 bits) per screen, and the screen is renewed 60 times per second. 4096 x 2160 x 60 x 24 = 12,740,198,400 or 12.7 Gbps BIT LENGTH (Wavelenght) 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. BANDWIDTH A digital signal is a composite analog signal with an infinite bandwidth. Why infinite bandwidth? A vertical line in the time domain means a frequency of infinity (sudden change in time); a horizontal line in the time domain means a frequency of zero (no change in time). Going from a frequency of zero to a frequency of infinity (and vice versa) implies all frequencies in between are part of the domain. BASEBAND TRANSMISSION Baseband transmission means sending a digital signal over a channel without hanging the digital signal to an analog signal. Requires we have a dedicated medium with a bandwidth constituting only one channel. Ex. the entire bandwidth of a cable In baseband transmission, the required bandwidth is proportional to the bit rate; if we need to send bits faster, we need more bandwidth. BROADBAND TRANSMISSION Broadband transmission or modulation means changing the digital signal to an analog signal for transmission. 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. PHYSICS ATTENUATION Means loss of energy -> weaker signal When a signal travels through a medium it loses energy overcoming the resistance of the medium Amplifiers are used to compensate for this loss of energy by amplifying the signal. DISTORTION Means that the signal changes its form or shape Distortion occurs in composite signals 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. DISTORTION (cont.) PHYSICS 2 NOISE There are different types of noise Thermal - random noise of electrons in the wire creates an NOISE There are different types of noise Thermal - random noise of electrons in the wire creates an extra signal Induced - from motors and appliances, devices act are transmitter antenna and medium as receiving antenna. NOISE There are different types of noise Thermal - random noise of electrons in the wire creates an extra signal Induced - from motors and appliances, devices act are transmitter antenna and medium as receiving antenna. Crosstalk - same as above but between two wires. NOISE There are different types of noise Thermal - random noise of electrons in the wire creates an extra signal Induced - from motors and appliances, devices act are transmitter antenna and medium as receiving antenna. Crosstalk - same as above but between two wires. Impulse - Spikes that result from power lines, lightning, etc. NOISE (cont.) PERFORMANCE One important issue in networking is the performance of the network—how good is it? Some of the performance measurements we will discuss are: Bandwidth Throughput Latency (Delay) Jitter BANDWIDTH In networking, we use the term bandwidth in two contexts. Bandwidth in Hertz The range of frequencies contained in a composite signal or the range of frequencies a channel can pass. Bandwidth in Bits per Seconds Refers to the speed of bit transmission in a channel or link. THROUGHPUT The throughput is a measure of how fast we can actually send data through a network. Bandwidth ≠ Throughput In other words, the bandwidth is a potential measurement of a link; the throughput is an actual measurement of how fast we can send data. LATENCY (DELAY) The latency or delay defines how long it takes for an entire message to completely arrive at the destination from the time the first bit is sent out from the source. JITTER We can roughly say that jitter is a problem if different packets of data encounter different delays and the application using the data at the receiver site is time- sensitive (audio and video data, for example).