Optically Switched Networking

1. Optically Switched Networking Michael Dales Intel Research Cambridge

2. 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

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. 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 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 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! Current demonstrator

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