Optical Network Resilience Resilient Network Design

Optical Network ResilienceResilient Network Design Massimo Tornatore, Politecnico di Milano, tornator@elet.polimi.it Guido Maier, Politecnico di Milano, maier@elet.polimi.it

Outline • Introduction to WDM network design and optimization • Integer Linear Programming approach • Physical Topology Design • Dedicated path protection case • Shared path & link protection cases • References

WDM networks: basic conceptsLogical topology CR1 CR3 CR2 CR4 Electronic-layer connection request • Logical topology (LT): each link represent a lightpath that could be (or has been) established to accommodate traffic • A lightpath is a “logical link” between two nodes • Full mesh Logical topology: a lightpath is established between any node pairs • LT Design (LTD): choose, minimizing a given cost function, the lightpaths to support a given traffic Electronic switching node(DXC, IP router, ATM switch, etc.) Optical network access point WDM LOGICAL TOPOLOGY WDM network WDM network nodes

WDM networks: basic concepts Physical topology • Physical topology: set of WDM links and switching-nodes • Some or all the nodes may be equipped with wavelength converters • The capacity of each link is dimensioned in the design phase WDM optical-fiber link Optical path termination Optical Cross Connect (OXC) Wavelength converter WDM PHYSICAL TOPOLOGY

WDM networks: basic concepts Mapping of logical over physical topology CR1 CR3 CR2 CR4 l l l 1 2 3 LP1 LP3 LP2 LP1 LP3 LP2 LP4 LP4 Mapping is differentaccording to the factthat the network isnot (a) or is (b) providedwith wavelengthconverters • Solving the resource-allocation problem is equivalent to perform a mapping of the logical over the physical topology • Also called Routing Fiber and Wavelength Assignment (RFWA) • Physical-network dimensioning is jointly carried out LP = LIGHTPATH (b) (a) Optical wavelength channels

Static WDM network planningProblem definition • Input parameters, given a priori • Physical topology (OXC nodes and WDM links) • Traffic requirement (virtual topology) • Connections can be mono or bidirectional • Each connection corresponds to one lightpath than must be setup between the nodes • Each connection requires the full capacity of a wavelength channel (no traffic grooming) • Parameters which can be specified or can be part of the problem • Network resources: two cases • Fiber-constrained: the number of fibers per link is a preassigned global parameter (typically, in mono-fiber networks), while the number of wavelengths per fiber required to setup all the lightpaths is an output • Wavelength-constrained: the number of wavelengths per fiber is a preassigned global parameter (typically, in multi-fiber networks) and the number of fibers per link required to setup all the lightpaths is an output

Static WDM network planning Problem definition (II) • Physical constraints • Wavelength conversion capability • Absent (wavelength path, WP) • Full (virtual wavelength path, VWP) • Partial (partial virtual wavelength path, PVWP) • Propagation impairments • The length of lightpaths is limited by propagation phenomena (physical-length constraint) • The number of hops of lightpaths is limited by signal degradation due to the switching nodes • Connectivity constraints • Node connectivity is constrained; nodes may be blocking • Links and/or nodes can be associated to weights • Typically, link physical length is considered

Static WDM network optimization Problem definition (III) • Routing can be • Constrained: only some possible paths between source and destination (e.g. the K shortest paths) are admissible • Great problem simplification • Unconstrained: all the possible paths are admissible • Higher efficiency in network-resource utilization • Cost function to be optimized (optimization objectives) • Route all the lightpaths using the minimum number of wavelengths (physical-topology optimization) • Route all the lightpaths using the minimum number of fibers (physical-topology optimization) • Route all the lightpaths minimizing the total network cost, taking into account also switching systems (physical-topology optimization)

Protection in WDM NetworksMotivations • Today WDM transmission systems allow the multiplexing on a single fiber of up to 160 distinct optical channels • recent experimental systems support up to 256 channels: • A single WDM channel carries from 2.5 to 40 Gb/s (ITU-T G.709) • The loss of a high-speed connection operating at such bit rates, even for few seconds, means huge waste of data !! • The increase in WDM capacity associated with the tremendous bandwidth carried by each fiber and the evolution from ring to mesh architectures brought the need for suitable protection strategies into foreground.

