Photonic slot routing

1. Photonic slot routing By Alisa Javadi

2. Photonic slot routing • PSR • Wavelength-routing AONs interconnect pairs of source & destination nodes via all-optical point-to-point lightpaths • Due to cost & efficiency reasons, it is impossible to interconnect each pair of nodes by a dedicated lightpath • Possible solutions • Use of multiple lightpaths => loss of transparency • Electronic traffic grooming at each source node (grooming: many small=>large nits) • Alternative solution: Photonic slot routing (PSR) • Avoids loss of transparency & need for electronic traffic grooming • Intermediate nodes switch entire slots, each carrying multiple packets on distinct wavelength channels, all-optically & individually without OEO conversion • Allows traffic aggregation to be done optically without electronic traffic grooming

3. Photonic slot routing • Photonic slot • In PSR networks, time is divided into fixed-size slots • Each slot spans all W wavelengths => photonic slot • Photonic slot • May contain a single data packet on each wavelength • All packets in a given photonic slot are required to be destined for the same node • Each photonic slot may be destined for a different node • Routed as a single entity • Advantages • No wavelength-sensitive components needed at intermediate nodes => lower costs & avoidance of interchannel switching crosstalk • Reduced complexity of switching operation & electronic control by factor W (number of wavelengths) • Cost-effective realization by using simple optical components

4. Photonic slot routing • PSR functions • (a) photonic slot switching • Contention resolution • (b) photonic slot copying • Multicasting • (c) photonic slot merging

5. Photonic slot routing • Synchronization • To achieve PSR functions, photonic slots must arrive synchronized at PSR nodes • Dispersion compensation is a must • Synchronization approaches • Optical synchronizer (local) • Use of fiber delay lines (FDLs) at input ports of PSR nodes to delay arriving photonic slots • Network-wide photonic slot synchronization • Briefly having a signal to sync the slot all over network

6. Photonic slot routing • Access control • Each photonic slot has its own destination address • Access control • Source node is allowed to send packet on any free wavelength of arriving photonic slot if its destination address matches the packet’s destination address

7. Photonic slot routing • Sorting access protocol • Used by each source node to ensure collision-free wave-length channel access & organize packet transmission • Operation • Destination address of first packet transmitted in a given photonic slot determines its destination address • Each source node stores packets in separate transmission buffers according to their destination address • Packet selection at source node • If arriving photonic slot is assigned a destination • Head-of-line packet of buffer associated with destination is selected for transmission, provided not all wavelengths are used • If arriving photonic slot is not assigned a destination • Oldest packet among head-of-line packets is selected for transmission

8. Photonic slot routing • PSR node • Coupler taps part of incoming signal off input fiber link • Slot detector finds out • Whether arriving photonic slot is destined for local node • Which wavelengths in arriving photonic slot carry packets • Based on this information • Switch is set by electronic control • Locally generated packet is inserted using another coupler

9. Photonic slot routing • PSR bridge • Interconnects different PSR network segments • Example: 2x2 PSR bridge • Equipped with photonic 2x2 cross-bar switch • Two pairs of control receiver RXC and control transmitter TXC • RXC inform bridge control unit of destination of photonic slots arriving on each network segment • Electronic control sets switch in bar or cross state

10. Photonic slot routing • PSR bridge: Contention • In PSR networks, photonic slots may experience contention at intermediate PSR bridges • Contention • Occurs when more than one photonic slot simultaneously arriving at a given PSR bridge need to be switched to the same output port • Possible solutions • Only one photonic slot is switched while remaining ones are dropped by PSR bridge & retransmitted by source PSR nodes • Alternatively, PSR bridge resolves contention (e.g. SDL)

11. Photonic slot routing • SDL bridge • Contention can be mitigated by adding switched delay line (SDL) to PSR bridge => SDL bridge (as buffer)

12. Photonic slot routing • Multiport SDL bridge/PSR node

13. Photonic slot routing • Contention resolution • Schemes to resolve contention in mesh PSR networks • Retransmission of dropped photonic slots by source PSR nodes • Buffering contending photonic slots at SDL bridges • Deflection routing • One photonic slot is routed through output fiber link specified by routing algorithm whereas other contending slots are routed through any of remaining free output fiber links • Deflection counter in photonic slots prevents slots from being deflected too often • Benefits heavily depend on topology & routing algorithm • Results for shortest path routing • Under low load: Buffering & deflection routing achieve similar throughput-delay performance • With increasing load: Deflection routing may outperform buffering by means of load balancing

14. Photonic slot routing • Pros & cons of PSR • PSR networks have pros & cons • Pros • Can be realized using inexpensive wavelength-insensitive devices & cross-connects based on proven technologies • Cons • PSR nodes ready to send packets to destinations other than that of photonic slot are prevented from using it, even though most wavelengths may be free • This inefficiency can be avoided by neglecting the requirement that all wavelengths need to be destined for same node • As a consequence, each individual wavelength can be accessed independently from each other => individual wavelength switching (IWS)

15. Photonic slot routing • IWS • Cost of optical components & switches expected to decrease significantly in the long run • With optical technology advances, cost-effective wavelength-sensitive optical packet switches become feasible . • Resultant wavelength-sensitive IWS switch able to switch optical fixed-size packets on each individual wavelength • Results • Network capacity can be increased significantly by carefully replacing a relatively small percentage of conventional PSR switches with IWS switches according to given traffic demands and/or cost constraints • Thus, IWS enables cautious upgrade & smooth migration paths from PSR networks to synchronous fixed-size optical packet switching (OPS) networks

