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Optical Fibre Communication Systems Lecture 7 – Optical Switches Professor Z Ghassemlooy Northumbria Communications Laboratory School of Computing, Engineering and Information Sciences The University of Northumbria U.K. http://soe.unn.ac.uk/ocr Prof. Z Ghassemlooy 1 Contents Network Systems Network Trends Switch Fabric Type of Switches Optical Cross Connects Optical Cross Connects Architecture Large Scale Switches Optical Router Applications Prof. Z Ghassemlooy 2 Development Milestones 2004 International Engineering Consortium Prof. Z Ghassemlooy 3 Network Network Connectivity – Point to Point: one to one – Broadcast: one to many – Multicast: many to many Network Span – Local / Metro Area Network – Wide Area Network – Long Haul Network Data Rates – Voice 64kbps – Video 155Mbps, etc. Service Types – Constant or Variable bit rate – Messaging – Quality of Service Prof. Z Ghassemlooy 4 Fully Connected, Un-switched Network Ports Ports Problem - limited and could not scale to thousands or millions of users Solution - switched network Prof. Z Ghassemlooy 5 Switched Network Pervasive, high-bandwidth, reliable, transparent Prof. Z Ghassemlooy 6 Optical Network - Issues Capacity 2.5 Gb/s 10 Gb/s 40 Gb/s Larger Control (switching) – Electronics • 10 Gb/s (GaAs, InP) can deliver low order optical cross connects (16 x 16) • > 10 Gb/s ??(mainly power dissipation) – Optical Reconfiguration: – Static or dynamic Prof. Z Ghassemlooy 7 Optical Network Elements Dense Wavelength Division Multiplexing Optical Add/Drop Multiplexers (OADM) Optical Gateways: – A critical network element. – A common transport structure to cater for • variety of bit rates and signal formats, ranging from asynchronous legacy networks to 10–Gbps SONET systems, • a mix of standard SONET and ATM services. Prof. Z Ghassemlooy 8 Switching - Electrical Right now, the optical switches have electrical core, where – Light pulses are converted back into electrical signals so that their route across the middle of the switch can be handled by conventional ASICs (application specific integrated circuits). This has a number of advantages: • Enabling the switches to handle smaller bandwidths than whole wavelengths, which fits in with current market requirements. • Easier network management, because standards are in place and products are available. Optical equivalents are not, at present. But, there are concerns that electrical cores won’t be able to cope with the explosion in the number of wavelengths in telecom networks (deployment of DWDM). Until recently, state-of-the-art ASIC technology wouldn’t support anything more than a 512-by-512-port electrical core, and carriers demanding for at least double this capacity. Prof. Z Ghassemlooy 9 Optical Network Elements - Switches Optical Bidirectional Line Switched Rings Optical Cross-Connect (OXC) – Efficient use of existing optical fibre facilities at the optical level becomes critical as service providers started moving wavelengths around the glob. Routing and grooming are key areas, and that is where OXCs are used. International Engineering Consortium, 2004 Prof. Z Ghassemlooy 10 Optical Switches • To provide high switching speed • To avoid the electronics speed bottleneck • I/O interface and switching fabric in optics • Switching control and switching fabric in optics • Switches act as routers and redirect the optical signals in a specific direction. • It uses a simple 2x2 switch as a building block Main feature: Switching time (msecs - to- sub nsecs) Prof. Z Ghassemlooy 11 All Optical Switches That’s the theory. But, things are turning out a little different in practice. – Vendors are finding ways of building larger scale electrical cores, with switch of many thousands of ports. – This may encourage carriers to put off decisions on moving to