Optimization problem solution Mathematical programming solving • WDM-network static-design problem can be solved with the mathematical programming techniques • In most cases the cost function is linear  linear programming • Variables can assume integer values  integer linear programming • NP-complete/hard problems  computational complexity grows faster than any polynomial function of the size of the problem • Matches very well with algebraic network modeling • LP solution • Variables defined in the real domain • The well-known computationally-efficient Simplex algorithm is employed • Exponential complexity, but high efficiency • ILP solution • Variables defined in the integer domain • The optimal integer solution found by exploring all integer admissible solutions • Branch and bound: admissible integer solutions are explored in a tree-like search

ILP application to WDM network design Approximate methods • An arduous challenge • NP-completeness/hardness coupled with a huge number of variables • In many cases the problem has a very high number of solutions (different virtual-topology mappings leading to the same cost-function value) • Practically tractable for small networks • Simplifications • RFWA problem decomposition: first routing and then f/w assignment • Route formulation with constrained routing • Relaxed solutions • ILP, when solved with approximate methods, loses one of its main features: the possibility of finding a guaranteed minimum solution

ILP application to WDM network design ILP relaxation • Simplification can be achieved by removing the integer constraint • Connections are treated as fluid flows (multicommodity flow problem) • Can be interpreted as the limit case when the number of channels and connection requests increases indefinitely, while their granularity becomes indefinitely small • Fractional flows have no physical meaning as they would imply bifurcation of lightpaths on many paths • LP solution is found • In some cases the closest upper integer to the LP cost function can be taken as a lower bound to the optimal solution • Not always it works… • See [BaMu96]

Notation • l,k: link identifiers (source and destination nodes) • Fl,k: number of fibers on the link l,k • l,k: number of wavelengths on the link l,k • cl,k: weight of link l,k (es. length, administrative weight, etc.) • Usually equal to ck,l k l

Cost functionsSome examples • RFWA • Minimum fiber number M • Terminal equipment cost • VWP case (cost of transponder) • Minimum fiber mileage (cost) MC • Line equipment [BaMu00] • RWA • Minimum wavelength number • Minimum wavelength mileage • [StBa99],[FuCeTaMaJa03] • Minimum maximal wavelength number on a link • [Mu97]

Approaches to WDM design • Two basilar and well-known approaches [WaDe96],[Wi99] • FLOW FORMULATION (FF) • ROUTE FORMULATION (RF)

Variable xl,s,d: flow on link i associated to source-destination couple s-d Variable rp,s,d: number of connections s,d routed on the admissible path p Flow (FF) vs Route (RF) Formulation 1 2 s d 4 3 ROUTE FLOW r1,s,d s d r3,s,d r2,s,d • Fixed number of variables • Unconstrained routing • Constrained routing -k-shortest path • Sub-optimality?

Outline • Introduction to WDM network design and optimization • Integer Linear Programming approach • Physical Topology Design • Dedicated path protection case • Flow formulation • Virtual Wavelength Path • Wavelength Path • Route formulation • Shared path & link protection cases • References

Dedicated Path Protection (DPP) • 1+1 or 1:1 dedicated protection (>50% capacity for protection) • Both solutions are possible • Each connection-request is satisfied by setting-up a lightpath pair of a working + a protection lightpaths • RFWA must be performed in such a way that working and protection lightpaths are link disjoint • Additional constraints must be considered in network planning and optimization • Transit OXCs must not be reconfigured in case of failure • The source model can not be applied to this scenario

Flow variable Flow on link (l,k) due to a request generated by to source-destination couple (s,d) Flow Formulation (FF) 1 2 s d 4 3 • Fixed number of variables • Unconstrained routing

ILP application to WDM network designFlow Formulation (FF) • Variables represent the amount of traffic (flow) of a given traffic relation (source-destination pair) that occupies a given channel (link, wavelength) • Lightpath-related constraints • Flow conservation at each node for each lightpath (solenoidal constraint) • Capacity constraint for each link • (Wavelength continuity constraint) • Integrity constraint for all the flow variables (lightpath granularity) • Allows to solve the RFWA problems with unconstrained routing • A very large number of variables and constraint equations

Solenoidal constraint Guarantees spatial continuity of the lightpaths (flow conservation) For each connection request, the neat flow (tot. input flow – tot. output flow) must be: zero in transit nodes the total offered traffic (with appropriate sign) in s and d Capacity constraint On each link, the total flow must not exceed available resources(# fibers x # wavelengths) Wavelength continuity constraint Required for nodes without converters ILP application to WDM network designFlow formulationfundamental constraints s d 2 4 1 6 5 3

