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How ADSL Modems Work

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Home/asymmetric: ADSL and VDSL. Cable modems. Cell phones. First generation ... ADSL Standards. Maximum data rates. supported (ideal case) 13.38 Mbps downstream ... – PowerPoint PPT presentation

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Title: How ADSL Modems Work


1
How ADSL Modems Work
  • Prof. Brian L. Evans
  • Dept. of Electrical and Computer Engineering
  • The University of Texas at Austin

bevans_at_ece.utexas.eduhttp//www.ece.utexas.edu/b
evans
2
Signals
  • Continuous-timesignals are functionsof a real
    argument,e.g. x(t)
  • Discrete-time signalsare functions of aninteger
    argument,e.g. xn

3
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
  • yn xn xn-1
  • yn x2n

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
Digital Subscriber Line (DSL) Broadband Access
Internet
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
  • ADSLcable modem market12 in US 51 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
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
Corporate Technical Ladder
  • Test Engineer
  • BS degree
  • Test other peoples 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
  • Ph.D. based on system prototyping
  • Ph.D. with significant theoretical results
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