all-optical switches. Does this mean that is the end of the idea of alloptical networks? – Well, not really. All that it might do is delay things. Prof. Z Ghassemlooy 12 Electrical vs. Optical - Cross Connects Optical Electrical Limits Number of ports 1024 512 256 • High power consumption: e.g. 1024x1024: 4 kW • Jitter: very large • Large switches • Need OE/EO conversion 128 • Bipolar or GaAs 64 32 16 Electrical 8 10 MHz 100 MHz 1 GHz 10 GHz 100 GHz Data rate M C Wu DS3 OC3 OC12 OC48 OC192 Prof. Z Ghassemlooy 13 Switching: Types Circuit Switching: E.g. Telephone – Continuous streams • no bursts • no buffers – Connections are created and removed - Buffering does not exist in circuit-switches Packet Switching: Uses store & forward - The configuration may change per packet - Switching/forwarding is based on the destination address mapping - Switching table is used to provide the mapping - Switching table changes according to network dynamics (e.g. congestion, failure) Prof. Z Ghassemlooy 14 Switching Fabric Electro-optical 2 x 2 switching elements are the key devices in the fabrication of N x N optical data path. The switching elements rely on the electro-optic effect (i.e., the application of an electric field to an electro-optical material changes the refractive index of the material). The result is a 2x2 optical switching element whose state is determined by an electrical control signal. Can be fabricated using LiNbO3 as well as other materials. Electrical control Optical input Electrical control Optical output Optical input Prof. Z Ghassemlooy Optical output 15 Switching Fabric – contd. Input interface Output interface Switching fabric Switching control Prof. Z Ghassemlooy 16 Switching Fabric – contd. ... Optical Crossconnect (OXC) Transponders ... ... ... ... Optical transport system (1.55 mm WDM) ... 1.3 mm intra-office Terminating equipment | SONET, ATM, IP... Prof. Z Ghassemlooy 17 Connectivity Since a switch work as a permutation that routes input to the outputs, therefore it needs to provide at least N! different configuration A minimum number of Log2(N!) is needed to configure N! different permutation Blocking Non-Blocking Prof. Z Ghassemlooy 18 Connectivity - Blocking Occurs when one reduces the number of crosspoints in order to achieve low crosstalk and less complexity. In some switching architecture internal blocking may be reduced to zero by: – Improving the switching control: Wide sense nonblocking – Rearranging the switching configuration: Rearrangeably non-blocking Prof. Z Ghassemlooy 19 Connectivity– Non-blocking A new connection can always be made without disturbing the existing connections: Strictly Non-blocking – A connection path can always be found no matter what the current switching configuration is or what switching control algorithm is used Wide-Sense Non-blocking – A connection path can always be found regardless of the current switching configuration provided a good switching control algorithm is employed – No re-routing of the existing connections Rearrangeably Non-blocking – The same as wide-sense, but requires re-routing of the existing connections to avoid blocking – Use fewer switches – Requires more complex control algorithm Prof. Z Ghassemlooy 20 Time Division Switching Interchanges sample (slot) position within a frame: i.e. time slot interchange (TSI) – when demultiplexing, position in frame determines output link – read and write to shared memory in different order 1 M U X N 4 3 2 1 TSI 2 4 1 3 1 2 3 4 Prof. Z Ghassemlooy D E M U X 1 N 21 TSI - Properties Simple Time taken to read and write to memory is the bottle-neck For 120,000 telephone circuits – each circuit reads and writes memory once every 125 ms. – number of operations per second : 120,000 x 8000 x2 – each operation takes around 0.5 ns => impossible with current technology Prof. Z Ghassemlooy 