Notation • c: node pair (source sc and destination dc) having requested one or more connections • xl,k,c: number of WDM channels carried by link (l,k) assigned to a connection requested by the pair c • Al: set of all the nodes adjacent to node i • vc: number of connection requests having sc as source node and dc as destination node • W: number of wavelengths per fiber • λ: wavelength index (λ={1, 2 … W})

Dedicated Path Protection (DPP) Flow formulation, VWP • New symbols • xl,k,c,t= number of WDM channels carried by link (l,k) assigned to the t-th connection between source-destination couple c • Rationale: for each connection request, route a link-disjoint connection  route two connection and enforce link-disjoint constraint

Extension to WP case • In absence of wavelength converters, each lightpath has to preserve its wavelength along its path • This constraint is referred to as wavelength continuity constraint • In order to enforce it, let us introduce a new index in the flow variable to analyze each wavelength plane • The structure of the formulation does not change. The problem is simply split on different planes (one for each wavelength) • Thevctraffic is split on distinct wavelengths vc,λ • The same approach will be applied for no-flow based formulations

Dedicated Path Protection (DPP) Flow formulation, WP • New symbols • xl,k,c,t ,λ= number of WDM channels carried by wavelength λ on link l,k assigned to the t-th connection between source-destination couple c • Vc,λ= traffic of connection c along wavelength λ

Outline • Introduction to WDM network design and optimization • Integer Linear Programming approach • Physical Topology Design • Dedicated path protection case • Flow formulation • Route formulation • Virtual Wavelength Path • Wavelength Path • Shared path & link protection cases • References

WDM mesh network designRoute formulation • All the possible paths between each sd-pair are evaluated a priori • Variables represent which path is used for a given connection • rpsd: path p is used by rpsd connections between s and d • Path-related constraints • Routing can be easily constrained (e.g. using the K-shortest paths) • Useful to represent path-interference • Physical topology represented in terms of interference (crossing) between paths (e.g. ipr = 1 (0) if path p has a link in common with path r) • Number of variables and constraints • Very large in the unconstrained case, • Simpler than flow formulation when routing is constrained

Dedicated Path Protection (DPP)Route formulation (RF), VWP • New symbols • rc,n,t= 1 if the t-th connection between source destination node couple c is routed on the n-th admissible path • R(l,k)= set of all admissible paths passing through link (l,k)

ILP application to WDM network design Route formulation (RF) II, VWP • Substitute the single path variable rc,nby a protected route variable r’c,n(~ a cycle) • No need for representation in terms of interference (crossing) between paths • Identical formulation to unprotected case

Dedicated Path Protection (DPP)Route formulation (RF), WP

ILP application to WDM network design Route formulation (RF) II, WP • rc,n,λ= number of connections between source-destination couple c routed on the n-th admissible couple of disjoint paths having one path over wavelength λ1and the other over λ2

Shared PathProtection (SPP) • Protection-resourcessharing • Protection lightpaths of different channels share some wavelength channels • Based on the assumption of single point of failure • Working lightpaths must be link (node)disjoint • Very complex control issues • Also transit OXCs must be reconfigured in case of failure • Signaling involves also transit OXCs • Lightpath identification and tracing becomes fundamental • Sharing is a way to decrease the capacity redundancy and the number of lightpaths that must be managed

Link Protection N o r m a l C o n n e c t i o n • Link protection (≥50% capacity for protection) • Each link is protected by providing an alternative routing for all the WDM channels in all the fibers • Protection switching can be performed by fiber switches (fiber cross-connects) or wavelength switches • Signaling is local; transit OXCs of the protection route can be pre-configured • Fast reaction to faults • Some network fibers are reserved for protection • Link-shared protection (LSP) • Protection fibers may be used for protection of more than one link (assuming single-point of failure) • The capacity reserved for protection is greatly reduced O M S p r o t e c t i o n ( l i n k p r o t e c t i o n )

Link Protection • Different protected objects • Fiber level • Wavelength channel level FAULT EVENT (1) (2)

Link ProtectionSummary results • Comparison between different protection technique on fiber needed to support the same amount of traffic • Switching protection objects at fiber or wavelength level does ot sensibly affects the amount of fibers. • This difference increases with the number of wavelength per fiber

