4 Communication Equipment

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????? 4 ??????????????(Communication Equipment)

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????????????(Objective)

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3
????????????????????????????????????????
(Equipment for WAN)
4
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(No Transcript)
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?????????????????????????????????? (WAN)

• ??????????????? (Multiplexer)
• ?????????????? (Concentrator)
• ???????????????????????(Front-End Processor
  FEP)
• ??????????????????????

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??????????????? (Multiplexer)

• ??????????????? (Multiplexer) ???????????? ???
  (mux) ???????????????????????????????????????????
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Transmission Efficiency Multiplexing

• To terminal users, the multiplexor appears to
  function as though there were several physical
  lines to the host instead of just one
• Several data sources share a common transmission
  medium simultaneously
• Line sharing saves transmission costs
• Higher data rates mean more cost-effective
  transmissions
• Takes advantage of the fact that most individual
  data sources require relatively low data rates

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Direct Point-to-Point
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Multiplexer
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Multiplexing Diagram
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Kinds of Multiplexors

• A communication link is divided among several
  users in two basic ways
• Frequency division multiplexing (FDM) which
  separates the links available bandwidth into
  sub-channels, one for each incoming line
• Time division multiplexing (TDM) which separates
  link into time slots. Each incoming line is given
  a time slot for transmitting a byte (or bit)

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Frequency Division Multiplexing

• Requires analog signaling transmission
• Total bandwidth sum of input bandwidths
  guardbands
• Modulates signals so that each occupies a
  different frequency band
• Standard for radio broadcasting, analog telephone
  network, and television (broadcast, cable,
  satellite)

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Figure 10-9
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FDM Example ADSL

• ADSL uses frequency-division modulation (FDM) to
  exploit the 1-MHz capacity of twisted pair.
• There are three elements of the ADSL strategy
• Reserve lowest 25 kHz for voice, known as POTS
• Use echo cancellation 1 or FDM to allocate a
  small upstream band and a larger downstream band
• Use FDM within the upstream and downstream bands,
  using discrete multitone

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Discrete Multitone (DMT)

• Uses multiple carrier signals at different
  frequencies, sending some of the bits on each
  channel.
• Transmission band (upstream or downstream) is
  divided into a number of 4-kHz subchannels.
• Modem sends out test signals on each subchannel
  to determine the signal to noise ratio it then
  assigns more bits to better quality channels and
  fewer bits to poorer quality channels.

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Time-Division Multiplexing (TDM)

• Used in digital transmission
• Requires data rate of the medium to exceed data
  rate of signals to be transmitted
• Signals take turns over medium
• Slices of data are organized into frames
• Used in the modern digital telephone system
• US, Canada, Japan DS-0, DS-1 (T-1), DS-3 (T-3),
  ...
• Europe, elsewhere E-1, E3,

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Statistical Time Division Multiplexing

• Intelligent TDM
• Data rate capacity required is well below the sum
  of connected capacity
• Digital only, because it requires more complex
  framing of data
• Widely used for remote communications with
  multiple terminals

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STDM Cable Modems

• Cable TV provider dedicates two channels, one for
  each direction.
• Channels are shared by subscribers, so some
  method for allocating capacity is
  needed\--typically statistical TDM

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Statistical TDM

• A major shortcoming of TDM is revealed when
  attached lines do not have data to transmit
• Times slots allocated to idle lines go unused
• Communication circuit is not used to its fullest
  extent in this circumstance
• A statistical time division multiplexor (STDM)
  improves on TDM efficiency by transmitting data
  only for lines with data to send by reallocating
  time slots so that idle lines take up none of the
  carrying capacity of the communication circuit

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Wavelength Division Multiplexing (WDM)

• Wavelength division multiplexing (WDM) is a
  relatively new multiplexing technique for optical
  transmissions over fiber optic cables
• Traditionally, a single laser operating at a
  single wavelength has been used to transmit
  signals over a fiber optic cable. WDM
  multiplexors, however, leverage multiple lasers
  operating at multiple wavelengths to transmit
  several simultaneous signals
• WDM enables carriers to increase transmission
  capacity without having to install more fiber
  optic cables
• Dense WDM (DWDM) combines WDM and TDM to further
  increase the amount of data that can be
  transmitted over a single fiber
• Today, more than 10 gbps can be transmitted over
  each DWDM circuit and more than 40 DWDM circuits
  can be created on each fiber
• Experts expect DWDM technologies to achieve 128
  simultaneous 10 gbps circuits in a few years

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Multiplexor Configurations

• Multiplexors can be added in a daisy-chain
  fashion (see Figure 10-13)
• A variety of multiplexors are found in todays
  WANs including
• Inverse multiplexors (see Figure 10-14)
• Data/voice multiplexors
• T-n (T-1, FT-1, and T-3) multiplexors
• Frame relay multiplexors
• IDSN multiplexors
• Local/short distance multiplexors
• E.g. Fiber optic multiplexors
• RS232 multiplexors
• DSL routers and multiplexors

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Figure 10-14
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?????????????? (Concentrator)

• ?????????????? (Concentrator) ????????????
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??????????????????????? (FEP)

• Front-End Processor ?????????????????????????????
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  ????????????????????????????????????? ATM

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?????????????????????? (Protocol Converter)

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