16. Photonic slot routing • Implementation • To date, PSR networks were experimentally investigated only to a limited extent • Wavelength stacking • Enables PSR nodes to send/receive multiple packets in each photonic slot using only one tunable transceiver

17. Photonic slot routing • Wavelength stacking

18. Optical flow switching By Alisa Javadi

19. Optical flow switching • Electro-optical bottleneck • Unlike individual wavelength switching (IWS) electronic IP packet switching networks provide several benefits • Network-wide synchronization is not required • Support of variable-size IP packets • Simpler & more efficient contention resolution by using electronic random access memory (RAM) • However, due to steadily growing line rates & amount of traffic electronic routers may become bottleneck in high-speed optical networks => electro-optical bottleneck

20. Optical flow switching • OFS • One of the main bottlenecks in today’s Internet is (electronic) routing at IP layer • Methods to alleviate routing bottleneck • Switching long-duration flows at lower layers ,routers are offloaded & electro-optical bottleneck is alleviated • Concept of lower-layer switching can be extended to switching large transactions and/or long-duration flows at optical layer => optical flow switching (OFS) • Definition of flow • Unidirectional sequence of IP packets between given pair of source & destination IP routers • Both source & destination IP addresses, possibly together with additional IP header information such as port numbers and/or type of service (ToS), used to identify flow

21. Optical flow switching • OFS • In OFS, a lightpath is established for the transfer of large data files or long-duration & high-bandwidth streams • Forms of OFS • Use of entire wavelength for a single transaction • Flows with similar characteristics may be aggregated & switched together by means of grooming in order to improve lightpath utilization • Issues of OFS • How to recognize start & end of flows • Size of flow should be in the order of the product of round-trip propagation delay & line rate of set-up lightpath

22. Optical flow switching • OFS vs. electronic routing • In OFS, data is routed all-optically in order to bypass & offload routers • Set-up lightpath eliminates need for packet buffering & processing at intermediate routers • OFS can be • End-user initiated • IP-router initiated

23. Optical flow switching • Advantages • Mitigation of electro-optical bottleneck by optically bypassing & thus offloading electronic IP routers • OFS represents highest-grade QoS • Established lightpath provides dedicated connection not impaired by presence of other users • Issues • Set-up of lightpaths must be carefully determined since wavelengths are typically a scarce resource • Without use of wavelength converters, wavelength continuity constraint further restricts number of available wavelengths

24. Optical flow switching • Integrated OFS approaches Dynamic lightpath set-up in OFS networks involves three steps • Routing • Wavelength assignment • Signaling • Integrated OFS approaches for end-user initiated lightpath set-up • Tell-and-go (TG) reservation • Reverse reservation (RR)

25. Optical flow switching • Tell-and-go (TG) reservation • Distributed algorithm with no wavelength conversion based on link state updates • Updates processed at each network node to acquire & maintain global network state • Given the network state, TG uses combined routing & wavelength assignment strategy • K shortest path routing with first-fit wavelength assignment • Optical flow is dropped if no route with available wavelength can be found • Connection set-up achieved using tell-and-go signaling • One-way reservation • Control packet precedes optical flow along chosen route in order to establish lightpath for trailing optical flow • Control packet & optical flow are terminated if not sufficient resources available at any intermediate node

26. Optical flow switching • Reverse reservation (RR) • Unlike TG, RR does not require (periodic or event-driven) updates to acquire & maintain global network state • Initiator of optical flow sends information-gathering packets, so-called info-packets, to destination node on K shortest paths • Info-packets record link state information at each hop • After receiving all K info-packets, destination node performs routing & first-fit wavelength assignment • Connection established via reverse reservation • Destination node sends reservation control packet along chosen route in reverse • Control packet configures intermediate switches & finally informs initiator about lightpath set-up • Otherwise, reservation is terminated & all resources held by reservation are released by sending additional control packets if control packet does not find sufficient resources

27. Optical flow switching • Implementation • OFS experimentally investigated in Next Generation Internet Optical Network for Regional Access using Multiwavelength Protocols (NGI ONRAMP) testbed • Bidirectional feeder WDM ring (8 wavelengths in each direction) connecting 10-20 access nodes (ANs) & backbone network • ANs serve as gateways to attached distribution networks of variable topologies, each accommodating 20-100 users • AN • Consists of IP router & ROADM(reconfigurable optical add drop MUX) • Routes optical wavelength channels & IP packets inside wavelength channels between feeder ring, IP router, and distribution network • Services • IP service • Involves electronic routing • Optical service • OFS with all-optical end-to-end connection

28. Optical flow switching • NGI ONRAMP

29. Optical flow switching • Flow detection • Flow detection that triggers the dynamic set-up of lightpaths is critical in OFS networks • Example of flow detection • x/y classifier • x denotes number of passing packets belonging to a given flow • y denotes prespecified period of time • Depending on whether value of classifier is above or below predefined threshold, flow is considered active or inactive, respectively • Node detects beginning of flow if value exceeds threshold • Node assumes end of flow if value falls below threshold

30. Thanks