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How ADSL Modems Work

How ADSL Modems Work. Prof. Brian L. Evans Dept. of Electrical and Computer Engineering The University of Texas at Austin. bevans@ece.utexas.edu http://www.ece.utexas.edu/~bevans. x ( t ). t. x [ n ]. n. Signals. Continuous-time signals are functions of a real argument, e.g. x ( t )

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How ADSL Modems Work

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  1. How ADSL Modems Work Prof. Brian L. Evans Dept. of Electrical and Computer Engineering The University of Texas at Austin bevans@ece.utexas.eduhttp://www.ece.utexas.edu/~bevans

  2. x(t) t x[n] n Signals • Continuous-timesignals are functionsof a real argument,e.g. x(t) • Discrete-time signalsare functions of aninteger argument,e.g. x[n]

  3. y(t) y[n] x(t) x[n] System System Systems • A system generates an output signal from an input signal • Continuous-time systems with input signal x and output signal y (a.k.a. the response): y(t) = x(t) + x(t-1) y(t) = x2(t) • Discrete-time system examples y[n] = x[n] + x[n-1] y[n] = x2[n]

  4. Communication Systems • Voiceband modems (56k) • Digital subscriber line (DSL) modems • ISDN: 144 kilobits per second (kbps) • Business/symmetric: HDSL and HDSL2 • Home/asymmetric: ADSL and VDSL • Cable modems • Cell phones • First generation (1G): AMPS • Second generation (2G): GSM, IS-95 (CDMA) • Third generation (3G): cdma2000, WCDMA Analog Digital

  5. downstream upstream Internet Digital Subscriber Line (DSL) Broadband Access DSLAM Central Office DSL modem DSL modem Voice Switch LPF LPF Customer Premises Telephone Network DSLAM - Digital Subscriber Line Access Multiplexer LPF – Low Pass Filter

  6. ADSL Standards • Maximum data ratessupported (ideal case) • 13.38 Mbps downstream • 1.56 Mbps upstream • Widespread deployment inUS, Canada, Western Europe,Hong Kong • ADSL:cable modem market1:2 in US & 5:1 worldwide

  7. Multicarrier Modulation • Divide channel into narrowband subchannels • Based on the Fourier series • Standardized for ADSL channel carrier magnitude subchannel frequency Subchannels are 4.3 kHz wide in ADSL

  8. y(t) x(t) cos(2 p fc t) Amplitude Modulation (AM) • Radio station frequency fc AM stations are spaced 10 kHz apartfrom 550 kHz to 1700 kHz Input is an audio signal x(t) withfrequencies from 0 to 5 kHz Output is the transmitted signal: y(t) = x(t) cos(2p fc t) • What if x(t) were a single musical tone? x(t) = cos(2p fm t) y(t) = x(t) cos(2p fc t) = cos(2p fm t) cos(2p fc t) y(t) = ½ cos(2p (fc – fm)t) + ½ cos(2p (fc + fm)t)

  9. Test Engineer BS degree Test other people’s designs Starting salary: $55,000 Design Engineer MS degree, or BS degree plus 2 years experience and design short courses Design new products Starting salary: $70,000 What about the Ph.D.? ¾ of Ph.D.’s to industry ¼ of Ph.D.’s to academia CTO VP, Eng. Technical Staff (R&D) PhDEE2 Director Eng. Project Management PhDEE1 MSEE Design Eng. Test Eng. BSEE BSEE Tech. Technician Corporate Technical Ladder • Ph.D. based on system prototyping • Ph.D. with significant theoretical results

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