1. Optically Switched Networking
Michael Dales
Intel Research Cambridge

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

3. 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

4. Part 1 – Technology overview

5. 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

6. 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

7. 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

8. 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

9. 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(?)

10. 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

11. 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

12. 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

13. 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

14. 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

15. 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

16. 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

17. Part II - Example

18. 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

19. 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

20. 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

21. Overview

22. 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

23. 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

24. 55us/div – Packet is 94.72us data, 1.28 us guard band
Switch fabric
Demonstrated switching 10 * 10 Gbps channels through an SOA
55us/div – Packet is 94.72us data, 1.28 us guard band

25. 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

26. Wavelength striping Incoming packet From arbiter grant
To arbiter Request
Incoming packet
To optical switch

27. 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

28. 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

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

30. 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

31. 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