22 Space Division Switching Crossbar Clos Benes Spank - Benes Spanke Prof. Z Ghassemlooy 23 Crossbar Architectures Each sample takes a different path through the switch, depending on its destination Crossbar: – Simplest possible space-division switch – Wide- sense blocking: When a connection is made it can exclude the possibility of certain other connections being made Crosspoints – can be turned on or off 1 2 Input ports 3 4 Sessions: (1,4) (2,2) (3,1) (4,3) 1 2 3 4 Output ports Prof. Z Ghassemlooy 24 Crossbar Architectures - Blocking Input channels 2 N X N matrix S/W 3 4 Output channels - Bars Input channels 1 M inputs x N outputs Switch configuration: “set of input-output pairs simultaneously connected” that define the state of the switch For X crosspoints, each point is either ON or Off, so at most 2X different configurations are supported by the switch. Case 1: - (3,2) ok Optical switching element 1 2 3 4 - (4,3) blocked Output channels - Cross Prof. Z Ghassemlooy 25 Crossbar Architecture - Wide-Sense Nonblocking Rule: To connect ith input to Input channels the jth output, the algorithm 1 sets the Input channels switch in the ith row and jth column at the “BAR” state and 2 sets all other switches on its left and below at the “CROSS” 3 state. Case 2: 4 1 2 3 4 - (2,4) ok - (3,2) ok - (4,3) ok Output channels Prof. Z Ghassemlooy 26 Crossbar Architectures – 2 Layer Only uses 6 x 9 = 54 cross points rather than 9 x 9 = 81 Penalty is loss of connectivity 2 3x3 5 Prof. Z Ghassemlooy 27 Crossbar Architectures - 3 Layer 1 2 3 4 5 6 4 5 6 7 8 9 7 8 9 Blocking still possible Prof. Z Ghassemlooy Output ports Input port 1 2 3 http://www.aston.ac.uk/~blowkj/index.htm 28 Crossbar Architectures - 3 Layer * 1 2 3 1 2 3 Blocking The first four connections 4 have made it 5 impossible for 6 3rd input to be connected to 7th 7 * output 4 5 6 7 8 9 8 9 The 3rd input can only reach the bottom middle switch The 7th output line can only be reached from the top output switch. Prof. Z Ghassemlooy 29 Crossbar Architecture - Features Architecture: Switch element: Switch drive: Switch loss: SNR: Wide Sense Non-blocking N2 (based on 2 x 2) N2 (2N-1).Lse +2Lfs XT – 10log10(N-1) Where XT; Crosstalk (dB), Lse; Loss/switch element Lfs; Fibre-switch loss Prof. Z Ghassemlooy 30 Crossbar Architecture - Properties Advantages: – – – – simple to implement simple control strict sense non-blocking Low crosstalk: Waveguides do not cross each other Disadvantages – – – – number of crosspoints = N2 large VLSI space vulnerable to single faults the overall insertion loss is different for each inputoutput pair: Each path goes through a different number of switches Prof. Z Ghassemlooy 31 Time-Space Switching Arch. 1 2 3 4 time 1 M U X 2 1 TSI 2 1 M U X 4 3 TSI 3 4 time 1 3 1 2 4 Each input trunk in a crossbar is preceded with a TSI Delay samples so that they arrive at the right time for the space division switch’s schedule Note: No. of Crosspoints N = 4 (not 16) Prof. Z Ghassemlooy 32 Time-Space Switching Arch. Can flip samples both on input and output trunk Gives more flexibility => lowers call blocking probability TSI Complex in terms of: TSI TSI TSI TSI TSI TSI TSI - Number of cross points - Size of buffers -Speed of the switch bus (internal speed) Prof. Z Ghassemlooy 33 Clos Architecture 1 nxp kxk pxn 1 1 1 n 32 33 2 2 2 64 32 64 32 993 k p k Stage 1 Stage 2 Stage 3 N= 1024 Prof. Z Ghassemlooy •It is a 3-stage network n - 1st & 2nd stages are fully connected - 2nd & 3rd stages are fully connected - 1st & 3rd stages are not directly connected Defined by: (n, k, p, k, n) e.g. (32, 3, 3, 3, 32) (3, 3, 5, 2, 2,) • Widely used • Stage 1 (nxp) • Stage 2(kxk) • Stage 3 (pxn) 34 Clos Architecture In this 3-stage configuration N x N switch has: 2pN + pk2 crosspoints (note N = nk) (compared to N2 for a 1-stage crossbar) If n = k, then