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E E 681 - Module 13 p-Cycles Jens Myrup Pedersen Aalborg University © Wayne D. Grover 2002, 2003 ( Version for book website Dec. 2003) E E 681 - Module 13 © Wayne D. Grover 2002, 2003 ‹#› Background and Motivation “ Ring “ “Mesh” A. 50 msec restoration times B. Complex network planning and growth C. High installed capacity for demand-served D. Simple, low-cost ADMs E. Hard to accommodate multiple service classes F. Ring-constrained routing G. Up to 1.5 sec restoration times H. Simple, exact capacity planning solutions I. well under 100% redundancy J. Relatively expensive DCS/OXC K. Easy / efficient to design for multiple service classes L. Shortest-path routing “ Shopping list” : A, D, H, I, L (and K) please...keep the rest E E 681 - Module 13 © Wayne D. Grover 2002, 2003 ‹#› Relative Characteristics of Known Schemes 1+1 APS, Rings 100% redundancy: “the dividing line” Capacity Redundancy p-Cycles Mesh Span Restoration Shared Backup Path Protection (SBPP) True Mesh Path Restoration Restoration Times E E 681 - Module 13 © Wayne D. Grover 2002, 2003 ‹#› p-Cycles E E 681 - Module 13 © Wayne D. Grover 2002, 2003 ‹#› p-Cycles - an “on-cycle” failure loopback loopback Reaction to an “on-cycle” failure is logically identical to a unit-capacity BLSR loopback reaction E E 681 - Module 13 “on-cycle” spans have both working and spare capacity like a BLSR © Wayne D. Grover 2002, 2003 ‹#› p-Cycles - a “straddling span” failure Break-in Break-in Reaction to a straddling span failure is to switch failed signals onto two protection paths formed from the related p-cycle Straddling spans have two protected working signal units and have no2002, spare capacity E E 681 - Module 13 © Wayne D. Grover 2003 ‹#› How much difference can this make ? x2 A lot ! x2 Re-consider the example: x2 x2 It consumes 13 unit-hops of spare capacity It protects one working signal on 13 spans and two working on 9 spans x2 E E 681 - Module 13 x2 x2 x2 i.e., spare / working ratio = 13 / (13*1 + 9*2 ) = 42% x2 A fully-loaded Hamiltonian p-cycle reaches the redundancy limit, 1/(d-1) © Wayne D. Grover 2002, 2003 ‹#› Example of a whole p-cycle network design Working span capacities arising from one unit of demand on each node-pair: 10 Total working capacity: 158 units 6 4 6 7 4 8 1 6 9 9 14 4 10 5 13 7 3 E E 681 - Module 13 7 11 © Wayne D. Grover 2002, 2003 7 5 2 ‹#› Design Solution: 53.8 % overall redundancy p-Cycle Copies A B C D E Total: 1 1 1 2 2 7 Total protection capacity: 85 units Redundancy: 53.8% Optimal configuration dynamically computable or self-organized E E 681 - Module 13 © Wayne D. Grover 2002, 2003 ‹#› Summary: Important Features of p-Cycles • • • • Working paths go via shortest routes over the graph p-Cycles are formed only in the spare capacity Can be either OXC-based or based on ADM-like nodal devices a unit-capacity p-cycle protects: – one unit of working capacity for “on cycle” failures – two units of working capacity for “straddling” span failures • Straddling spans: – there may be up to N(N-1)/2 -N straddling span relationships – straddling spans each bear two working channels and zero spare – -> mesh capacity efficiency • Only two nodes do any real-time switching for restoration – protection capacity is fully pre-connected – switching actions are known prior to failure – -> BLSR speed • “pre-configured protection cycles” p - cycles E E 681 - Module 13 © Wayne D. Grover 2002, 2003 ‹#› p-Cycle Capacity Design A. Form the spare capacity into a particular set of pre-connected cycles ! i j If span i fails, p-cycle j provides one unit of restoration capacity A span on the cycle fails - 1 Restoration Path, BLSR-like j i A p-cycle If span i fails, p-cycle j provides two units of restoration capacity A span off the p-cycle fails - 2 Restoration Paths, Mesh-like E E 681 - Module 13 © Wayne D. Grover 2002, 2003 " xi , j 1 " case " xi , j 2 " case ‹#› Optimal Spare capacity design Typical Results • “Excess Sparing” = Spare Capacity compared to Optimal SpanRestorable Mesh Test Network Excess spare capacity # of unitcapacity p-cycles formed # of distinct cycles used 1 2 3 4 5 9.09 % 3.07 % 0.0 % 2.38 % 0.0 % 5 88 250 2237 161 5 10 10 27 39 i.e., “mesh-like” capacity E E 681 - Module 13 © Wayne D. Grover 2002, 2003 ‹#› Understanding why p-cycles are so efficient... Spare UPSR or BLSR p-Cycle …with same spare capacity Working Coverage 9 Spares cover 9 Workers E E 681 - Module 13 “the clam-shell diagram” © Wayne D. Grover 2002, 2003 9 Spares cover 29 working channels on 19 spans ‹#› Further comparing p-cycles to rings E E 681 - Module 13 © Wayne D. Grover 2002, 2003 ‹#› A Priori p-Cycle Efficiency: AE(p) • AE (p) measures a cycle’s potential to provide protection relationships for working channels X p ,i 2 S S , p SC , p iS AE p SC,p = # on