Optical Networks: from fiber transmission to photonic switching

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e-mail: lastname@polito.it http://www.tlc-networks.polito.it/ fabio.neri@polito.

1 Optical Networks: from fiber transmission to photonic switching Wavelength-Routing Networks Fabio Neri and Marco Mellia TLC Networks Group Electronics Department tel tel Wavelength Routing Networks - 1

2 Course program outline Introduction and motivations Signal propagation in optical fibers Components for photonic systems First-generation optical networks Broadcast-and-select WDM networks Multi-hop WDM networks Wavelength-routing networks Optical access networks Protocol architectures in optical networks Network management, reliability and fault recovery Optical packet switching Wavelength Routing Networks - 3

3 Wavelength Routing (WR) networks WDM technologies are exploited to route and switch information in the optical domain using wavelengths Transparent or opaque optical circuits, called lightpaths, are used to connect network nodes. Lightpaths are optical circuits that may span one or more hops in the physical topology, and may cross switching elements in the optical layer Traffic carried by a lightpath may be packet-based, e.g., IP datagrams, or circuit oriented, e.g., telephone streams. The optical network is not aware of traffic formats Wavelength Routing Networks - 4

4 WR devices In a wavelength routed network, we distinguish among Optical Line Terminal (OLT): line terminal, taking care of physical functions, signal regeneration, wavelength adaptation, amplification, traffic multiplexing/demultiplexing Optical Add-Drop Multiplexer (OADM): it allows to add and drop traffic carried by one or more wavelengths in a (bidirectional) WDM link Optical Cross Connect (OXC): switches incoming wavelengths to multiple outgoing fibers These devices are similar to the equivalent firstgeneration SONET/SDH devices Wavelength Routing Networks - 5

5 WR network architecture OLT OLT OXC OXC IP router OADM OADM OADM SONET terminal OADM Wavelength Routing Networks - 6

6 WR network example To give the intuition of the flexibility of the WR solution, we start from an example, modifying a traditional first-generation telephone network (which has only point-to-point optical links and electronic switching) Typical current solutions consider a SONET/SDH ring topology, in which OC48- STM-16 (2.5 Gb/s) links are used Only add-drop multiplexers (ADM) and digital cross-connects (DCS) are used Wavelength Routing Networks - 7

7 Traditional SONET ring ADM = Add-Drop Multiplexer DCS = Digital Cross-Connect Wavelength Routing Networks - 8

8 Traditional SONET ring The following (normalized) traffic matrix is carried: A B C D A B C D If traffic grows: A B C D A B C D a WDM upgrade may be introduced Wavelength Routing Networks - 9

9 WDM ring Wavelength Routing Networks - 10

10 Traffic routing becomes: WDM ring flow wavelength # OC-48 AB λ 1 1 BD λ 1 1 AD λ 1 1 AC λ 2 2 BC λ 3 1 BD λ 3 1 CD λ 3 1 We need 3 wavelengths Wavelength Routing Networks - 11

11 WR ring By using WR, we build a logical topology, in which links are optical lightpaths, on top of the physical topology, in which point-to-point connections on a ring are available Wavelength Routing Networks - 12

12 WR ring Optical switches may be used to add flexibility: in case of traffic changes, or to manage faults, the logical topology may change Wavelength Routing Networks - 13

13 Second example A B C Physical topology Three IP routers, namely A, B e C, with10 Gb/s interfaces 50 Gb/s of traffic for each router pair router router router without OADM router router With OADM OADM router Wavelength Routing Networks - 14

14 Second example 10 wavelengths are required in both fibers Without OADM, the physical topology and the logical topology are the same, being both a bus topology; router B has 20 interfaces With OADM, the logical topology is a ring; router B has only 10 interfaces The cheapest solution depends on the relative cost of components (OADMs vs. router ports) A B C A 5 B 5 5 C Wavelength Routing Networks - 15

