1. Photonic Packet Switches: Architectural and design issues Yatindra Nath Singh Electrical Engineering Dept./ACES IIT Kanpur-208016 Email: ynsingh@ieee.org http://home.iitk.ac.in/~ynsingh

2. copyright 2001, YNSPhotonic Packet Switching2 Outline of the talk - What is packet switch? - Functionalities of packet switch - Why optical packet switching? - Basic switching elements and buffering - Various architectures - What we have been doing? - Conclusion

3. copyright 2001, YNSPhotonic Packet Switching3 NxN switch 1 N 1 N Input ports Output ports What is a packet switch? - Packet arrivals at inputs - Header analysis and routing to designated output port - Header replacement

4. copyright 2001, YNSPhotonic Packet Switching4 Functions in a packet switch -Routing - communication between neighbors - resulting in provisioning of network connectivity information - ultimately gives routing tables -Forwarding - analysis of header - comparison of destination info with routing table - decision regarding destination output port.

5. copyright 2001, YNSPhotonic Packet Switching5 - Switching - directing each packet to proper output (as defined by forwarding process) - needs switching and interconnect elements -Buffering - resolving contention by storing packets

6. copyright 2001, YNSPhotonic Packet Switching6 Problems with optical circuit switching - BW granularity is poor. - IP Router A not connected to IP Router B. Both connected to IP Router C. - Packet from A to B goes via C. - AC and CB light path may share some physical link. - If traffic between AB high, Light path should be adjusted. - Average traffic per physical link should be minimized.

7. copyright 2001, YNSPhotonic Packet Switching7 Why optical packet swtiching? clock skew cheaper electronic interfaces bit rate, modulation and format need not be standard Need to be agreed between two edge routers only. leads to payload transparency.

8. copyright 2001, YNSPhotonic Packet Switching8 Basic elements needed for the switching - SOA - Electro-optic switch (based on 2x2 coupler) - Spatial Light modulator - Tunable wavelength converters alongwith wavelength filter/ AWGM - Fixed wavelength converter with tunable filter

9. copyright 2001, YNSPhotonic Packet Switching9 Basic generic packet switch Synchroni- zing block Header Replace- ment block Switching And Buffering block Switch controller 1 N 1 N

10. copyright 2001, YNSPhotonic Packet Switching10 - Routing and forwarding - difficult to implement using optical technology at the moment - Switching and buffering - can be implemented optically as well as electronically For photonic packet switch - hybrid approach preferred Routing and forwarding - using electronics switching and buffering - optically

11. copyright 2001, YNSPhotonic Packet Switching11 Format for packets For implementing certain optical header regeneration techniques - packet format may be different. This structure need to be standardized over the network. - for payload only duration need to be standardized.

12. copyright 2001, YNSPhotonic Packet Switching12 For buffering - No equivalent of RAM in optical domain. - Bits can be stored in bistable laser diodes or flip-flops made using optical logic gates. (Technology is not matured for implementing large optical RAMs.) - Optical fiber delay lines

13. copyright 2001, YNSPhotonic Packet Switching13 Effective Refractive index in fiber ~ 1.5 Speed of light in fiber ~ 2x10 8 m/s For packet with 1024 bytes = 8192 bits ~ 9000 bits (overhead bits, synchronization bits etc.), transmission rate 1Gb/s, duration of transmission = 9x10 -6 secs (slot period) Fiber length to delay the packet by one slot = 1.8 km l - length of loop, c - Speed of light, n - R.I. of fiber, b - number of bits in packet, T - transmission rate

14. copyright 2001, YNSPhotonic Packet Switching14 Various types of buffers

15. copyright 2001, YNSPhotonic Packet Switching15 Other devices available - WDM (multiplexers, demultiplexers), - Couplers, - filters, - Add drop multiplexers using these and other elements switch architectures can be build.

16. copyright 2001, YNSPhotonic Packet Switching16 Wavelength routed all-optical packet switch

17. copyright 2001, YNSPhotonic Packet Switching17 Frontiernet architecture : another wavelength routed switch

18. copyright 2001, YNSPhotonic Packet Switching18 Broadcast and select type of switch

19. copyright 2001, YNSPhotonic Packet Switching19 Fiber loop memory based switch

20. copyright 2001, YNSPhotonic Packet Switching20 Architectural issues - Processing time for a header - Switch throughput – limited by number of headers that can be processed by controller. - Payload should be sufficiently large in size. - Ideally header processing time should be less than packet duration. - Header replacement technique

21. copyright 2001, YNSPhotonic Packet Switching21 - Guard band duration. - too small, tight tolerances on fiber delay lines and switch control implementation. - too large, channel utilization low. - Control points - more separation of control points - control synchronization required. - Architectures with control points at inputs or output only – simpler to implement.

22. copyright 2001, YNSPhotonic Packet Switching22 - Noise accumulation - ASE (amplified spontaneous emission noise) of Optical amplifiers - Gain crosstalk of optical amplifier - Gain in loop if maintained equal to loss of loop, minimization of noise accumulation.

23. copyright 2001, YNSPhotonic Packet Switching23 What we have been done so far? (using analysis and simulations – in fiber loop buffer memory switch) - degradation due to ASE noise with number of recirculations - control algorithm for the switch (with and without priority mechanisms -queuing performance analysis (exact and approaximate methods) – appeared in IEEE comm. Letters, May 2001. Detailed paper submitted in IEEE/OSA Journal of Lightwave technology.

24. copyright 2001, YNSPhotonic Packet Switching24 - gain control techniques for EDFA to optimize switch performance - gain in the loop should be maintained equal to loss for optimized operation test

25. copyright 2001, YNSPhotonic Packet Switching25 Recent results on this submitted to Electronics Letters.

26. copyright 2001, YNSPhotonic Packet Switching26 Summary and future scope -Optical switching can be a viable technique for switching in backbone -Investigation on queuing performance for multiple priority traffic/ multicast traffic need to be done. - Investigations in high speed optical memories needed. - control function of routing and forwarding using optical processing (need investigation) - Switching architecture using free space optics