WDM rings for metro and distributed switching applications

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WDM rings for metro and distributed switching applications Fabio Neri Politecnico di Torino Italy www.polito.it www.

1 WDM rings for metro and distributed switching applications Fabio Neri Politecnico di Torino Italy Telecommunication Networks Group Optical Communications Group Politecnico di Torino

2 Outline Optical WDM packet networks at Politecnico di Torino: the Wonder project Wonder network architecture and access protocols Experimental efforts and results Optics in switching 2

3 The WONDER project Optical metro network architectures studied and prototyped at Politecnico di Torino, Italy Support from national research funding (PRIN projects RingO, WONDER, OSATE) and from the FP6 NoE e-photon/one WONDER: Wdm Optical Network DEmonstrator over Rings Project ( ) run by a consortium of Italian Universities Politecnico di Torino (Fabio Neri, Pierluigi Poggiolini, Claudio Sansoè) Politecnico di Milano (Achille Pattavina) Università di Parma (Alberto Bononi) WONDER project main target: Prototyping of the architecture Implementation of fully equipped nodes and network, capable of transferring real application data (to/from Linux PCs) 3

4 Main features The focus is on single-hop WDM/TDM packet networks on ring topologies interface photonic domain Ethernet-over-optics approach The considered architectures look for an optimal compromise between photonic and electronic technologies, aiming at the best trade-off between costs and performance: the aggregate bandwidth is handled in the photonic domain statistical multiplexing in data channels, and packet buffering, are controlled in the electronic domain at node interfaces located at the boundary with the photonic domain Fully distributed resource allocation 4

5 The Wonder experimental testbed The core of the project is the experimental testbed in the PhotonLab at Politecnico di Torino, focused on the experimental demonstration of the physical and MAC layer of the proposed architecture 5

6 Design goals Network designed for metro applications (10 50 km) IP directly over WDM (no SONET, ATM, etc.); nodes comprise hardware that in principle is much simpler and cheaper than in an equivalent SONET/SDH network All-optical packet-mode transport between ingress node and egress node (no electro optical conversions in the network) The optical network does not loose packets because of congestion: a transmitted packet is guaranteed to reach its destination Efficient multicast support With current technology, it is reasonable to have on each fiber: wavelengths 10 Gbit/s per wavelength number of nodes equal to and more 6

7 Wonder topology TX RX M λ 1 RX TX Double ring (reconfigurabily) folded to a double bus One transmission ring and one reception ring λ N F T λ i Priority to upstream traffic in transmission WDM (multi-channel) packet-mode operation RX T RX TX λ j TX 7

8 Network architecture Slotted transmission; slot alignment among all wavelengths multi-slots Fixed-size packets (in the current version) A tunable transmitter and a fixed receiver per node (minimal tuneability requirements and limited electronic bandwidth equal to one WDM channel) Channel inspection mechanism (also called λ-monitor or carrier sense) and priority to in-transit traffic In each node, packets are stored in different FIFO queues per destination (or per groups of destinations) to avoid Head-Of-the-Line (HOL) blocking Efficient multicast: a single transmission can reach several destinations 8

9 The WONDER topology: TX/RX space separation Packet collision is avoided at the transmitter using an empty-slot strategy based on carrier-sensing Node i (folding point) TX RX λ i TX R X Node j Node n (synch gen) TX R X λ k λ k Packet from n to m λ j TX R X λ i TX RX Node m λ j Node k TX RX Node l 9

10 The WONDER topology: TX/RX space separation Time is slotted by a global node synchronization (which is easier on linear topologies) Node i (folding point) TX RX λ i TX R X Node j Node n (synch gen) TX R X λ k λ k λ j TX R X λ i TX RX Node m λ j Node k TX RX Node l 10

11 The WONDER topology: TX/RX space separation Passing-through packets do not require electronic processing at intermediate nodes (except for carrier sensing) Node i (folding point) TX RX λ i TX R X Node j Node n (synch gen) TX R X λ k λ k λ j TX R X λ i TX RX Node m λ j Node k TX RX Node l 11

12 The WONDER topology Transmissions and receptions happen on separate resources. This separation can be either in the space domain (two fibers) or in the wavelength domain (two wavebands) The separation of transmissions and receptions prevents space reuse, but brings a number of advantages (e.g., fault recovery, and much better transmission properties) Upgradeability: WDM channels can be shared by several receivers, so that new nodes can be added without changing the existing equipment and infrastructure 12

