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A novel switching paradigm for buffer-less WDM networks Myungsik Yoo and Chunming Qiao EE and CSE Departments University at Buffalo (SUNY) Why WDM Optical Internet ? • • • • • • Explosion of the Internet traffic Advances in DWDM networking technology Emergence of Terabit routers with OC-48 line speed Reduced capital equipment and operating costs Current incarnation: IP routers over WDM links Envisioned Next Generation Optical Internet: – IP over an all-optical WDM layer Existing switching paradigm • Optical circuit switching – set-up delay = round-trip time – a limited number of wavelengths and thus limited connectivity – not suitable for bursty traffic and short-lived sessions • Optical packet/cell switching – a payload and its header are sent together (tightly coupled) – need to buffer the payload while processing the header – high control overhead due to small packet/cell size • Optical Burst Switching (OBS) – combines the best of the two while avoiding their shortcomings Comparison of three paradigms Optical switching paradigms Bandwidth Utilization Latency (setup) Circuit Low High Optical Buffer Not required Packet/Cell High Low Low OBS High Proc./Sync. Adaptivity Overhead (traffic & fault) (per unit data) Low Low Required High High Not required Low High OBS based on Just-Enough-Time Unique Features • Loosely coupled bursts and their control packets – uses an offset time + out-of-band control • Offset time : T H , where H is number of hops to go k 1 – A burst is buffered only at the source node • Delayed Reservation (DR) of BW at node i – reserved from tb to tb+l (offset time at node i : i T(i) T • Efficient utilization of BW (as well as FDLs if any) k 1 ) BW Utilization vs Latency • Comparing JET and NoDR OBS for IP over WDM • Run IP on top of WDM switches • Use a dedicated control wavelength between the IP entities – for routing control packets • To send data, a control packet first sets up a connection – one-way reservation based on offset time and DR • Then, the data burst cuts through intermediate IP entities – reduces latency & processing load at IP routers • Similar to multi-protocol label switching (MPLS) Support QoS at the WDM Layer • • • • • • • • IP provides only best-effort services A WDM layer supporting basic QoS is useful E.g. to let Class 1 have a higher priority than Class 0 Existing approaches (e.g. Fair Queueing) require buffer Challenge: support priority in a buffer-less WDM layer Solution: assign Class 1 bursts an extra offset time Class 1 bursts can reserve BW much in advance Class 0 bursts can only “buy tickets at door” Extra Offset Time for Priority Differentiated Services • Assume: the length of Class 0 bursts is exponentially distributed with an average of L – L may be a few Kbits or a few microseconds at OC-48 • At least 95% of Class 0 bursts are shorter than 3L – and at least 99% of them are shorter than 5L • So, if the extra offset time = 3L, at least 95% of Class 0 bursts will not block a Class 1 burst • Blocking probability of Class 1 bursts will be reduced – average blocking probability (over all bursts) unchanged QoS Performance Improvement • Blocking probability after 6 hops (offered load = 0.8) Conclusion • Optical Burst Switching (OBS) is a novel switching paradigm for the Next Generation Optical Internet • A buffer-less WDM layer can support basic QoS • Future work on supporting multiple classes • Also to interwork with other approaches to achieve absolute vs. relative QoS guarantee