15 Third example Physical topology 3 λs are required Logical topology Wavelength Routing Networks - 16

16 Fourth example Three different logical topologies overlaid on the same bidirectional ring physical topology. Compare them considering: the number of interfaces at routers the number of wavelengths the distance (number of hops) in the logical topology Traffic is uniform, so that each router transmits t% of the capacity of a WDM channel to every other router (t/n-1 from router to router) WDM point-to-point (PWDM) hub (single hub) full-mesh (fully optical) hub (an additional hub router is added) Wavelength Routing Networks - 17

17 Fourth example Number of router ports (Q) PWDM Hub Full-mesh Q = 2W Q = t Q = (N-1) t/(n-1) Number of wavelengths (W) W = t/8 (N+1+1/(N-1)) W = t N/2 W = t / N-1 (N 2 /8+N/4) Considering N=8, we compare results for different values of t In the design of an optical network, it is probably better to minimize the number of ports rather than optimizing bandwidth usage Wavelength Routing Networks - 18

18 Fourth example: # of ports (total traffic times average distance over number of links) Wavelength Routing Networks - 19

19 Fourth example: # di λ t Wavelength Routing Networks - 20

20 Grooming The term grooming refers to the multiplexing of multiple traffic flows in a single wavelength. Typically, OXCs perform traffic grooming A single lightpath can therefore carry more than a single traffic flow, by using a multiplexing technology Electro-optical conversion allow to perform grooming using electronic functionalities. In previous examples, the all-optical solution suffers from limited grooming (so that for small values of t, lightpaths are underutilized); we notice a coarse quantization (or coarse bandwidth granularity) in the plots Wavelength Routing Networks - 21

21 Wavelength Routing network Optical Cross-Connects OXC or Wavelength Cross-Connects WXC are linked by optical fibers The network offers lightpaths (bandwidth pipes) between nodes. Up to few hundred lightpaths per fiber are possible today B C λ 1 E λ 1 A lightpath λ 2 D WDM cross-connect It is very important to select the wavelength allocation to maximize the spatial reuse Wavelength Routing Networks - 22

22 Optical layer A wavelength routing network forms an optical layer (with switching capabilities) which offers lightpaths to upper layers Both static and reconfigurable solutions are available Technology keys: Spatial reuse of wavelengths Optional wavelength conversion Level of transparency Circuit switching Fault management (protection may be implemented in the optical layer) Wavelength Routing Networks - 23

23 Wavelength conversion Wavelength converters may be used to better exploit network resources and simplify interconnection between equipments of different vendors B E λ 3 λ 1 λ 1 C A λ 2 D Wavelength Routing Networks - 24

24 Wavelength conversion Why use a wavelength converter? Add flexibility to WDM layer in a wavelength routing network Data may be sourced from a λ which is not compatible to WR network devices In the interconnection of different providers networks Four different conversion schemes: Input λ Output λ variable fixed fixed variable fixed fixed variable variable Wavelength Routing Networks - 25

25 Wavelength conversion Wavelength Routing Networks - 26

26 Wavelength converter Opto-electronic converters (OEO): RX regen TX They offer also a regeneration function They allow to introduce (variable) delays to resynchronize No switching is needed in the optical domain Lightpaths loose in transparency Wavelength Routing Networks - 27

27 Spatial diversity Often there is more than one fiber in a cable. This offers a space diversity which is equivalent to a wavelength diversity by changing space switching into wavelength converters fiber 1 fiber 1 fiber 1 fiber 1 λ 1 λ 1 λ 1 λ 1 λ 2 λ 3 λ 2 λ 3 λ 2 λ 3 λ 2 λ 3 λ 1 λ 2 λ 3 λ 1 λ 2 λ 3 λ 4 λ 5 λ 6 λ 4 λ 5 λ 6 fiber 2 fiber 2 switch λ-converter Wavelength Routing Networks - 28

