Networking Concepts

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Networking Concepts

• Basic techniques and concepts
• Multiplexing
• Demultiplexing
• Routing
• Contention

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Multiplexing

• Most users need relatively low bandwidth
• Transmission is more economical at higher
  bandwidth
• So we need to be able to combine different
  channels to create larger bandwidths

• We also need to be able to separate the channels
  when required

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TDM devices

• A simple optical coupler will achieve TDM
  multiplexing
• remember that the power loss depends on the
  number of channels multiplexed

(Min)Insertion loss -10logN
N
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Pedestal Requirement
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Wavelength Division Multiplexing

• Individual channels consist of different
  wavelengths of light
• any format is possible in principle
• When combined, the channels overlap in time

• Need to ensure that the channels are far enough
  apart to avoid crosstalk

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Coarse WDM

• Originally used to refer to two wavelengths
• 1300nm and 1550nm
• Latest ITU standards allow for 20nm spacing
• Cover 1270nm to 1610
• Wide channel spacing allows for lower cost
  lasers
• ITU-T G.695 standard

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Dense WDM

• Closely spaced channels
• 50GHz spaced frequencies defined
• Lasers require frequency stabilisation
• Preferred option for long haul transmission
• ITU-T G.694.2

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WDM devices

• A wavelength dependent optical coupler will
  achieve WDM multiplexing
• the power loss does not necessarily depend on
  the number of channels multiplexed

?1
?1,..., ?N
?N
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WDM sources

• ITU specifications
• various grids are specified with e.g 200GHz
  spacing
• note spacing is constant in frequency, not
  wavelength
• Limitations
• stability
• linewidth

?1
?N
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Demultiplexing

• Inverse operation to multiplexing
• Time domain, need to be able to access a single
  channel
• need access to some form of modulator switch
• need for channel identification
• need for clock recovery
• Wavelength domain, need to be able to filter out
  a single channel
• need for good bandpass filters

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TDM devices
Detect
Clock
Modulate

• This diagram shows a typical TDM demultiplexer
• Some signal is extracted and detected
  electrically
• The electrical signal is used to derive a
  suitable clock
• The clock is used to drive a modulator
• Clock recovery
• Typically involves dividing down
• Binary cascades are common

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AWG mux/demux
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Sink Trees

• Optimality principle
• if route ABC is optimal from A to C then AB is
  optimal and so is BC
• independent of definition of optimal
• Sink Tree
• set of optimal routes to a given destination

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Contention

• Generic network problem
• two channels at the input want to use the same
  output
• Bandwidth management
• Simple to solve, only consider ?free? resources
• Packet switching
• Need for memory (buffer)
• bad news for optics, no static memories exist
• use delay lines
• channel translation

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Other techniques

• Wavelength routing
• uses wavelength as a physical address
• typically requires wavelength conversion
• Negotiated wavelength transmission
• Before transmitting a wavelength channel is
  allocated
• requires some form tunable source
• Amplification
• will always be required on global scale networks
• need wide-band flat gain spectrum

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Wavelength Conversion

• O-E-O switch
• hybrid solution involving electronic detection
  followed by re-modulation on a new wavelength
• tends to lock in many properties of the system
• expensive in terms of device count
• all-optical methods
• must be optically nonlinear
• four wave mixing, cross modulation

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Next lecture

• Switching architectures
• Blocking
• Buffering
• Add/Drop Multiplexing