the total number of crosspoints = 3pN, which is < N2 if 3p < N. Problem: Internal blocking Larger number of crossovers when p is large. Prof. Z Ghassemlooy 35 Clos Architecture – Blocking If p < 2n-1, blocking can occur as follows: - Suppose input 1 want to connect to output 1 (these could be any fixed input and outputs. - There are n-1 other inputs at k-switch (stage 1). Suppose they each go to a different switch at stage 2. - Similarly, suppose the n-1 outputs in the first switch other than output 1 at the third stage are all busy again using n1 different switches at stage 2. - If p < n -1 + n -1 +1 = 2n -1 then there will be no line that input 1 can use to connect to output 1. If p = 2n -1, then – Total Switch Element: 2kn(2n-1) + (2n -1)k2 Prof. Z Ghassemlooy 36 Clos Architecture – Blocking If p = 2n -1, then – Total Switch Element: 2kn(2n-1) + (2n -1)k2 Since k = N/n, therefore – the number of switch elements is minimised when n ~(N/2) 0.5. Thus the number switch elements = 4 (2)0.5 N3/2 – 4N, which is less than N2 for the crossbar switch Prof. Z Ghassemlooy 37 Clos Architecture – Non-blocking If p 2n -1, the Clos network is strict sense nonblocking (i.e. there will free line that can be used to connect input 1 to output 1) If p n, then the Clos network is re-arrangeably non-blocking (RNB) (i.e. reducing the number of middle stage switches) Prof. Z Ghassemlooy 38 Clos Architecture – Example If N = 1000 and and n = 10, then the number of switches at the: – – – – 1st & 3rd stages = N/n = 1000/10 = 100 1st stage = 10 x p 3rd stage = p x 10 2nd stage = p x k x k. If p = 2n -1 = 19, then the resulting switch will be non-blocking. If p < 19, then blocking occurs. For p = 19, the number of crosspoints are given as follow:Prof. Z Ghassemlooy 39 Clos Architecture – Example contd. In the case of a full 1000 x 1000 crossbar switch, no blocking occurs, requiring 106 crosspoints. For n = 10 and p = 19, the number of crosspoints at – 1st and 3rd stages = no. of stages x (n x p) x k = 2 x (10 x 19) x 100 = 38,000 crosspoints – 2nd stage (p = 19 crossbars each of size 100 x 100, because N/n = 100. = p x k x k = 19 x 100 x 100 = 190000 crosspoints. The total no. of crosspoints = 38000 + 190000 = 228000 Vs. the 106 points used by the complete crossbar. Prof. Z Ghassemlooy 40 Clos Architecture – Example contd. Since k = N/n, the number of switch elements k is minimised when n ~(N/2)0.5 = (1000/2) 0.5 =~ 23 instead of 19. then k = N/n = 1000/23 =~ 44 switches in the 1st & 3rd stages, and p = 2(23) -1 = 45. the number of crosspoints at 1st and 3rd stages = no. of stages x (n x p) x k = 2 x (23 x 45) x 44 = 91080. the number of crosspoints at 2nd stage = p x k x k = 45 x 44 x 44 = 87120. Since n = 23 does not divide 1000 evenly, we actually have 12 extra inputs and outputs that we could switch with this configuration ( 23x44=1012 and 1012 - 1000 = 12). Thus the total number of crosspoints = 91090 + 87120 = 178200 best case for a non-blocking switch as compared with the: 1,000,000 for the complete crossbar and about 190,000 for n = 10. This is a factor of over 11 less equipment needed to switch 1000 customers! Prof. Z Ghassemlooy 41 Benes Architecture 22 22 N/2 N/2 Benes N N N/2 N/2 Benes NxN switch (N is power of 2) RNB built recursively from Clos network: 1st step Clos(2, N/2, 2, N/2, 2) Rearrangably non-blocking Prof. Z Ghassemlooy 42 Benes Architecture - contd. Number of stages = 2.log2N - 1 Number of 2x2 switches /each stage = N/2 Total number of crosspoints ~N.(log2N -1)/2 For large N, total number of crosspoint = N.log2N Benes network is RNB (not SNB) and so may need re-routing: Modular switch design Multicast switches can be built in a modular fashion by including a copy module in front of the point-to-point switch Prof. Z Ghassemlooy 43 Benes Architecture - contd. 