13 Advantages of the ring topology The ring topology, thanks to its linearity, helps to: distribute synchronization information implement distributed Medium Access Control (MAC) protocols improve resilience The availability of good optical amplifiers (EDFAs, SOAs, LOAs) permits to recover node insertion losses Attention must be paid to avoid transmission impairments accumulations due to signal recirculations on the ring 13

14 Node input λ 1...λ Ν EDFA Logical structure Delay of line WONDER nodes TX FIBER 90/10 50/50 Node output 90/10 RX FIBER EDFA Filter AWG tunable transmitter fixed receiver λ drop High bit-rate burst mode receiver Output DATA λ 1 λ 2 λ Ν DC-coupled Photodiode array Threshold comparator λ 1 External Modulator MUX... Laser Array λ Ν High bit-rate Data source λ-monitor Node Controller 14

Access protocol Input-queuing protocol Small coordination required between nodes Carrier-sensing is obtained by directly inspecting optical power (for each wavelength and each slot) Virtual Output

15 Access protocol Input-queuing protocol Small coordination required between nodes Carrier-sensing is obtained by directly inspecting optical power (for each wavelength and each slot) Virtual Output Queuing (VOQ) : more than one input queue per node Head-of-the-line (HOL) blocking is virtually eliminated VOQ allows to achieve nearly 100% throughput with a small increase in complexity 4-channels λ-monitor board 15

16 Access protocols Basic mechanisms: priority to in-transit traffic the packet to be transmitted is chosen from the heads of node s FIFO queues, giving priority to larger fanouts and to oldest packets multicast with fanout splitting it is possible to add fairness control and slot reservation mechanisms A strong analogy exists with crossbar-based inputqueued cell switches, and similar resource allocation algorithms can be used 16

17 FROM PC - PCI BUS Access Protocol FAN OUT SET SDU 1011 INPUT FIFO BUFFER Concurrent processes: I/O with PCI bus separate queuing MAC PROTOCOL selection of packet to be transmitted HARDWARE (longest queue) IMPLEMENTATION reception of packets UNICAST MULTICAST L0 L1 L2 L3 L4 L5 L6 L7 Q0 Q1 Q2 Q3 Q4 Q5 Q6 Q7 SDU SDU SDU SDU SDU SDU SDU SDU λ 1 QUEUE λ 2 QUEUE FAN OUT SET Q FAN OUT SET Q FAN OUT SET Q λ 3 QUEUE λ 4 QUEUE MULTICAST QUEUE I CHANNEL 1000 L0 STATE (FROM λ-monitor) L MULTICAST QUEUE II MULTICAST QUEUE III MULTICAST QUEUE IV LASER DRIVER LASER 1 ON (λ1) LASER 2 OFF (λ2) FAN OUT SET Q3 FAN OUT SET Q L4 L4 LASER 3 ON (λ3) LASER 4 OFF (λ4) FAN OUT SET Q L5 FAN OUT SET Q6 FAN OUT SET Q SUPPORT VECTOR L7 MAX RESEARCH 0100 NEW FAN OUT SET Q4 17

18 Fairness control The first nodes on the transmission ring have better access chances Two distributed fairness control schemes were considered: Multi-Metaring: a control message per wavelength channel is circulated among nodes to distribute a transmission quota, and can be retained by nodes experiencing access difficulties Multi-Fasnet: on each wavelength channel, access is organized in trains of packets; each node can append a limited number of packets to each train; suitable train start policies can be devised 18

19 Variable-size packets The slotted operation eases the implementation of resource allocation strategies, and the implementation of the opto-electronic hardware The slot size must be chosen as a function of the interpacket guard times (due to tuning times, dispersion, switching times), of segmentation overheads, and of throughput efficiency in presence of small packets Long, variable-size packets can be transmitted after segmentation in sequences of contiguous slots Multi-Fasnet has the potential for unslotted operation 19

20 Fault protection The folded bus configuration easily offers fault protection (Slow) optical switch to dynamically adjust the folding point 20

21 Fault recovery algorithm Considered Faults Fiber cuts Node failures (shutdown) Distributed (i.e., per node state-machine) algorithm to riconfigure the network and isolate the fault The scheme operates at the physical layer All channels are operated in 1R mode (signal amplification only) Node senses the presence/absence of the synch signal on tx/rx bus Sequences of switching actions at each node to converge to an operational condition 21

