Komunikasi Data (TEU 611) Data and Signals Dr. I Wayan Mustika, ST., M.Eng. Jurusan  Teknik  Elektro  dan  Teknologi  Informasi     FT  UGM   Outline Ø Analog and Digital Ø Periodic Analog Signal Ø Digital Signals Ø Transmission Impairment Ø Data Rate Limits Ø Performace Analog and Digital §  Data Ø Analog data are continuous and take continuous values Ø Digital data have discrete states and take discrete values §  Signals Ø Analog signals can have an infinite number of values in a range Ø Digital signals can have only a limited number of values Comparison of Analog and Digital Signal In data communication, we commonly use periodic analog signals and aperiodic digital signals. A Periodic Sine Wave Unit of Periods and Frequencies Unit Equivalent Unit Equivalent Seconds (s) 1 s hertz (Hz) 1 Hz Milliseconds (ms) 10–3 s Kilohertz (KHz) 103 Hz Microseconds (ms) 10–6 s Megahertz (MHz) 106 Hz Nanoseconds (ns) 10–9 s Gigahertz (GHz) 109 Hz Picoseconds (ps) 10–12 s Terahertz (THz) 1012 Hz Summary of Periods and Frequencies §  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 §  If a signal does not change at all, its frequency is zero §  If a signal changes instantaneously, its frequency is infinite Phase •  Phase describes the position of the waveform relative to time zero Example of Sine Wave The Time-Domain and FrequencyDomain Plots Summary of Sine Wave •  A single-frequency sine wave is not useful in data communications; we need to change one or more of its characteristics to make it useful •  When we change one or more characteristics of a singlefrequency signal, it becomes a composite signal made of many frequencies. •  According to Fourier analysis, any composite signal can be represented as a combination of simple sine waves with different frequencies, phases, and amplitudes. A Composite Periodic Signal 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 The Decomposition of A Composite Periodic Signal The Time and Frequency Domains of A Nonperiodic Signa l Bandwidth The bandwidth is a property of a medium: It is the difference between the highest and the lowest frequencies that the medium can satisfactorily pass Example §  A signal has a spectrum with frequencies between 1000 and 2000 Hz (bandwidth of 1000 Hz). A medium can pass frequencies from 3000 to 4000 Hz (a bandwidth of 1000 Hz). Can this signal faithfully pass through this medium? 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. Two Signal Levels and Four Signal Levels Example •  What is the bit rate for high-definition TV (HDTV)? Solution •  HDTV uses digital signals to broadcast high quality video signals. The HDTV screen is normally a ratio of 16 : 9. There are 1920 by 1080 pixels per screen, and the screen is renewed 30 times per second. Twenty-four bits represents one color pixel. The TV stations reduce this rate to 20 to 40 Mbps through compression. Digital Signal as a Composite Analog Signal Transmission of Digital Signals Ø Baseband Transmission Ø Broadband Transmission (using modulation) Baseband Transmission Baseband Transmission using a Dedicated Medium Broadband Transmission (Using Modulation) Modulation of a Digital Signal for Transmission on a Bandpass Summary of Transmission of Digital Signals §  The analog bandwidth of a medium is expressed in hertz; the digital bandwidth, in bits per second §  Digital transmission needs a low-pass channel §  Analog transmission can use a band-pass channel Transmission Impairment Attenuation §  Loss of energy Decibel §  Measures the relative strengths of two signals or one signal at two different point dB = 10log P2 P1 Distortion §  The signal changes its form or shape Noise §  Example: thermal noise, induced noise, crosstalk, and impulse noise Signal-to-Noise Ratio (SNR) SNR = average signal power average noise power 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) §  Theorem: §  Nyquist à Noiseless channel §  Shannon à Noisy channel Noiseless Channel: Nyquist Bit Rate §  Theoretical maximum bit rate §  Note: Increasing the levels of a signal may reduce the reliability of the system bandwidth of the channel Bit rate = 2×bandwidth×log2 L number of signal levels Noisy Channel: Shannon Capacity §  In reality, channel is always noisy §  Example: Assume that SNRdB = 36 dB and the channel bandwidth is 2 MHz. The theoretical channel capacity can be calculated as C = B log2(1 + SNR) Performance Ø Bandwidth Ø Throughput Ø Latency (Delay) Ø Bandwidth-Delay Product Bandwidth Two different context: §  Bandwidth in hertz, refers to the range of frequencies in a composite signal or the range of frequencies that a channel can pass §  Bandwidth in bits per second, refers to the speed of bit transmission in a channel or link Throughput §  A measure of how fast we can actually send data through a network §  Example A network with bandwidth of 10 Mbps can pass only an average of 12,000 frames per minute with each frame carrying an average of 10,000 bits. What is the throughput of this network? Throughput = Amount of Data Time Latency Latency = Tpropagation + Ttransmission + Tqueuing + Tprocessing Tpropagation = Distance Propagation Speed Ttransmission = Message Size Bandwidth Bandwidth-Delay Product •  The number of bits that can fill the link Concept: Assignment 2 1.  A non-periodic composite signal contains frequencies from 10 to 30 KHz. The peak amplitude is 10 V for the lowest and the highest signals and is 30 V for the 20-KHz signal. Assuming that the amplitudes change gradually from the minimum to the maximum, draw the frequency spectrum. 2.  What is the total delay (latency) for a frame of size 5 million bits that is being sent on a link with 10 routers each having a queuing time of 2 µs and a processing time of 1 µs. The length of the link is 2000 Km. The speed of light inside the link is 2 x 108 m/s. The link has a bandwidth of 5 Mbps. Which component of the total delay is dominant? Which one is negligible?