1 1 2 2 3 3 4 4 5 5 6 6 X 7 7 8 8 •e.g. Connection sequence 2 to 1 1 to 5 3 to 3 4 to 2 Fails Note there is no way 4 to 2 connection could be made Prof. Z Ghassemlooy 44 Benes Architecture –Non-blocking contd. • Now use different connections • e.g. 2 to 1 1 to 5 3 to 3 Prof. Z Ghassemlooy 4 to 2 OK 45 Three Building Blocks for OXC International Engineering Consortium, 2004 Prof. Z Ghassemlooy 46 Optical Switches - Tow-Position Switch Control Signal Input port Ii Optical Switch I1 Output ports I2 The input signal can be switched to either of the output ports without having any access to the information carried by the input optical signal • In the ideal case, the switching must be fast and low-loss. • 100% of the light should be passed to one port and 0% to the other port. Prof. Z Ghassemlooy 47 Two Position Switch - contd. The two-position switch requires three fibres with collimating lenses and a prism. Lens B A Prisem Light arriving at port A needs to be switched to port C. C Fibre B A C Prof. Z Ghassemlooy 48 Optical Switches - Applications Provisioning: Used inside optical cross connects to reconfigure them and set-up new path. [1 - 10 msecs] Protection Switching: To switch traffic from a primary fibre onto another fibre in the case of a failure. [1 to 10 usecs] Packet Switching: 53 byte packet @ 10 Gb/s. [1 nsecs] External Modulation: To switch on-off a laser source at a very high speed. [10 psecs << bit duration] Network performance monitoring Reconfiguration and restoration: Fibre networks Prof. Z Ghassemlooy 49 Optical Switching - Technologies Slow Switches (msecs) – Free space – Mechanical – Solid state Fast Switches (nsecs) – LiNbO – Non-linear – InP Prof. Z Ghassemlooy 50 Optical Switches - Criteria Maximum Throughput: – Total number of bits/sec switched through. – To increase throughput: • Increase the number of I/O ports • Bit rate of each line Maximum Switching Speed – Important: • Packet switched • Time division multiplexed Minimum Number of Crosspoints – As the size of the switch increases, so does the number of crosspoints, thus high cost – Multistage switching architecture are used to reduce the number of crosspoints. Prof. Z Ghassemlooy 51 Criteria - contd. Minimum Blocking Probability: Important in circuit switching – External blocking: when the incoming call request an output port that is blocked. • Subject to external traffic conditions – Internal blocking: when no input port is available. • Subject to the switch design Minimum Delay and Loss Probability – Important in packet switching, where buffering is used, which will introduce additional delay. Scalability – Replacing an old switch with a new larger switch is costly. – Incrementally increasing the size of the existing switching as traffice grows is desirable Broadcasting and Multicasting – To provide conferencing and multimedia applications Prof. Z Ghassemlooy 52 Criteria - contd. • Optical switches with low insertion loss and low crosstalk are needed in broadband optical networks – Restoration – Reprovisioning – Bandwidth on demand • Conventional optical switches cannot satisfy all the requirements: – Solid-state guided-wave switches (electro-optic, thermo-optic, SOA): limited expandability due to high crosstalk, loss, and power consumption – Optomechanical switches: excellent insertion loss and crosstalk, but are bulky, expensive, and suffer from poor reliability and scalability Prof. Z Ghassemlooy 53 Optical Switches - Types Waveguide Electro-optic effect - Semiconductor optical amplifier - LiNbO - InP Thermo-optic effect - SiO2 / Si - Polymer Free Space - Fast - Complex - Maturing - Lossy - Slow - Maturity - Reliable - Slow - Low loss & crosstalk - Inherently scalable - Liquid crystal - Mechanical / fibre - Micro-optics (MEM’s) Prof. Z Ghassemlooy 54 Optical Switches - Thermo-Optic Effect Some materials have strong thermo-optics