cycle SC , p ci Xp,i = 1 if on cycle Xp,i = 2 if straddler E E 681 - Module 13 iS X p ,i 1 SS,p = # straddlers ci = unit cost of i SS,p = 3 SS,p = 4 SC,p = 9 SC,p = 10 AE(p) = 1.67 AE(p) = 1.80 © Wayne D. Grover 2002, 2003 ‹#› Demand-weighted p-Cycle Efficiency: Ew(p) • Ew(p) measures a cycle’s actual efficiency in providing protection relationships for uncovered working channels Ew Xp,i = 1 if on cycle Xp,i = 2 if straddler 3 1 2 2 3 iS i 2 1 4 p ,i ) i wi = working on i ci = unit cost of i iS X p ,i 1 3 2 2 4 min( w , X p c AE(p) = 1.67 Ew(p) = 3.78 1 2 2 4 2 2 1 2 3 3 2 AE(p) = 1.67 Ew(p) = 3.67 1 E E 681 - Module 13 © Wayne D. Grover 2002, 2003 ‹#› Self-organization of the p-cycles ... • p-cycles certainly could be centrally computed and configured. – based on the preceding formulation However, an interesting option is to consider if the network can adaptively and continually selforganize - a near-optimal set of p-cycles within itself, - for whatever demand pattern and capacity configuration it currently finds. E E 681 - Module 13 © Wayne D. Grover 2002, 2003 ‹#› Self-organization of the p-cycles • Based on an extension / adaptation of SHN™ distributed mesh restoration algorithm – “DCPC” = distributed cycle pre-configuration protocol • Operates continually in background – Non-real time phase self-organizes p-cycles – Real time phase is essentially BLSR switching – p-cycles in continual self-test while in “storage” • Centralized “oversight” but not low-level control – Method is autonomous, adaptive • Networks actual state on the ground is the database E E 681 - Module 13 © Wayne D. Grover 2002, 2003 ‹#› Key concepts of DCPC protocol • Node roles: – Cycler node state , Tandem node state • DCPC implemented as event-driven Finite State Machine (FSM) • Nodal interactions are (directly) only between adjacent nodes – Indirectly between all nodes (organic self-organization) – via “statelets” on carrier / optical signal overheads • Three main steps / time-scales / processes – Each nodes act individually, “exploring” network from its standpoint as cycler node. – All nodes indirectly compare results – Globally best p-cycle is created E E 681 - Module 13 © Wayne D. Grover 2002, 2003 ‹#› Overview of DCPC protocol E E 681 - Module 13 © Wayne D. Grover 2002, 2003 ‹#› How DCPC discovers “best p-cycles” (2) E E 681 - Module 13 © Wayne D. Grover 2002, 2003 ‹#› How DCPC discovers “best p-cycles” (1) E E 681 - Module 13 © Wayne D. Grover 2002, 2003 ‹#› DCPC Performance studies E E 681 - Module 13 © Wayne D. Grover 2002, 2003 ‹#› Illustrating the Real time phase E E 681 - Module 13 © Wayne D. Grover 2002, 2003 ‹#› Adapting p-cycles to the IP-layer … E E 681 - Module 13 © Wayne D. Grover 2002, 2003 ‹#› IP Network Restoration • IP Networks are already “Restorable” • Restoration occurs when the Routing protocol updates the Routing Tables • This update can take a Minute or more - Packets are lost until this happens • Speed-up of IP Restoration is needed • Not losing packets would be great too • Also some control over capacity / congestion impacts needed • p-cycles proposed as “fast” part of a fast + slow strategy that retains normal OSPF-type routing table re-convergence E E 681 - Module 13 © Wayne D. Grover 2002, 2003 ‹#› Operation of IP-layer p-cycles Data Encapsulation Router Failed Link Data De-Encapsulation Router p-cycle (a) On-Cycle Failure (1 restoration Path) E E 681 - Module 13 (b) Straddling Failure (2 Restoration paths) © Wayne D. Grover 2002, 2003 ‹#› Router Failure Restoration using “Node-Encircling” p-Cycles • • Node Encircling p-Cycles. Each Node has a p-Cycle dedicated to its failure For each Node, a p-Cycle is chosen which includes all logically “Adjacent” Nodes but not the Protected Node NodeEncircling pcycle E E 681 - Module 13 © Wayne D. Grover 2002, 2003 Other Nodes Encircled Node ‹#› Router Restoration using “Node-Encircling” p-Cycles Node Failure p-Cycles are Virtual Circuits/Protection Structures which can redirect Packets around Failures – Plain IP is Connectionless but p-Cycles can be realized with MPLS, IP Tunneling/Static Routes E E 681 - Module 13 © Wayne D. Grover 2002, 2003 ‹#› Concluding Comments • p-cycles offer new approaches to both WDM and IP-layer transport – “ mesh-like efficiency with ring-like speed ” • Capacity-planning theory – for 100% span restoration in WDM / Sonet with mesh sparing – for controlled worst-case over-subscription in IP-layer • “Node-encircling” p-cycles – fast integrated restoration against either router or link-failures • Nortel has implemented span-restoration via IP p-cycles – ~ 10 msec restoration time, no packet loss in their experiments • Ongoing studies: • Integrated planning of composite node / link restoration p-cycles • Availability analysis of p-cycles E E 681 - Module 13 © Wayne D. Grover 2002, 2003 ‹#›