28 Wavelength Cross-Connect network element manager Backbone interface WDM cross-connect Backbone interface Local interface Optical cross-connects can offer different levels of regeneration (and transparency) : 1R: only reception and transmission of amplified optical signals (analog operation) 2R: signal amplification and reshaping 3R: amplification, reshaping and resynchronization Wavelength Routing Networks - 29

29 Wavelength Cross-Connect wavelength demux wavelength mux 1 λ 1 λ 2... λ W λ 1 λ 2... λ W 1 2 λ 1 λ 2... λ W electronic cross connect λ 1 λ 2... λ W 2 M λ 1 λ 2... λ W λ 1 λ 2... λ W M receivers transmitters Wavelength Routing Networks - 30

30 Wavelength Cross-Connect wavelength demux wavelength mux 1 λ 1 λ 2... λ W λ 1 λ 2... λ W 1 2 λ 1 λ 2... λ W optical switch λ 1 λ 2... λ W 2 M λ 1 λ 2... λ W λ 1 λ 2... λ W M wavelength converters (tunable fixed) Wavelength Routing Networks - 31

31 Wavelength Cross-Connect 1 λ 1 λ 2... λ W λ 1 λ 1 λ 2... λ W 1 2 λ 1 λ 2... λ W λ 2 λ 1 λ 2... λ W 2 M λ 1 λ 2... λ W λ W λ 1 λ 2... λ W M demux switch mux (no wavelength conversion) Wavelength Routing Networks - 32

32 Wavelength Cross-Connect Technology Optical Electronic Transparency yes difficult Wavelength conversion difficult simple Bit rate > 10 Gb/s 10 Gb/s Physical size small large Power supply requirements small large Physical layer design difficult simpler Monitoring limited complete Required components: mux/demux yes yes optical switches yes no electronic switches no yes rx/tx no yes wavelength converters may be no The complexity of optical solutions is by a large extent independent of the bit rate, but requires 3R at the optical layer Today electro-optical solutions are cheaper Wavelength Routing Networks - 33

33 Reconfigurability WR networks may be static (without OXCs): cheaper, no flexibility, no fault management reconfigurable (with OXCs): more expensive, high flexibility, more robust A static network may be modeled by a connectivity matrix or by a bipartite (multi)graph Wavelength Routing Networks - 34

34 Static WR topology mux λ 1 λ 2 λ 3 λ 1 λ 2 λ 3 λ 1 λ 2 λ 3 coupler λ 1 λ 2 λ 3 λ 1 λ 2 λ 3 λ 1 λ 2 λ 3 coupler demux λ 3 λ 1 λ 3 λ 2 λ 2 λ 2 λ 3 λ 2 λ 1 λ λ 1 λ Fixed OXC Bipartite graph Wavelength Routing Networks - 35

35 Reconfigurable WR network There is an equivalence between wavelength agility and space switching (i.e., between wavelength number and number of switching stages) Reconfigurability in short times transform the off-line problem (in which the traffic matrix is fully known) into an on-line problem (in which lightpaths change slot-by-slot) Two problems can be faced: Logical (Virtual) Topology Design (LTD) Routing and Wavelength Assignment (RWA) Wavelength Routing Networks - 36

36 Design of a WR network Logical Topology Design (LTD) problem: given a node-node traffic, find the best logical topology (best set of lightpaths in terms of cost, price, performance, ) Routing and Wavelength Assignment (RWA) problem: given a physical topology and a set of lightpaths (end-to-end), find a route and a wavelength for each lightpath, so as to minimize the required total number of wavelengths, subject to the wavelength continuity and uniqueness constraints Wavelength Routing Networks - 37

37 Logical Topology Design Upper layer devices (e.g., IP routers or SONET/SDH nodes) are connected by means of lightpaths, which are point-to-point logical channels There is a physical topology, i.e., the set of fibers and OXCs, and a logical topology, i.e., the set of lightpaths and upper layer nodes 1 2 lightpath fiber Optical WXC SDH switch lightpath logical topology 4 3 physical topology Wavelength Routing Networks - 38