effect that could be used to guide light in a waveguide. The thermo-optic coefficient is: – Silica glass dn/dt = 1 x 10-5 K-1 – Polymer dn/dt = -1 x 10-5 K-1 Difference thermo-optic effect results in different switch design. +v Electrodes Prof. Z Ghassemlooy 55 Thermo-Optic Switch - Silica Mach – Zehnder Configuration Input Ii Outputs I1 Heater I2 I1 sin 2 ( / 2) Ii I2 cos 2 ( / 2) Ii Directional coupler Prof. Z Ghassemlooy 56 Thermo-Optic Switch - Polymer Y – Junction Configuration PH1 I1 Ii PH2 I2 • If PH1 = PH2 = 0, then I1 = I2 = Ii /2 • If PH1 = Pon & PH2 = 0, then I1 = 0, and I2 = Ii • If PH1 = 0 & PH2 = Pon, then I1 = Ii, and I2 = 0 Prof. Z Ghassemlooy 57 Thermo-Optic Switch - Characteristics Parameters Switch Size 2x2 Si Poly. Si No. of S/W 1 1 64 112 256 Insertion Loss (dB) 2 0.6 4 10 18 Crosstalk 22 39 18 17 13 S/W time (ms) 2 1 ~3 1.5 ~4 S/W power (W) 0.6 0.005 5 4.5 15 Prof. Z Ghassemlooy 8x8 Poly. 16 x 16 Si 58 Mechanical Switches 1st Generation – Mid. 1980’s Loss Speed Size Reliability Applications: Low (0.2 – 0.3 dB) slow (msecs) Large Has moving part - Instrumentation - Telecom (a few) Size: Loss: Crosstalk: Switching time: 8X8 3 dB 55 dB 10 msecs Prof. Z Ghassemlooy 59 Micro Electro Mechanical Switches Combines optomechanical structures, microactuators, and micro-optical elements on the same substrate Input fibres Made using micro-machining Free-space: polarisation independent Independent of: – Bit-rate – Wavelength – Protocol Speed: 1 10 ms Output fibres Lens Flat mirror 4 x 4 Cross point switch Raised mirror Prof. Z Ghassemlooy 60 Micro Electro Mechanical Switches This tiny electronically tiltable mirror is a building block in devices such as all-optical cross-connects and new types of computer data projectors. I/O Fibers Reflector Imaging Lenses MEMS 2-axis Tilt Mirrors Lightwave Prof. Z Ghassemlooy 61 Micro Electro Mechanical Switches Monolithic integration --> Compact, lightweight, scalable Batch fabrication --> Low cost Share the advantages of optomechanical switches without their adverse effects General Characteristics: + Low insertion loss (~ 1 dB) + Small crosstalk (< - 60 dB) + Passive optical switch (independent of wavelength, bit rate, + + + – modulation format) No standby power Rugged Scalable to large-scale optical crossconnect switches Moderate speed ( switch time from 100 nsec to 10 msec) Prof. Z Ghassemlooy 62 Large Optical Switches - Optical Cross Connects Switch sizes > 2 X 2 can be implemented by means of cascading small switches. Used in all network control Bit rate at which it functions depends on the applications. – 2.5 Gb/s are currently available Different sizes are available, but not up to thousands (at the moment) Control 1 2 N 1 2 N X N Cross Connect Prof. Z Ghassemlooy N 63 Optical Cross Connects Prof. Z Ghassemlooy 64 Optical Switches Electrical switching and optical cabling: inputs come from different clock domains resulting in a switch that is generally timing-transparent. Optical switching and optical cabling, clocking and synchronization are not significant issues because the streams are independent. Inputs come from different clock domains, so the switch is completely timing-transparent. Prof. Z Ghassemlooy 65 Optical Switches - System Considerations For a given switch size N, – the number of 2x2 switches should be as small as possible. When the number is large it will result in: • high cost • large optical power loss and crosstalk. A switch with reduced number of crosspoints in each configured path, can have a large internal blocking probability In some switching architectures, the internal blocking probability can be reduced to zero by: – using a good switching control – or rearranging the current switch configuration Prof. Z Ghassemlooy 66 Optical Routers In the core large optical-switching elements have already started to appear to