CS 313 Introduction to Computer Networking Telecommunication Modulation

CS 313 Introduction to Computer Networking & Telecommunication Modulation, Multiplexing, & Public Switched Telephone Network Chi-Cheng Lin, Winona State University

Topics l Modulation l Multiplexing l Telephone System l Switching 2

Modulation Analog signals are devised to send digital information l Digital modulation l q. Process of converting between bits and signals that represent them l Schemes q. Baseband transmission q. Passband transmission (our focus) 3

Passband Transmission l Fact q. Baseband signal from 0 to B Hz and be shifted up to a passband of S to S+B Hz for transmission q. Amount of information carried not affected q. Receiver can shift the signal back down to 0 to B Hz for decoding 4

Passband Transmission l Modulation q. Superimpose information signals on to the carrier signal at transmitting end l Basic modulation techniques q. Amplitude shift keying (ASK) ØDifferent amplitudes representing different symbols (e. g. , 0 and 1) ØProblem: vulnerable to noise q. Frequency shift keying (FSK) ØDifferent frequencies representing different symbols ØProblem: limited by physical capacity of carrier q. Phase shift keying (PSK) ØDifferent phases representing different symbols ØProblem: Hard to distinguish small phase shift 5

Binary Signal ASK FSK PSK 6

Passband Transmission l Quadratic phase shift keying (QPSK) q 4 symbols 2 bits per symbol l Combination of ASK and PSK q. QAM-16: 16 symbols 4 bits per symbol q. QAM-64: 64 symbols __ bits per symbol l Constellation patterns q. Diagrams showing legal combinations of amplitude and phase 7

More Constellation Diagrams (a) QPSK (4 -QAM). (b) QAM-16. (c) QAM-64. 8

Passband Transmission l Gary-code q Adjacent symbols differ in only one bit position q At most 1 bit error in the expected case 9

Multiplexing l Multiplexing q. Set of techniques allowing multiple signals to share a single data link q. Dividing total available bandwidth over a link into multiple channels l Why multiplexing? 10

Multiplexing l Frequency Division Multiplexing (FDM) q. Dividing bandwidth of a link into separate channels l Orthogonal FDM (OFDM) q. Problem of FDM: guard bands needed to separate channels well bandwidth wasted q. OFDM uses subcarriers that send data independently without using guard bands l Time Division Multiplexing (TDM) q. Combining signals from low speed channels to share time on a high-speed link 11

Frequency Division Multiplexing (FDM) (a) The original bandwidths. (b) The bandwidths raised in frequency. (c) The multiplexed channel. Computer Networks, Fifth Edition by Andrew Tanenbaum and David Wetherall, © Pearson Education-Prentice Hall, 2011

OFDM l Used in 802. 11, cable networks, power line networking, (& 4 G cellular systems) Computer Networks, Fifth Edition by Andrew Tanenbaum and David Wetherall, © Pearson Education-Prentice Hall, 2011

Time Division Multiplexing l Round-robin q. Each user takes turn q. Each one periodically gets the entire bandwidth l Used in telephone and cellular networks Computer Networks, Fifth Edition by Andrew Tanenbaum and David Wetherall, © Pearson Education-Prentice Hall, 2011

Structure of the Telephone System (a) Fully-interconnected network. (b) Centralized switch. (c) Two-level hierarchy. 15

Structure of the Telephone System l A typical circuit route for a mediumdistance call. 16

Major Components of the Telephone System l Local loops q. Analog twisted pairs going to houses and businesses – “last mile” l Trunks q. Digital fiber optics connecting the switching offices l Switching offices q. Where calls are moved from one trunk to another 17

Digital Transmission l Why digital? q. Low error rate q. Mix signals from different sources (multimedia) q. Cheaper q. Maintenance is easier 18

Modem • The use of both analog and digital transmissions for a computer to computer call. Conversion is done by the modems and codecs. 19

Modem l l How can we transmit digital data over analog local loop? Modulator-demodulator q. A device that accepts a serial stream of bits as input and produces a modulated (analog) carrier signal as output (or vice versa) q. Each high-speed modem standard has its own constellation pattern l Baud rate q. Number of signal units transmitted per sec q. Number of symbols per second 20

Examples l A modem uses QPSK and sends data at the rate of 2400 baud. What is the data rate? l Compute the bit rate for a 2400 -baud 16 -QAM signal. l Compute the bit rate for a 2400 -baud 64 -QAM signal. 21

Telephone Modems l A telephone line has a bandwidth of q 3000 Hz (3300 – 300) for voice q 2400 Hz (3000 – 600) for data l Modem standards q. V. 32: 9, 600 bps q. V. 32 bis: 14, 400 bps q. V. 34 bis: 28, 800 ~ 33, 600 bps q. V. 90: download up to 56 kbps (56 K modem) q. V. 92: adjustable speed, call waiting, etc. 22

Trellis Coded Modulation l l 1 parity per symbol to reduce error Examples q. V. 32 standards uses 32 constellation points to transmit data at the rate of 2400 baud and 1 bit per symbol. What is the bit rate? Ans: 2400 x (log 2 32 - 1) = 2400 x (5 - 1) = 9600 bps q. V. 32 bis standards uses 128 constellation points to transmit data at the rate of 2400 baud and 1 bit per symbol. What is the bit rate? Ans: 2400 x (log 2 128 - 1) = 2400 x (7 - 1) = 14400 bps 23

Trunk and Muliplexing l TDM – Why is T 1 line 1. 544 Mbps? l Frame size / duration of frame = 193 bits / (125 10 -6 sec) = 1. 544 Mbps 24

TDM l Multiplexing T 1 streams onto higher carriers 24 voice channels ___ voice channels Capacity of each digital channel = 64 Kbps ___ voice channels 25

Wavelength Division Multiplexing (WDM) l Used over fiber optics, similar to FDM 26

Switching l Switch q. Device creating connections between devices linked to it l Switching q. Forwarding data from a switch to another device 27

Switching Techniques l Techniques q. Circuit switching q. Packet switching l Circuit switching q. End-to-end path has to be set up BEFORE any data can be sent q. Data follow the same path q. No danger of congestion (except in path setup phase) 28

Switching Techniques l Packet switching q. Store-and-forward q. Tight upper limit on block size allowing packets to be buffered in router main memory q. No single block can tie up a link for too long q. Shorter delay and higher throughput 29

Circuit Switching Vs. Packet Switching Circuit switching Packet switching 30

# hops=3 Circuit switching Packet switching 31

Circuit Switching Vs. Packet Switching 32