Kommunikcis rendszerek alapjai 9. eloads ADSL

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Kommunikációs rendszerek alapjai9. eloadásADSL

• Takács György

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2004
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2006. november
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2007. december
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2008. november
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What is ADSL?

• Asymmetric Digital Subscriber line,
• A modem technology,
• Convert existing twisted-pair telephone lines
  into access paths for multimedia and high speed
  data communication,
• Can transmit up to 6-25Mbps downstream,
• Can transmit up to 0,8-4 Mbps upstream,
• Transform the existing PSTN network to a
  powerful system capable of bringing multimedia,
  full motion video to the subscribers home.

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ADSL system components
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ADSL components at the subscriber
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ADSL components at SOHO
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ADSL components at Central Office
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DSL access and IP/content provider network
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What is the future of the ADSL?

• Will play a crucial role over the next ten years
  for telephone companies and other service
  providers
• New broadband cabling will take decades to reach
  all prospective subscribers.
• EU regulation document on local loop unbundling
  because no competitive technology!

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What can we learn from ADSL story?

• No ultimate technology!!
• Frequency division multiplexing, time division
  multiplexing, modulation, error control, flow
  control, scrambling, signal processing,
  adaptation, STM-ATM, trellis coding, in-service
  performance monitoring and surveillance,
  initialization, handshaking, channel analysis,
  are mixed in ADSL
• More room for further development.

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key requirements (1)

• Test loops makeup and topology (to ensure
  adequate penetration).
• Cross talk or steady state noise margin (to
  allow for interactions from other DSL in a
  multi-pair cable).
• Data rates (both line and payload).
• Impulsive or transient noise margin (to allow
  for noise spikes e.g. ringing).
• Transmitter power spectral density limits (to
  ensure spectral compatibility and minimise
  unwanted RF emissions).
• Return loss (to ensure good line matching and
  signal power transfer).
• Line interface balance (to prevent EMC
  problems).

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key requirements (2)

• Framing and data scrambling (to prevent
  cyclo-stationary effects e.g. line spectra).
• Latency (to minimise delay e.g. for voice
  traffic).
• Jitter and wander (to minimise data loss).
• Start up protocols (handshaking).
• Warm/cold start limits (time taken to
  synchronise and achieve reliable bit transport
  to minimise circuit unavailability).
• Line coding (to achieve efficiency in terms of
  bits/s/Hz)
• Duplexing (e.g. time, frequency, echo
  cancellation).

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key requirements (3)

• Forward error correction (to self-correct
  physical layer transmission errors and  not
  burden higher layer protocols with data
  re-transmission.)
• Embedded operations and maintenance (for the
  transfer of service related  information e.g.
  QoS).

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ADSL bitrate as a function of line length
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Discrete MultiTone

• Copper lines have a frequency spectrum of 1.1Mhz
  which can be used to data communication under two
  main limitations   1.The lower 4Khz are being
  used by the POTS. 2.The amplification isn't the
  same in all frequencies.

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The technology being used is DMT which divides
the frequency range to 256 sub-frequencies from
64Khz to 1.1Mhz Each sub-ferqency is an
independent channel and has it own stream of
signals . The ADSL protocol defines a basic
stream of data which is known to both endpoints
in advanced and enables them to find the specific
SNR for each  sub-frequency , and uses this
information to split the data over the
sub-frequencies
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dividing the spectrum to 256 subfrequencies
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       adaptive SNR discovery
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Code Error Correction One of the most
important technology breakthrough that helped the
ADSL is the coding . Using a method called
consellation encoding and based on Reed Solomon
coding decoding the information on the line can
be damaged and yet the decoder rebuild the
information in a very high reliability.To improve
the performance of ADSL system some companies use
16 state 4 dimensional trellis code on top of the
consellation encoding. Another useful method to
increase the ADSL systems reliability is Forward
Error Correction (FEC).
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Framing Scrambling   As most of the Computer
communication networks the ADSL uses a specific
framing method . The main frame is called
Superframe and it is composed of 68 ADSL data
frames , the ATU-C sends a superframe every 17
msec . Each data frame gets his information from
two data buffers (interleaved buffer and fast
buffer)which are scrambled at a specific sequence
, this scrambling method makes the error
correction and coding more efficient.
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ADSL 2/2 bitrate as a function of line length
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