Chapter Twelve: Digital Modulation and Modems

Introduction Most systems use analog carriers but digital modulation RF (sine wave) carriers are used Carrier amplitude, frequency, and phase can be modulated Vector or quadrature modulators are common

Information Shannon-Hartley Theorem C = 2Blog 2 M C = information capacity in bps B = bandwidth in Hz M = Number of states per symbol (2 for binary, 3 for ternary, 4 for quaternary, etc.) Shannon Limit C = Blog 2 (1 + S/N) S/N = signal to noise ratio

Eye Diagram Synchronized to the symbol rate S/N ratio Jitter Intersymbol interference etc

FSKFSK (Frequency-Shift Keying) The simplest form of digital modulation Two frequencies are generated, –a binary zero (space) –a binary one (mark) High noise immunity Applications –Caller ID –HF Radio teletype

FSK Spectrum Modem Standards

AFSKAFSK Using FM Equipment

GMSKGMSK (Gaussian Minimum-Shift Keying) Used in GSM cellular phones Gaussian refers to the shape of the baseband filterGaussian –to reduce the transmitted bandwidth –Mark & space frequencies are separated by ½ the bit rate f m - f s = 0.5 f b

PSKPSK (Phase-Shift Keying) Supports higher data rates DPSK – differential phase-shift keying QPSK – quadrature phase-shift keyingQPSK Each symbol represents two bits (dibit) –The bit rate is twice the baud rate

QPSK Each symbol represents two bits (dibit) –The bit rate is twice the baud rate The signal amplitude may pass through zero

QPSK Variations π /4 Delta Phase-Shift Keying –Transmitted signal does not go to zero amplitude –Two 4 star constellations alternate –Nonlinear amplifiers may be used O-QPSK –The I & Q channels are offset by ½ bit period –Only 1 bit of the dibit changes at a time

Telephone Modems Telephone modems use: –FSK –PSK –QAM Bandwidth of about 3.1 kHz Requires sophisticated modulation schemes

Bell Modems Bell 202 –Used for call display –Simplex Bell 103 –300 baud –Full duplex Bell 212A –PSK –1200 bps

QAM - Quadrature Amplitude Modulation Increase the number of bits/symbol Combination of amplitude and phase modulation A constellation diagram shows all signal states Connection quality determines data rate

56K Modem Dial-up Signal passes through the BORSCHT codec FCC limitation is 54 kb/s (downstream) maximum of 33.6 kb/s upstream Speed varies according to line noise & distance

V.90 Modem System

Fax Modems Group 3 –Digitally codes the document –Analog QAM modulation up to 14.4 kb/s Group 4 –ISDN up to 64 kb/s

Error Correction and Compression Error-correction schemes –MNP 2, 3, 4, and 10MNP 2, 3, 4, and 10 –ITU V.42ITU V.42 –LAPM - Link Access Procedure for ModemsLAPM –All of these schemes use CRC to check each packet Popular compression methods –V.42 bis and MNP5

Modem-to-Computer Connections UART the parallel and serial transmission interfaceUART Most popular is the RS-232C (EIA 232D)RS-232C –Defines the voltages and pin numbers on a serial port

Cable Modems Cable Modems &Digital Subscriber LinesDigital Subscriber Lines CATV systems are broadband HFC

Cable Modems Use one television channel for downstream and another for upstream On large cable modem systems, distribution hubs are used to add the data from several channels to the fiber-optic backbone The equipment that does this is called a cable modem terminal server (CCMTS)

DSLDSL - Digital Subscriber Line High-speed over single twisted-pair telephone lines Requires a modem at the CO to bypass the BORSCHT codec Symmetry –Symmetrical - equal upstream and downstream data rates –Asymmetrical - more common, but lower upstream than downstream speeds

Comparison of Cable Modems &ADSL Both are always-on connections Neither interferes with the original use of the service (telephone or CATV) Cable modems offer somewhat higher speeds than ADSL, but are subject to congestion from multiple users