38 WR network design RWA and LTD design problems are indeed coupled problems: LTD is the input of the RWA, but it may be unfeasible In reality, they are disjointly solved: first the LTD problem is solved, then the RWA problem is solved. In case the RWA is unfeasible, a new LTD can be generated with additional constraints This is also due to the fact that often the owner of the physical infrastructure is a service provider which has to solve the RWA problem, and offers a service (ATM, SONET, IP links) to customers that must solve the LTD problem Wavelength Routing Networks - 39

39 Logical Topology Design The design goal is to minimize the cost, finding the best compromise between lightpath cost and the switching cost at IP, SONET/SDH or ATM layers. Often reliability constraints are considered It is possible to give a mixed integer-linear programming formulation of the LTD problem It results in a NP-complete problem, which leaves room for several heuristic algorithms. Finding the optimal solution is often impossible Wavelength Routing Networks - 40

40 Constraints and utility formalization Constraints: limited number of transmitters/receivers per node, or in the network coarse grooming of traffic over lightpaths routing algorithms used to route traffic NP-HARD problem Utility function: minimize the (maximum, average, ) length of multi-hop paths in the logical topology minimize the electronic switching minimize (maximum, average, ) congestion on lightpaths minimize the monetary cost of laying or renting lightpaths, and of devices... Wavelength Routing Networks - 41

41 A possible problem formalization Given: A physical WDM optical network in which node i has δ i O transmitters and δ i I receivers A traffic matrix T sd =[t sd ]; Σ d t sd = t s A routing algorithm on the logical topology Find The lightpath set that satisfies traffic requests and minimizes the maximum congestion level on lightpaths f max, which is given by the sum of all traffic flowing on a lightpath Wavelength Routing Networks - 42

42 Problem formalization Utility function: min f max f ij f i, max j Flow conservation at each node: j s s s t if s = i f ij f ji = s, i si j t if s i s fij = fij i, s s s fij bijt i, j, s j Connectivity constraints: j i b b ij ij δ δ i O j I i j b ij { 0,1} i, j b ij = 1 if there is a lightpath between node i and node j f s ij = amount of traffic from node s routed over lightpath (i,j) Wavelength Routing Networks - 43

43 Multi-hop routing of traffic In the previous formulation, routing of traffic assumes i) traffic splitting and ii) multipath routing It may not be a realistic assumption for a given technology, i.e., IP routing is i) minimum cost path, ii) (often) single path. If no traffic splitting is admitted, the routing problem is a well known multicommodity problem, which in its general formulation is a NP-Hard problem Routing constraints must be therefore included in the formalization, making the problem formulation even more complex Wavelength Routing Networks - 44

44 Optimization techniques Greedy heuristics Given the problem input and description, a custom algorithm is used to solve the problem Best Random solution (BR): generate N random topologies and keep the best one Single hop traffic maximization (Scom): add lightpaths to the largest traffic flows t sd Route and Remove (R&R): remove least used lightpaths, then reroute traffic Use regular logical topologies, and optimize node placement Wavelength Routing Networks - 45

45 Optimization techniques Metaheuristic approaches: explore the space of possible solutions in a smart way Simulated annealing: Given the current solution, generate a similar solution. If it is better (f max is smaller) keep the current solution. If it is worse, keep it with a probability (which decreases with time) Tabu search: Given the current solution, generate all the neighboring solutions (obtained by a deterministic modification of the current solution). Keep the best within the neighborhood. Use a tabu list to avoid local minimum entrapment Wavelength Routing Networks - 46

46 Ball on terrain example Local Search vs Greedy Algorithms A ball is initially placed at a random position on the terrain From the current position, the ball should be fired such that it can only move one step left or right What algorithm should we follow for the ball to finally settle at the lowest point on the terrain? Wavelength Routing Networks - 47

