Contents

1. Contents • Network Systems • Network Trends • Switch Fabric • Type of Switches • Optical Cross Connects • Optical Cross Connects Architecture • Large Scale Switches • Optical Router • Applications

2. Development Milestones 2004 International Engineering Consortium

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

4. Ports Ports Fully Connected, Un-switched Network Problem • limited and could not scale to thousands or millions of users Solution - switched network

5. Switched Network Pervasive, high-bandwidth, reliable, transparent

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

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.

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.

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

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)

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 all-optical networks? • Well, not really. All that it might do is delay things.

12. Optical • Electrical Limits • High power consumption: e.g. 1024x1024: 4 kW • Jitter: very large • Large switches • Need OE/EO conversion • Bipolar or GaAs 1024 512 256 128 Number of ports 64 32 16 Electrical 8 10 MHz 100 MHz 1 GHz 10 GHz 100 GHz Data rate DS3 OC3 OC12 OC48 OC192 Electrical vs. Optical - Cross Connects M C Wu

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)

14. Electrical control Electrical control Optical input Optical output Optical input Optical output 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.

15. Input interface Output interface Switching fabric Switching control Switching Fabric – contd.

16. ... ... 1.3 mm intra-office Transponders ... ... Optical Crossconnect (OXC) ... ... Optical transport system (1.55 mm WDM) Terminating equipment | SONET, ATM, IP... Switching Fabric – contd.

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

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 non-blocking • Rearranging the switching configuration: Rearrangeably non-blocking

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

20. 1 1 D E M U X M U X TSI 4 3 2 1 N N 1 2 3 4 2 4 1 3 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

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

22. Space Division Switching • Crossbar • Clos • Benes • Spank - Benes • Spanke

23. 1 2 Input ports 3 4 1 2 3 4 Output ports 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 Sessions: (1,4) (2,2) (3,1) (4,3)

24. Input channels 1 2 Input channels N X N matrix S/W Output channels - Bars 3 4 1 2 3 4 Optical switching element Output channels - Cross Crossbar Architectures - Blocking • 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

25. Input channels 1 2 Input channels 3 4 1 2 3 4 Output channels 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. • Case 2: • - (2,4) ok • (3,2) ok • (4,3) ok

26. 2 3x3 5 Crossbar Architectures – 2 Layer • Only uses 6 x 9 = 54 cross points rather than 9 x 9 = 81 • Penalty is loss of connectivity

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

28. 1 1 2 2 3 3 * 4 4 5 5 6 6 7 * 7 8 8 9 9 Crossbar Architectures - 3 Layer Blocking • The first four connections have made it impossible for 3rd input to be connected to 7th output The 3rd input can only reach the bottom middle switch The 7th output line can only be reached from the top output switch.

29. 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

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

31. time 1 1 MUX time 1 2 1 2 1 2 TSI TSI 3 MUX 4 3 3 4 4 3 1 2 4 Time-Space Switching Arch. • 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)

32. TSI TSI TSI TSI TSI TSI TSI TSI Time-Space Switching Arch. • Can flip samples both on input and output trunk • Gives more flexibility => lowers call blocking probability • Complex in terms of: - Number of cross points - Size of buffers -Speed of the switch bus (internal speed)

33. nxp kxk pxn 1 1 1 1 n n 32 33 2 2 2 64 32 32 64 993 k p k N= 1024 Stage1 Stage 2 Stage 3 Clos Architecture • It is a 3-stage network • - 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.

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 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. • If p = 2n -1, then • Total Switch Element: 2kn(2n-1) + (2n -1)k2

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)0.5N3/2 – 4N, which is less than N2 for the crossbar switch

37. 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, thenthe Clos network is re-arrangeably non-blocking (RNB) (i.e. reducing the number of middle stage switches)

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:-

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.

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!

41. 2 2 2 2 N/2 N/2 Benes N/2 N/2 Benes N N Benes Architecture • 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

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

43. 1 1 2 2 3 3 4 4 5 5 X 6 6 7 7 8 8 4 to 2 Fails 2 to 1 1 to 5 3 to 3 Benes Architecture - contd. • e.g. Connection sequence Note there is no way 4 to 2 connection could be made

44. 4 to 2 OK 2 to 1 1 to 5 3 to 3 Benes Architecture –Non-blockingcontd. • Now use different connections • e.g.

45. Three Building Blocks for OXC International Engineering Consortium, 2004

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

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

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

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

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.