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Transcript
Photonic slot routing By Alisa Javadi 1 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 2 traffic grooming 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 3 Photonic slot routing • PSR functions – (a) photonic slot switching • Contention resolution – (b) photonic slot copying • Multicasting – (c) photonic slot merging 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 5 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 Photonic slot routing • Sorting access protocol – Used by each source node to ensure collision-free wavelength 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 7 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 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 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) 10 Photonic slot routing • SDL bridge – Contention can be mitigated by adding switched delay line (SDL) to PSR bridge => SDL bridge (as buffer) Photonic slot routing • Multiport SDL bridge/PSR node 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 13 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) 14 Photonic slot routing • IWS – Cost of optical components & switches expected to decrease significantly in the long run – With optical technology advances, cost-effective wavelengthsensitive 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 15 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 16 Photonic slot routing • Wavelength stacking Optical flow switching By Alisa Javadi 18 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 19 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 20 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 roundtrip propagation delay & line rate of set-up lightpath 21 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 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 23 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) 24 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 25 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, socalled 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 26 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 27 » OFS with all-optical end-to-end connection Optical flow switching • NGI ONRAMP Optical flow switching • Flow detection – Flow detection that triggers the dynamic setup 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 29 – Node assumes end of flow if value falls below threshold Thanks 30