47 Local Search vs Greedy algorithms Initial position of the ball Initial position of the ball Simulated Can Annealing escape from explores more. Chooses local entrapment this move with a small probability (Hill Climbing) Greedy Algorithm gets stuck here! Locally stuck here! Optimum Solution. Greedy algorithm gets Locally optimum solution After may iteration, the best solution can be found Upon a large no. of iterations, SA converges to this solution. Wavelength Routing Networks - 48

48 Local search algorithms Given an initial solution X Define a modification of X (move) Evaluate the neighbors (A,B,C,D) Select one of them Best among all (steepest descent) First improving neighbor (first improvement) Stop after a number of iterations Problem: possible entrapments in local minima B C x A F... D E Wavelength Routing Networks - 49 Opt

49 Tabu Search and Simulated Annealing To avoid local entrapment Recall the visited path: use memory (Tabu search) Break deterministic behavior introducing randomness: Simulated Annealing Wavelength Routing Networks - 50

50 Background Tabu search is a metaheuristic that guides a local search procedure to explore the solution space beyond local optimality Memory-based strategies are the hallmark of tabu search approaches Wavelength Routing Networks - 51

51 Basic Concepts Solution Initial Current Best Move Attributes Value Neighborhood Original Modified (Reduced or Expanded) Tabu Status Activation rules Wavelength Routing Networks - 52

52 Tabu Search Store forbidden moves (not the solutions) Excludes from the neighborhood the already visited solutions by forbidding back moves Considering the LTD problem move: arc exchange (avoid to visit notadmissible solutions by enforcing the connectivity degree constraints) neighborhood: all the topologies obtained performing all possible arc exchanges Select the steepest descent Wavelength Routing Networks - 53

53 Simulated Annealing Select two nodes at random and perform an arc exchange Keep the neighbor if the obtained topology is better Otherwise keep it with a probability which evolves as p = e -(iteration)/kt T models a temperature T decreases with the time, simulating the annealing process For increasing times, it behaves as first improvement Wavelength Routing Networks - 54

54 Results obtained from a 24 node network Wavelength Routing Networks - 55

55 Optimization evolution Wavelength Routing Networks - 56

56 RWA: first example Source Destination Wavelength Routing Networks - 57

57 RWA: first example λ Used fibers 3 13 Wavelength Routing Networks - 58

58 RWA: another example t 1 r 1 t 2 r 2 t 3 r 3 t 4 r 4 t 5 r 5 A lightpath from i to n-i+1 is requested (n=5 in the example) n wavelengths are needed when no wavelength conversion is available With conversion and proper routing, we can have at most two lightpaths per link, so that two wavelengths are sufficient Wavelength Routing Networks - 59

59 Wavelength Assignment (WA) Wavelength Assignment problem is similar to the RWA problem, but routes are fixed Given a set of lightpaths and a set of routes, define the network load as L=max i l i in which l i is the number of lightpaths crossing fiber i The WA problem is simple in case of full wavelength conversion: at most L wavelengths are needed Otherwise, at least L wavelength are needed Wavelength Routing Networks - 60

60 WA and graph coloring λ 2 λ 1 λ RWA graph Lighpath graph The lightpath graph to be colored is obtained by having a node for each lightpath, and an arc whenever two lightpaths share a fiber Wavelength Routing Networks - 61

61 Utility function: RWA objective function Minimize load on link L (offline scenario) Minimize the blocking probability of future requests (online scenario) It can be proved that to route a lightpath set of load L considering a network G with M edges if the maximum number of hops of a lightpath is D the number of wavelengths is at most min [(L-1)D + 1, (2L-1) M L + 2] Wavelength Routing Networks - 62

62 Graph coloring Lightpath graph: each lightpath is modeled by a node. Edges represent sharing of fibers along the path Graph coloring problem: Each node must be colored using a different color for neighbors nodes. The minimum number of colors is called chromatic number of the graph Wavelength Routing Networks - 63