22 WONDER: fault recovery Through Through fault Folding Master 22

23 The Wonder node structure TX fiber S-EDFA splitter delay coupler coupler switch LTM: Lambda and Timing Monitor TT Timing Transmitter DT Data Transmitter Node Controller optics electronics NPS DR Data Receiver TR Timing Receiver AWG 50 S-EDFA switch RX fiber 50 23

24 The Wonder testbed True traffic exchange among LINUX machines using standard TCP/IP stack Master Node Through Node Folding Node 24

The WONDER testbed Three nodes prototype with the following features: 4 wavelengths, 100 GHz spacing 1 Gbit/s per wavelength (Gigabit Ethernet like), with provision for an upgrade to 2.

25 The WONDER testbed Three nodes prototype with the following features: 4 wavelengths, 100 GHz spacing 1 Gbit/s per wavelength (Gigabit Ethernet like), with provision for an upgrade to 2.5 Gbit/s (plus study of subsystems scalability to 10 Gbit/s) 1 μs packets The logic is implemented on a last-generation FPGA board (ALTERA Stratix GX) On-board 3.2 Gbit/s CDR circuits All used optical components are commercially available Commerical PCI-bus interface core in FPGA to interface PC 25

26 The Wonder testbed: main activities Node interface design, based upon the Altera Stratix GX FPGAs PCI interface WDM MAC protocol PCI bus FPGA channels VOQ system board burst-mode receiver electro-optical interface Development of Linux driver to run standard IP-based applications Design of distributed (slot and bit) synchronization circuits, based on a dedicated wavelength Study of cheap amplification schemes Design of distributed protocols to select the synchgenerating node and the folding point, and to recover from faults 26

WONDER subsystems Design and prototyping some of the key sub-systems Gain-locked optical amplification (based on Linear Optical Amplifiers, LOA, from Finisar, by

27 WONDER subsystems Design and prototyping some of the key sub-systems Gain-locked optical amplification (based on Linear Optical Amplifiers, LOA, from Finisar, by University of Parma) λ-monitor board (low-cost photodiode, glue logic, and threshold comparator for each channel) Packet duration: 1 μs Fast initial transient: 50 ns 27

28 WONDER subsystems: Packet transmitter Node Controller (programmable logic inside a PC) TTL levels Clock generator (for the slot) TTL levels Laser Analog levels Driver Laser Driver Laser Driver Laser Driver 2.5 Gb/s pattern generator ECL levels L Optical A λ 1 Coupler S 4x1 E R λ 2 λ 3 A R λ 4 R A Y Modulator External Driver Modulator Analog levels Fiber Out Packet envelope 15 ns rise time 9 ns fall time 28

Node transmitter architecture Efficient implementation of multicasting Same payload on multiple wavelengths in the same time slot Potential alternative are

29 Node transmitter architecture Efficient implementation of multicasting Same payload on multiple wavelengths in the same time slot Potential alternative are fast tunable lasers (studied in collaboration with UCL, UK) Pros: potentially less expensive for high channel count (>16) Cons: does not allow multicasting 29

30 Synchronization issues Bit frequency and slot frequency and phase are distributed to all nodes using a dedicated wavelength operated in continuous-wave mode Data channels are instead operated in burst mode at 1.25 Gbit/s with 8B/10B line coding The synch information is generated by the first node on the transmission ring, and received by the other nodes A (Fast Ethernet compatible) 125 Mbit/s data stream is transmitted on the synch wavelength, and is multiplied locally by each receiver to obtain the 1.25 Gbit/s bit frequency. It is also used for marking slot boundaries, and for control, measurement and node monitoring information The bit frequency synchronization is used to reduce the complexity of the 1.25 Gbit/s burst mode receiver 30

31 WONDER subsystems: Network synchronization Network synchronization: the master node sends reference 125 Mbit/s synch signals on a dedicated synchronization wavelength DFB laser and MZ integrated modulator 31

32 Burst mode receiver issues Two solutions for burst mode receiver at 1.25 Gbit/s were developed The first solution is based on the CDR and phase aligner available (for SONET/SDH) in the Stratix GX FPGA. This solution does not require extra components and is very cheap The second solution is based on a Zenko ( optical front-end and a Zenko fast phase aligner. This solution requires a dedicated board that was developed. The Zenko burst mode receiver is a commercial device for PONs (Passive Optical Networks) 32

