ENG735 – COMUNICAÇÕES ÓPTICAS CHAVEADORES ÓPTICOS http://soe.northumbria.ac.uk/ocr/teaching/fibre/pp/Components-L2.ppt http://soe.northumbria.ac.uk/ocr/teaching/fibre/pp/opticalsw.ppt Prof. Dr. Vitaly F. Rodríguez-Esquerre

Purely electronic (present situation) Switching is the process by which the destination of a individual optical information signal is controlled Types of Optical Switching Space Division Switching Wavelength Division Switching Time Division Switching Hybrid of Space, Wavelength and Time Switch control may be: Purely electronic (present situation) Hybrid of optical and electronic (in development) Purely optical (awaits development of optical logic, memory etc.)

Optical Switching Element Technologies Liquid High Loss Crystal Gel/oil based Not Scalable Polarization Dependent LiNbO 3 Mechanical Poor Reliability Indium Phosphide Optical Switching Element Technologies SiO 2 / Si SOA Micro-Optic Fibre ( acousto -optic) (MEMS) Thermo- optic Bubble Can be configured in two or three dimensional architectures Waveguide Free Space

Electro-optic Switch Use a directional coupler Its coupling ratio is changed by varying the refractive index Thermo-optic Switch Liquid-Crystal Switch Bubble Switch Acousto-optic Switch

Two axis motion Micro mirror

2D MEMS based Optical Switch Matrix SEM photo of 2D MEMS mirrors Output fibre Input fibre Mirrors have only two possible positions Light is routed in a 2D plane For N inputs and N outputs we need N2 mirrors Loss increases rapidly with N SEM photo of 2D MEMS mirrors

3D MEMS based Optical Switch Matrix SEM photo of 3D MEMS mirrors Mirrors require complex closed-loop analog control But loss increases only as a function of N1/2 Higher port counts possible SEM photo of 3D MEMS mirrors

Total Internal Reflection LC Switch

Liquid crystal (Total internal Reflection) The glass and nematic liquid crystal refractive indices are chosen to be equal in the transmittive state and to satisfy the total reflection condition in the reflective state Schematic diagram of the total reflection switch: 1- glass prisms; 2- liquid crystal layer; 3-spacers

Optomechanical Switch MEMS Electro-optic Switching time Milliseconds Nanoseconds Insertion loss Low High PDL high Scalability Bad Good

Optical Number of ports Electrical Data rate 1024 32 64 16 8 512 256 128 Optical 10 MHz DS3 100 MHz OC3 OC12 1 GHz OC48 OC192 100 GHz Electrical 10 GHz Data rate Electrical Limits High power consumption: e.g. 1024x1024: 4 kW Jitter: very large Large switches Need OE/EO conversion Bipolar or GaAs Provide fast switching speed No bottleneck due to electronics speed I/O interface and switching fabric in optics Switching control and switching fabric in optics Uses a simple 2x2 switch as a building block

Switching control Input interface Output Switching fabric Optical Electrical control Optical input output Electrical control Switching control Input interface Output Switching fabric

Optical Switches: - A comparison Characteristic Traditional Optical Switches Next Generation Optical Switches Switching Speed >1ms <1µsec Multicast Not available Dynamic power partition between ports Integrated VOA functionality High dynamic range VOA Reliability ~10 Million cycles (Mech.dev.) ~10 Billion cycles (Opto-elect.) Insertion loss Low Cross talk High Scalability Medium-High

Optical Switches - Tow-Position Switch Input port Ii Output ports I1 I2 Control Signal 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.

Two Position Switch - contd. The two-position switch requires three fibres with collimating lenses and a prism. B A C Lens Fibre Prism Light arriving at port A needs to be switched to port C. B A C

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

Optical Switching - Technologies Slow Switches (msecs) Free space Mechanical Solid state Fast Switches (nsecs) LiNbO Non-linear InP

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.

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

Optical Switches - Types Waveguide Electro-optic effect Semiconductor optical amplifier LiNbO - InP Thermo-optic effect - SiO2 / Si - Polymer Free Space - Liquid crystal - Mechanical / fibre - Micro-optics (MEM’s) - Fast - Complex - Maturing - Lossy - Slow - Maturity - Reliable - Slow - Low loss & crosstalk - Inherently scalable

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

Thermo-Optic Switch - Silica Mach – Zehnder Configuration Analogue I1 I2 Outputs Input Ii Heater Directional coupler

Thermo-Optic Switch - Polymer Ii I1 I2 PH1 PH2 Y – Junction Configuration Digital 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

Thermo-Optic Switch - Characteristics 15 5 4.5 0.6 0.005 S/W power (W) ~4 ~3 1.5 2 1 S/W time (ms) 13 18 17 22 39 Crosstalk 18 4 10 2 0.6 Insertion Loss (dB) 256 64 112 1 1 No. of S/W 16 x 16 Si 8 x 8 Si Poly. 2 x 2 Si Poly. Switch Size Parameters

Mechanical Switches 1st Generation – Mid. 1980’s Loss Low (0.2 – 0.3 dB) Speed slow (msecs) Size Large Reliability Has moving part Applications: - Instrumentation - Telecom (a few) Size: 8 X 8 Loss: 3 dB Crosstalk: 55 dB Switching time: 10 msecs

Micro Electro Mechanical Switches Input fibres Output fibres Lens Flat mirror Raised mirror Made using micro-machining Free-space: polarisation independent Independent of: Bit-rate Wavelength Protocol Speed: 1 10 ms 4 x 4 Cross point switch

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. Lightwave

Switching Fabric – contd. ... Optical transport system (1.55 mm WDM) 1.3 mm intra-office Optical Crossconnect (OXC) Transponders Terminating equipment | SONET, ATM, IP...