handle optical circuits, Large, centralized IP routers are also appearing, because they're an extremely efficient solution to IP routing. There are a variety of technologies and issues that influence the architecture for these types of network elements. To transport Tbps, new optical technologies have emerged to enable the economic transport of incredible bandwidth over single-mode optical fibrer, including DWDM and OTDM. That means individual optical links can sustain the enormous traffic needed to support the continuing growth of IP data. Prof. Z Ghassemlooy 67 Optical Routers High-power, low-noise optical amplifiers-or erbium-doped fiber amplifiers (EDFAs)-and pulseshaping technologies mean the high-bit-rate optical signals do not require electronic regeneration except on the very longest fiber spans. New fibres with larger cross-sectional areas mean a large number of high-bit-rate signals can be wavelength-multiplexed onto a single fiber. Thus, it is becoming affordable to actually construct links that can support Tbps of capacity between routing and switching centres. Prof. Z Ghassemlooy 68 Network Problems - Scalability The bottleneck at the core of the expanding network is at the junction points of the fibre bundles: I.e the switching and routing centres. With Tbps links, a huge amount of data converges into a single central office (CO) (see Figure 1). New routers emerge only to be swamped with traffic within months. Prof. Z Ghassemlooy 69 Network Problems - Scalability Solution: Use of cluster of several routers (or crossconnects). However, clustering is not a good long-term solution, because: • a cluster of crossconnects requires interconnecting links between the crossconnects. As the number of switches in the cluster grows beyond about 4 or 5, the interconnecting links consume most of the ports. Clustered routers have the same problem. • the IP traffic must transit more and more devices, and the latency (the delay of IP packets) and jitter (delay variance) of the cluster grow quickly. • the hot-spot problem, where one of the small routers in a cluster can be overwhelmed by temporary traffic dynamics in the network that do not exceed the combined node capacity. This swamping effect also increases the delay of that saturated small router. Prof. Z Ghassemlooy 70 Large, Centralized Router Current trend in XCs is to use large microelectromechanical systems (MEMS)-based OXCs for core node protection and grooming of DWDM traffic. Similarly, large centralized routers are an efficient alternative to solving bottleneck problems: – by avoiding the hot-spot problems of distributed routers, – eliminating clustering problems, and – permitting global scheduling. A centralized (single-hop), synchronous, large nonblocking switch fabric has the best latency and throughput performance of all router topologies. It also scales better than a clustered system-and it results in less complicated system software for the network element. Prof. Z Ghassemlooy 71 IP Routers + Optical Network Elements End Customer Router Router Router ONE Router Router ONE ONE Optical Network A V Lehmen, Telecordia Tech. Prof. Z Ghassemlooy 72 Optical Layer Capability: Reconfigurability IP Router IP Router IP Router OXC - A OXC - B IP Router OXC - C IP Router OXC - D Crossconnects are reconfigurable: Can provide restoration capability Provide connectivity between any two routers Prof. Z Ghassemlooy A V Lehmen, Telecordia Tech. 73 Architecture 1: Large Routers + High capacity Fibres Access lines A Z • All traffic flows through routers • Optics just transports the data from one point to another • IP layer can handle restoration • Network is ‘simple’ Access lines A V Lehmen, Telecordia Tech. • But….. - more hops translates into more packet delays - each router has to deal with thru traffic as well as terminating traffic Prof. Z