33 WONDER subsystems: Burst mode receiver Zenko burst mode receiver board 33

34 Performance of burst mode receivers Experimental results: both solutions were tested by sending one packet after a long inactivity period (corresponding to 1000 packet times) 1 Tpkt 1 Tpkt 1000 Tpkt 10 Tpkt The Stratix GX burst mode receiver has a very good performance (error free) only if is provided with a bit synchronization signal (by the synchronism circuitry) The Zenko burst mode receiver has a very good performance if is provided a bit synchronization and good performance (one packet lost every 1000 packets) without any synchronization signal 34

35 Key optical components Key optical components currently being assembled and customized for the project: Optical splitter and combiners Demultiplexers in arrayed waveguide (AWG) technology Array of fixed DFB lasers Optical switches External Lithium Niobate modulators & Drivers Receiver optoelectronics (PINFET, photo-receivers, clockrecovery, etc) FPGA control logics Optical amplifiers with gain locking 35

36 A feasible solution The design is based on currently commercially available optical components It does not require actual optical switching of packets It requires an electronic bandwidth per node equal to the single channel bit-rate, and NOT to the aggregate bit rate It balances the advantages of photonic and electronic domains Large effort in implementing the custom electronic control logic Many components of the Wonder node are also used in PONs (Passive Optical Networks) wrt PONs, larger losses (need for more amplifiers), but symmetrical system with fully distributed control 36

37 Features of the WONDER design Very clean data path (several 10s of nodes possible) despite the 1R architecture Transmission performance results (at 10 Gbit/s with gain-clamped EDFAs): 64 nodes and 16 wavelengths 32 nodes and 32 wavelengths The network architecture permits several receivers in the same wavelength channel: this largely improves the network scalability It is possible to allocate receivers to wavelength channels in an optimal fashion (the optimal receiver allocation minimizes packet losses for a given traffic scenario) It possible to introduce slowly tunable drop filters, leading to the possibility of dynamically allocating receivers to wavelength channels 37

38 Node input λ 1...λ Ν EDFA TX FIBER Slowly Delay line tunable receiver 90/10 50/50 Node output 90/10 RX FIBER EDFA slowly tunable receiver λ-monitor Tunable filter λ drop High bit-rate burst mode receiver Output DATA AWG λ 1 λ 2 λ Ν DC-coupled Photodiode array Threshold comparator Node Controller λ 1 External Modulator MUX... Laser Array tunable transmitter λ Ν High bit-rate Data source 38

39 Receiver allocation with static traffic Given A dual ring with W wavelengths λ j N nodes and their receiver loads (l i for node i) Find the allocation of the N nodes that minimizes the max wavelengths load (Meaningful when N > W) 39

40 Static traffic ILP formulation Max loaded wavelength L W max j = 1 x ij x ij N = i= 1 1 l i min x ij L max j, i, { 0,1} i, j j i = = 1,..., 1,..., W N x ij =1 if node i receives on wavelength λ j 40

41 Static case solution Well-known operations research problem: Scheduling jobs on identical parallel machines (NPhard) A simple and well-performing heuristic exists, called LPT (Longest Processing Time): 1. Order receiver's loads decreasingly 2 Assign the largest load to the least loaded wavelength 3 While unassigned loads exist, goto 1 41

42 Simulation scenario Folded Bus with 4 wavelengths 16 nodes & Two-Server Traffic 2 servers: transmit only to clients 14 clients: transmit only to servers Network 100% Comparison of: Random Allocation Simple Allocation (N/W nodes per wavelength) Longest Processing Time (LPT) 42

43 (Static) node allocation results 43

44 Optics in packet switches The architectures described above can be adapted to become an optical switching fabric inside a highperformance packet switch The Italian project dubbed OSATE (Optics in Switching Architectures: Theory and Experimentation) studied the implementation of an electro-optical Gbit Ethernet switch using the WONDER WDM ring as a switching fabric FPGA-based linecards interface Ethernet lines to the WDM ring; a PC attached to the WDM ring implements centralized management and control functions (management of Spanning Tree, forwarding tables, etc.) 44

45 OSATE switch design Commercial Gbit Ethernet core instead of PCI bus core in the WONDER FPGA 45

WONDER: A resilient WDM packet network for metro applications

Optical Switching and Networking 5 (2008) 19 28 www.elsevier.com/locate/osn WONDER: A resilient WDM packet network for metro applications A. Antonino a, A. Bianco a, A. Bianciotto b, V. De Feo a, J.M.