Space Division Switching Crossbar Clos Benes Spank - Benes Spanke

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 Input ports Output ports 1 2 3 4 Sessions: (1,4) (2,2) (3,1) (4,3)

Crossbar Architectures - Blocking 1 2 3 4 Input channels Output channels - Bars Output channels - Cross N X N matrix S/W 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 - (4,3) blocked Optical switching element

Crossbar Architecture - Wide-Sense Non-blocking Rule: To connect ith input to the jth output, the algorithm sets the switch in the ith row and jth column at the “BAR” state and sets all other switches on its left and below at the “CROSS” state. 1 2 3 4 Input channels Output channels Case 2: - (2,4) ok (3,2) ok (4,3) ok

Crossbar Architectures – 2 Layer Only uses 6 x 9 = 54 cross points rather than 9 x 9 = 81 Penalty is loss of connectivity 3x3 2 5 Inputs Outputs 1 4 N

Crossbar Architectures - 3 Layer 1 1 2 2 3 3 4 4 5 Output ports Input port 5 6 6 7 7 8 8 9 9 Blocking still possible http://www.aston.ac.uk/~blowkj/index.htm

Crossbar Architectures - 3 Layer Blocking 1 1 2 2 * 3 3 The first four connections have made it impossible for 3rd input to be connected to 7th output 4 4 5 5 6 6 7 7 * 8 8 9 9 The 3rd input can only reach the bottom middle switch The 7th output line can only be reached from the top output switch.

Crossbar Architecture - Features Architecture: Wide Sense Non-blocking Switch element: N2 (based on 2 x 2) Switch drive: N2 Switch loss: (2N-1).Lse +2Lfs SNR: XT – 10log10(N-1) Where XT: Crosstalk (dB), Lse: Loss/switch element Lfs: Fibre-switch loss

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 input-output pair: Each path goes through a different number of switches

Time-Space Switching Arch. MUX 1 2 3 4 2 1 3 4 TSI 4 3 time 1 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: 4 (not 16)

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: - Number of cross points - Size of buffers -Speed of the switch bus (internal speed)

Clos Architecture kxk nxp pxn Stage 1 Stage 3 Stage 2 Stage 1 (nxp) 32 64 33 N= 1024 993 n It is a 3-stage network - 1st & 2nd stages are fully connected - 2nd & 3rd stages are fully - 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)

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. Problems: Internal blocking Larger number of crossovers when p is large.

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 n-1 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.

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

Clos Architecture – Non-blocking If p  2n -1, the Clos network is strict sense non-blocking (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)

Clos Architecture – Example If N and n are equal to 100 and 10, respectively, then the number of switches at the 1st & 3rd stages are N/n = 1000/10 = 100. at the 1st stage, they are 10 x p switches at the 3rd stage they are p x 10 switches. the 2nd stage will have p switches of size 100 x 100. 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:-

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, each switch at stage 1 is a 10 x 19 crossbar requiring 190 crosspoints and there are 100 such switches. Same for the third stage. So the 1st & 3rd stages use 2x190x100 = 38,000 crosspoints altogether. The 2nd stage consists of p = 19 crossbars each of size 100 x 100, because N/n = 100. So stage 2 uses 190,000 crosspoints. Altogether, the Clos construction uses 228,000 crosspoints Vs. the 106 points used by the complete crossbar.

Clos Architecture – Example contd. Since k = N/n, therefore the number of switch elements is minimised when = n ~(N/2)0.5 = (500) 0.5 =~ 32 We would then use 44 switches in the 1st & 3rd stages and p = 2n-1= 2x23 – 1 = 45. Since n = 23 does not divide 1000 evenly, thus we actually have 12 extra inputs and outputs that we could switch with this configuration ( 23x44=1012 and 1012 - 1000 = 12). So we use 2x23x44x45=91,080 crosspoints in the 1st & 3rd stages and an additional 44x44x45=87,120 crosspoints in the 2nd stage. Thus the total number of crosspoints in the best Clos construction involves fewer than 180,000 crosspoints 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!

Benes Architecture 2  2 N/2  N/2 Benes N 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 (RNB)

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

Benes Architecture - contd. 1 2 3 4 5 6 7 8 X e.g. Connection sequence 4 to 2 Fails 2 to 1 1 to 5 3 to 3 Note there is no way 4 to 2 connection could be made

Benes Architecture –Non-blocking contd. Now use different connections e.g. 4 to 2 OK 2 to 1 1 to 5 3 to 3