63 Graph coloring problem It is a well know NP-complete problem in its general formulation There are several polynomial-time algorithms that guarantee results with a limited error In special cases, there are algorithms that guarantee to find the optimal solutions Several meta-heuristic approaches have been proposed in the literature Wavelength Routing Networks - 64

64 RWA on rings Rings are particularly interesting: they are the simplest bi-connected topologies, and are adopted by several standards, e.g., FDDI or SONET/SDH There are always two possible routes for each lightpath. The minimum load L can be obtained when not considering minimum distance routing, or 2L when adopting minimum distance routing Given the route set, the WA problem can be easily solved considering a ring topology under load L: no more than 2L-1 wavelengths are needed Wavelength Routing Networks - 65

65 WA on rings Cut Perform a ring cut at nodes with the minimum number of in transit lightpaths Lightpath set λ 3 λ 2 λ 1 greedy WA: add a λ for each interrupted lightpath (it guarantees no more than 2L-1 λs) Wavelength Routing Networks - 66

66 WA on general topologies Topology wavelength conversion no fixed full limited any min[(l-1)d+1, L L (2L-1) M-L+2] ring 2L-1 L+1 L L star 3L/2 L L tree 3L/2 L Wavelength Routing Networks - 67

67 RWA and wavelength converters A (limited) wavelength conversion capability simplifies the RWA problem, reducing the number of wavelengths required to solve the problem when considering the off-line problem Considering the on-line problem, with random lightpath requests generations, and accepting a limited request blocking probability, several (mostly simulation-based) studies in the literature showed that the benefit of wavelength conversion is limited Wavelength Routing Networks - 68

68 RWA on generic topologies Heuristic solutions are proposed Examples: First Fit strategy (SPFF) Given a lightpath from s to d Route it considering the shortest physical path Assign the first available wavelength considering all fibers along the path Max Fill strategy (MF) Consider a wavelength λ Given a lightpath from s to d Route it using λ, if there exists a path Wavelength Routing Networks - 69

69 Number of λ vs number of lightpaths SPFF MF 250 λ Number of lightpaths per node Wavelength Routing Networks - 70

70 λ utilization on links λ 20 SPFF 20 MF Arc id λ on arc average number of used λs Wavelength Routing Networks - 71

71 Example of LTD and RWA User network with N=4 nodes, called A,B,C,D Traffic matrix (full duplex): A 0.1 C B 0.3 D A B C D A B C D Wavelength Routing Networks - 72

72 LTD problem Constraints: no more than two lightpaths per node: δ Ii =δ Oi =2 At least 2 topologies: both consider two lightpaths with load 100%, and two lightpaths with load 50% A C A C B D B D Wavelength Routing Networks - 73

73 First LTD solution AB (1.0) [total flow1.0] AB (0.1) AC (0.1) AD (0.3) [total flow 0.5] AB (0.1) BC (0.1) BD (0.3) [total flow 0.5] AB (0.1) AC (0.1) BD (0.3) CD (0.5) [total flow 1.0] Wavelength Routing Networks - 74

74 RWA problem Given the following physical topology (fibers and OXCs): A C X Y B It is a bi-connected topology. By avoiding shortest path routing, it is possible to solve the RWA problem with a single wavelength Z D Wavelength Routing Networks - 75

75 RWA solution AXB λ 1 AYD λ 1 CXZB λ 1 CYZD λ 1 By using shortest path routing, two wavelengths are required to solve the RWA problem AXB λ 1 AYD λ 2 CXB λ 2 CYD λ 1 AYZB λ 1 AXZD λ 2 CYZB λ 2 CXZD λ 1 for fault protection Wavelength Routing Networks - 76

Some economical principles

Hints on capacity planning (and other approaches) Andrea Bianco Telecommunication Network Group firstname.lastname@polito.it http://www.telematica.polito.it/ Some economical principles Assume users have