Optically Switched Networking

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Optically Switched Networking

• Michael Dales
• Intel Research Cambridge

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Overview

• Part 1 Technology overview
• Optical fibre as a connection medium
• Optical switching fabrics
• Optical switches
• Part 2 Example network
• SWIFT Architecture overview
• Current work
• Research topics

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Recommended reading

• If optical networks turn you on then the
  following text book is worth seeking
• Optical Networks, A Practical Perspective by
  Rajiv Ramaswami and Kumar N. Sivarajan, Morgan
  Kaufman

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Part 1 Technology overview
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Optical fibre links

• Optical fibre yet another wire
• Advantages
• Capacity long haul links of 160 Gbps over a
  single fibre
• Range signal can travel further without
  regeneration
• Noise immunity does not suffer from EM
  interference
• Weight/space a lot lighter/smaller than copper
• Power
• Popular in the long haul network

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Optical fibre links

• Not all good some problems
• Polarisation sensitivity
• Chromatic dispersion
• Non-linear behaviour
• Fibre more delicate
• Cant be thrown around like copper
• Minimum coil radius
• Coupling/splitting costs

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Optical fibre links

• In copper we use TDM to multiplex multiple
  channels on a single link
• In fibre can also use Wavelength Division
  Multiplexing (WDM)
• Each wavelength (lambda, l) can carry a different
  channel
• Free extra wires!
• Can TDM each wavelength too

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Switched optical networks

• Optical links are common in high speed switched
  networks
• ATM, Infiniband, Fibre-channel
• But all these networks convert data back to
  electrons at the switch

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Switched optical networks

• O-E-O switch design makes it easy to design an
  optical network (just like copper ones!)
• Disadvantages
• Size/power need to duplicate electronics for
  each lambda
• Latency O-E-O conversion takes time
• Bandwidth for really high capacity, electronics
  can become the bottleneck(?)

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Optically switched networks

• A key focus of the optical network community is
  to find ways to make all optical networks
• Packets stay in photons from edge to edge
• Techniques used depend on traffic type circuit
  switching and packet switching have very
  different requirements
• Might want to move to different wavelength across
  switch

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Optical switch fabrics

• Switch fabric design covered later in course
• Here we look at switching elements for light
• Need a way to switch light from one port to
  another
• Many possible ways with varying loss, switching
  time, polarisation dependency, etc.
• Mechanical moveable mirrors
• Can uses MEMS devices for compactness (e.g.,
  glimmerglass)
• Thermo-optical heat it to change
• Electro-optical control by current

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Buffering?

• In an electronic switch we use buffering to
• Delay packet whilst we decide what to do with it
• Resolve contention when multiple packets want to
  go to the same place at the same time
• There is no optical equivalent of random access
  memory
• Best we have are Fibre Delay Lines
• Use a long loop of fibre to delay the signal for
  a while

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Optical switches

• The switching fabric is only half the story how
  do we decide where to switch the packet?
• In electronic switch read header and then route
  through fabric accordingly
• In optical switches we have three options
• Convert the header to electrons and process
  electronically
• Process the header optically using optical logic
• Forget it all and use some form of reservation

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Optical switches

• Use electronics to route packet
• Read header from photons and convert to electrons
• Use a FDL to buffer packet whilst switch makes
  decision

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Optical switches

• Alternatively use reservation signal ahead of
  time that a packet is coming, typically on a
  reserved l
• One popular technique is Optical Burst Switching
• Packets grouped into a burst at source to
  amortise overhead
• Control packet fired into network ahead of time
  passes through switches setting up a path
• A fixed-delay time later the burst is sent
  through network
• No guarantee that youll get through

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Optical switches

• Alternatively use photonic devices to perform
  optical header reading
• No need to convert to electrons
• Still not a prime time technology can only
  handle a couple of addressing bits

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Part II - Example
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SWIFT optical network

• SWIFT is a research project between Intel
  Research, University of Cambridge, Essex
  University, and Intense Photonics
• Aim to built a short range, high capacity,
  wavelength striped, optically switched, packet
  switched network
• Aim for 100 Gbps and up
• Use photonic devices under electronic control

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SWIFT motivation

• Optics traditionally used in long haul, but not
  in short range, where copper dominates
• but copper will eventually run out (eventually)
• SWIFT looks at applying optics to short range
• Device interconnects, Cluster/supercomputer
  interconnects, Storage Area Networks, etc.
• Want have optical data-path, but still use
  electronics for control

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Architecture overview

• A short range packet switched network based upon
• WDM to increase bandwidth per link
• An all optical data path
• A single switch for simplicity (for now)
• An electronic control plane
• Use WDM for l striping use all ls for one
  channel
• Create a light bus
• Reserve one channel for control

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Overview
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Switch design

• Optical data-path packets remain optical
  throughout the network
• Light-paths need to be constructed through the
  switch before packets arrive
• Asynchronous control signalling used to request
  switch configuration

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Switch fabric

• Many light switching technologies, ranging from
  mechanical mirrors to semiconductor solutions
• Switch response time is important for packet
  switching
• We use Semiconductor Optical Amplifiers (SOAs)
• Turn light on or off based on an electrical input
• Have a switching time of a few nanoseconds

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Switch fabric

• Demonstrated switching 10 10 Gbps channels
  through an SOA

55us/div Packet is 94.72us data, 1.28 us guard
band
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Host interface

• Host interface has two main tasks
• Taking packets and converting them to striped
  format and vice versa
• Negotiating with the switch for access
• When a node wishes to transmit it requests
  permission over the control channel and waits for
  a light-path to be setup

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Wavelength striping
From arbiter grant
To arbiter Request
Incoming packet
To optical switch
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Demonstrator

• Have built a test-bed network
• Goal is to allow practical evaluation at many
  levels
• Photonics evaluation
• MAC layer testing
• Real application performance
• Used to validate a simulation model for
  investigation of network scaling

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Testbed overview

• Built a 3 node test-bed
• Two main components host interfaces and switch
• Control electronics on FPGAs

• 2 data l in 1500nm range
• 1 control l in 1300 nm range
• Couplers/AWGs used to combine/split ls

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Current demonstrator

• Current setup seen here
• Three racks
• 1 switch
• 2 host interface board
• 3 host interface transceivers
• PCs off shot
• Large due to using off the shelf components!

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Status

• Recently got first stage working
• Switches packets between nodes
• Data striped over both wavelengths
• Can run TCP, UDP, ICMP, etc. end to end
• Currently tuning performance for benchmarking
• Have simulation model in NS2 ready to correlate
  against testbed

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Future work

• Looking at several areas, including
• Switch fabric design
• Photonic device control
• Current SOA configuration done manually
• Want to automate this process using electronics
• Network scheduling and management
• Improve on request/grant protocol