References • Articles • [WaDe96] N. Wauters and P. M. Deemester, Design of the optical path layer in multiwavelength cross-connected networks, Journal on selected areas on communications,1996, Vol. 14, pages 881-891, June • [CaPaTuDe98] B. V. Caenegem, W. V. Parys, F. D. Turck, and P. M. Deemester, Dimensioning of survivable WDM networks, IEEE Journal on Selected Areas in Communications, pp. 1146–1157, sept 1998. • [ToMaPa02] M. Tornatore, G. Maier, and A. Pattavina, WDM Network Optimization by ILP Based on Source Formulation, Proceedings, IEEE INFOCOM ’02, June 2002. • [CoMaPaTo03] A.Concaro, G. Maier, M.Martinelli, A. Pattavina, and M.Tornatore, “QoS Provision in Optical Networks by Shared Protection: An Exact Approach,” in Quality of service in multiservice IP Networks, ser. Lectures Notes on Computer Sciences, 2601, 2003, pp. 419–432. • [ZhOuMu03] H. Zang, C. Ou, and B. Mukherjee, “Path-protection routing and wavelength assignment (RWA) in WDM mesh networks under duct-layer constraints,” IEEE/ACM Transactions on Networking, vol. 11, no. 2, pp.248–258, april 2003. • [BaBaGiKo99] S. Baroni, P. Bayvel, R. J. Gibbens, and S. K. Korotky, “Analysis and design of resilient multifiber wavelength-routed optical transport networks,” Journal of Lightwave Technology, vol. 17, pp. 743–758, may 1999. • [ChGaKa92] I. Chamtlac, A. Ganz, and G. Karmi, “Lightpath communications: an approach to high-bandwidth optical WAN’s,” IEEE/ACM Transactionson Networking, vol. 40, no. 7, pp. 1172–1182, july 1992. • [RaMu99] S. Ramamurthy and B. Mukherjee, “Survivable WDM mesh networks, part i - protection,” Proceedings, IEEE INFOCOM ’99, vol. 2, pp. 744–751, March 1999. • [MiSa99] Y. Miyao and H. Saito, “Optimal design and evaluation of survivable WDM transport networks,” IEEE Journal on Selected Areas in Communications, vol. 16, pp. 1190–1198, sept 1999.

References • [BaMu00] D. Banerjee and B. Mukherjee, “Wavelength-routed optical networks: linear formulation, resource budgeting tradeoffs and a reconfiguration study,” IEEE/ACM Transactions on Networking, pp. 598–607, oct 2000. • [BiGu95] D. Bienstock and O. Gunluk, “Computational experience with a difficult mixed integer multicommodity flow problem,” Mathematical Programming, vol. 68, pp. 213–237, 1995. • [RaSi96] R. Ramaswami and K. N. Sivarajan, Design of logical topologies for wavelength-routed optical networks, IEEE Journal on Selected Areas in Communications, vol. 14, pp. 840{851, June 1996. • [BaMu96] D. Banerjee and B. Mukherjee, A practical approach for routing and wavelength assignment in large wavelength-routed optical networks, IEEE Journal on Selected Areas in Communications, pp. 903-908,June 1996. • [OzBe03] A. E. Ozdaglar and D. P. Bertsekas, Routing and wavelength assignment in optical networks, IEEE/ACM Transactions on Networking, vol. 11, no. 2, pp. 259-272, Apr 2003. • [KrSi01] Rajesh M. Krishnaswamy and Kumar N. Sivarajan, Design of logical topologies: A linear formulation for wavelength-routed optical networks with no wavelength changers, IEEE/ACM Transactions on Networking, vol. 9, no. 2, pp. 186-198, Apr 2001. • [FuCeTaMaJa99] A. Fumagalli, I. Cerutti, M. Tacca, F. Masetti, R. Jagannathan, and S. Alagar, Survivable networks based on optimal routing and WDM self-heling rings, Proceedings, IEEE INFOCOM '99, vol. 2, pp. 726-733,1999. • [ToMaPa04] M. Tornatore and G. Maier and A. Pattavina, Variable Aggregation in the ILP Design of WDM Networks with dedicated Protection , TANGO project, Workshop di metà progetto , Jan, 2004, Madonna di Campiglio, Italy

Optical Network Resilience Resilient Network Design