CS 656 Analog/Digital, Page 1 Physical Layer, Part 2 Analog and Digital Transmission These slides are created by Dr. Yih Huang of George Mason University. Students registered in Dr. Huang s courses at GMU can make a single machine-readable copy and print a single copy of each slide for their own reference, so long as each slide contains the copyright statement, and GMU facilities are not used to produce paper copies. Permission for any other use, either in machine-readable or printed form, must be obtained from the author in writing. CS 656 Analog/Digital, Page 2 Analog vs Digital Analog data continuous values ex. voice, video, temperature Digital data discrete values ex. ASCII data, numeric data Analog signal/transmission continuously varying signal canbeusedtotransmitanalogdata example?
CS 656 Analog/Digital, Page 3 canbeusedtotransmitdigitaldata example? use amplifiers to boost energy in signal due to attenuation amplification distorts analog signal because noise is also amplified. Digital signal/transmission (ideally) sequence of discrete values canbeusedtotransmitanalogdata example? canbeusedtotransmitdigitaldata example? repeaters used to restore signal periodically repeaters do not distort the signal (and data) CS 656 Analog/Digital, Page 4 Digital Transmission Digital transmission is the future. Due to VLSI technology, price of digital circuitry is continually dropping; not as drastic for analog equipment Multiplexing is easier, compared to analog transmission. multiplexing enables multiple low-bandwidth signals to share a single high-bandwidth transmission medium Encryption techniques can easily be applied (to both digital and analog data). Enable integration of multiple communication forms, such as human interaction and computer communications, in one network.
CS 656 Analog/Digital, Page 5 Digital Data, Digital Signals Data rate: number of bits/bytes transmitted per second Modulation rate (bouads): rate at which the signal is changed Encoding: mapping from data bits to signal elements CS 656 Analog/Digital, Page 6 Nonreturn to Zero (NRZ) Encoding a positive voltage represents 1; a negative voltage represents 0 simplest to implement efficient use of bandwidth (modulation rate equals data rate) no synchronization available from the signal
CS 656 Analog/Digital, Page 7 Manchester Encoding a downward transition represents 1; a upward transition represents 0 at least one transition per bit self-clocking/synchronization; error detection modulation rate twice the bit rate inefficient use of bandwidth used in some LANs CS 656 Analog/Digital, Page 8 Delay Modulation Encoding a transition in middle represents 1; no transition represents 0 must have at least one transition per two bits to enforce this, insert a transition at the beginning of a bit period when necessary
CS 656 Analog/Digital, Page 9 Bipolar Encoding 0 Volt represents 0; either positive or negative pulse represents 1 The 1 pulses must alternate in polarity. used in T1 lines CS 656 Analog/Digital, Page 10 Digital Data, Analog Signals Most common usage is in transmitting digital data using modem. Requires use of modem to convert digital data to analog signals, and vise versa. Uses modulation techniques to impress digital data on analog signals (e.g., voices). Amplitude-shift keying Frequency-shift keying Phase-shift keying combinations of above
CS 656 Analog/Digital, Page 11 Amplitude-Shift Keying vary amplitude of the signal (called carrier) commonly, one of the amplitudes is zero CS 656 Analog/Digital, Page 12 Frequency-Shift Keying vary frequency of the signal (called carrier) used in high-frequency radio transmission, among others
CS 656 Analog/Digital, Page 13 Phase-Shift Keying vary phase of the signal a simple PSK scheme works like this: 0 has the same phase of the previous bit; 1 has the opposite phase (180 0 ) CS 656 Analog/Digital, Page 14 90 0 180 360=0 270
CS 656 Analog/Digital, Page 15 more sophisticated schemes use phase shifts other than 180 degree for example: binary 00, 0 degree shift; 01, 90; 10, 180; 11, 270 1 0 1 1 0 0 0 1 1 0 CS 656 Analog/Digital, Page 16 Quadrature Amplitude Modulation A method of combining of amplitude-shift and phase-shift keying Example: to encode 3 bits per signaling element Bit Combination Phase Shift Amplitude 000 0 low 001 0 high 010 90 low 011 90 high 100 180 low 101 180 high 110 270 low 111 270 high
CS 656 Analog/Digital, Page 17 Analog Data, Digital Signals The transmission of digitized analog data (such as voice). Examples Internet telephony Surprise, telephone networks too! Internally, modern telephone networks use digital transmission A technique called Pulse Code Modulation (PCM) is used to digitize voice signals. each voice signal is supposed to have 4 khz bandwidth the signal is sampled 8000 times per second, 8-bit result per sample CS 656 Analog/Digital, Page 18 Analog Data, Analog Signal Examples: voice signals over the telephone wire in your house TV broadcast, AM/FM radio, etc. Modulation can be used so that multiple signals can share a single high-bandwidth medium (that is, multiplexing). amplitude modulation frequency modulation