Ghassemlooy 74 Architecture 2: Small Routers + OXC OXC OXC OXC OXC • Router interconnectivity through OXC’s • Only terminating traffic goes through routers • Thru traffic carried on optical ‘bypass’ • Restoration can be done at the optical layer • Network can handle other types of traffic as well A V Lehmen, Telecordia Tech. •But: network has more NE’s, and is more complicated Prof. Z Ghassemlooy 75 Dynamic Set-Up of Optical Connection IP Router IP Router IP Router OXC - A OXC - B IP Router OXC - C A V Lehmen, Telecordia Tech. 1. Router requests a new optical connection 2. OXC A makes admission and routing decisions 3. Path set-up message propagates through network 4. Connection is established and routers are notified Prof. Z Ghassemlooy 76 OXC – Router-Selector Architecture 1 1 N N 1 1 N N •Type I - 1 x N & N x 1 optical switches •Type II - 1 x N passive optical splitter - N x 1 Optical switches Prof. Z Ghassemlooy 77 OXC – Router - Feature Type I TypeII Strictly non-blocking Architecture Switch Element 2N(N-1) N(N-1) Switch Drive 2Nlog2N Nlog2N Switch Loss (2Nlog2N)Lse+4Lfs log2N(3+Lse)+2Lfs SNR 2XT-10log10(log2N) XT-10log10(log2N) Where XT; Crosstalk (dB), Lse; Loss/switch element Lfs; Fibre-switch loss Prof. Z Ghassemlooy 78 OXC + Wavelength Converters Prof. Z Ghassemlooy 79 Optical Switches: - A comparison Characteristic Traditional Optical Switches Next Generation Optical Switches >1ms <1µsec Multicast Not available Dynamic power partition between ports Integrated VOA functionality Not available High dynamic range VOA ~10 Million cycles (Mech.dev.) ~10 Billion cycles (Optoelect.) Insertion loss Low Low Cross talk High Low Scalability Low Medium-High Switching Speed Reliability Prof. Z Ghassemlooy 80 Optical Gateway Cross-Connect Performs digital grooming, traditional multiplexing, and routing of lowerspeed circuits in mesh or ring network configurations. Specifically, it brings in lower rate SONET/SDH layer OC-3/STM-1, OC-12/STM-4 and OC48/STM-16 rates and electrical DS-3, STS-1 and STM-1e rates and grooms them into higher rate optical signals. Alcatel. 2001 Prof. Z Ghassemlooy 81 IP-router with Tb/s throughput can be built with fast tunable lasers & NxN optical mux From Input Port Scheduler Buffer Output T-Tx 40 G mod 40G Rx T-Tx 40 G mod 40G Rx T-Tx 40 G mod 40G Rx T-Tx 40 G mod 40G Rx retiming Clock Yamada et al., 1998 Prof. Z Ghassemlooy 82 Router & Optical Switch CHIARO- OptIPuter Optical Switch Workshop Prof. Z Ghassemlooy 83 The Optical Future- Tomorrow's Architecture Services are consolidated onto a single access line at the user site and fed into a Sonet multi-service provisioning platform at the carrier’s POP (point of presence). Several POPs feed traffic into a terabit switch capable of handling all traffic— including IP, ATM and TDM. The terabit switches sit at the edge of a three-tier network of optical switches—local, regional and long distance-each of which has a mesh topology. DWDM is used throughout the network and access lines. Where fiber is scarce, FDM (frequency division multiplexing) is used to pack as much traffic as possible into wavelengths. Light signals no longer need regeneration on long distance routes. Prof. Z Ghassemlooy 84 Prof. Z Ghassemlooy Separate access networks carry telephony and data into the carrier’s point of presence. Voice traffic runs over a TDM (time division multiplexer) network running over a Sonet (synchronous optical network) backbone. IP traffic is shunted onto an ATM backbone running over other Sonet channels. The Sonet backbone comprises three tiers of rings at the local, regional and national level, interconnected by add-drop multiplexers and cross-connects. DWDM (dense wave division multiplexing) is in use in the regional and national rings, but not the local rings. Light signals need regenerating on long distance routes. 85