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
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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

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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
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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
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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

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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
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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)

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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