Optically Switched
Networking
Michael Dales
Intel Research Cambridge
www.intel.com/research
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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
 Can’t 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
PD
PD
PD
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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 you’ll 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
 2 data l in 1500nm range
 Two main components: host
interfaces and switch
 1 control l in 1300 nm
range
 Control electronics on FPGAs  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
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