Understand p-Cycles, Enhanced Rings, and Oriented Cycle Covers Wayne D. Grover TRLabs and University of Alberta TRLabs and University of Alberta Edmonton, AB, Canada web site for related papers etc: web site for related papers etc: ICOCN 2002, November 11-14, Singapore

Wayne D. Grover ICOCN ‘02 Singapore 2 Outline What are p- Cycles ? –Why do we say they offer “mesh-efficiency with ring-speed ?” Why are p-cycles so efficient ? Comparison to rings and “enhanced rings” Comparison to oriented cycle-covering techniques

Wayne D. Grover ICOCN ‘02 Singapore 3 The context The domain for all that follows is the problem of network protection at the transport capacity layer. i.e…. –Layer 3 inter-router lightwave channels –OBS-service layer working channels –Direct transport lighpaths –any other services or layers employing lightwave channels or paths All these sum to produce a certain number of working lightwave channels on each span Philosophy: Protect the working capacity directly and it doesn’t matter what the service type is

Rings... Fast, but not capacity - efficient

Wayne D. Grover ICOCN ‘02 Singapore 5 Two main types of “survivable ring”....(1) UPSR Unidirectional Path-switched Ring...Principle of operation

Wayne D. Grover ICOCN ‘02 Singapore 6 Protection fibre Working fibre  UPSR Animation... Tail-end Switch

Wayne D. Grover ICOCN ‘02 Singapore 7 UPSR (OPPR)...line capacity requirement Consider a bi-directional demand quantity between nodes A, B: d A,B. - A to B may go on the short route - then B to A must go around the longer route Thus, every (bi-directional) demand pair circumnavigates the entire ring. Hence in any cross section of the ring, we would find one unidirectional instance of every demand flow between nodes of the ring. Therefore, the line capacity of the UPSR must be: A D E B C A -> B B -> A “ The UPSR must have a line rate (capacity) greater (or equal to) the sum of all the (bi-directional) demand quantities between nodes of the ring. “

Wayne D. Grover ICOCN ‘02 Singapore 8 Protection fibres Working fibres Loop-back  (4 fiber) BLSR…(or OPSR)

Wayne D. Grover ICOCN ‘02 Singapore 9 BLSR …(OPSR) line capacity requirement both directions of a bi-directional demand can follow the short (or long) route between nodes “Bandwidth reuse” The line capacity of the BLSR must be: A D E B C A -> B B -> A “ The BLSR must have a line rate (capacity) greater (or equal to)the largest sum of demands routed over any one span of the ring. “

Wayne D. Grover ICOCN ‘02 Singapore 10 A particular issue in multi-ring network design... Ring 8 Ring 7 Ring 6 Example of 3 (of 7) rings from an optimal design for network shown Ring span overlaps Ideally, BLSR-based networks would be 100% redundant. Span overlaps and load imbalances mean in practice they can be up to 300% redundant

Mesh... Capacity - efficient, but (traditionally argued to be) slower, and have been hampered by DCS / OCX port costs

Wayne D. Grover ICOCN ‘02 Singapore 12 Concept of a span- (link-) restorable mesh network (28 nodes, 31 spans) 30% restoration 70% restoration 100% restoration span cut

Wayne D. Grover ICOCN ‘02 Singapore 13 Basics of Mesh-restorable networks (28 nodes, 31 spans) span cut 40% restoration 70% restoration 100% restoration

Wayne D. Grover ICOCN ‘02 Singapore 14 Basics of Mesh-restorable networks Spans where spare capacity was shared over the two failure scenarios ?..... This sharing efficiency increases with the degree of network connectivity “nodal degree”

Wayne D. Grover ICOCN ‘02 Singapore 15 Mesh networks require less capacity as graph connectivity increases ~ 3x factor in potential network capacity requirement

“ p-cycles “.. Fast, and capacity efficient.... Now we also have “ p-cycles “..

Wayne D. Grover ICOCN ‘02 Singapore 17 Background - ideas of mesh “preconfiguration”

Wayne D. Grover ICOCN ‘02 Singapore 18 Protection using p-cycles If span i fails, p-cycle j provides one unit of restoration capacity If span i fails, p-cycle j provides two units of restoration capacity i j i j

Wayne D. Grover ICOCN ‘02 Singapore 19 Optimal Spare capacity design with p-cycles

Wayne D. Grover ICOCN ‘02 Singapore 20 Optimal Spare capacity design - Typical Results “Excess Sparing” = Spare Capacity compared to Optimal Span- Restorable Mesh i.e., “mesh-like” capacity

Wayne D. Grover ICOCN ‘02 Singapore 21 Corroborating Results: COST239 European Study Network Pan European optical core network 11 nodes, 26 spans Average nodal degree = 4.7 Demand matrix –Distributed pattern –1 to 11 lightpaths per node pair (average = 3.2) 8 wavelengths per fiber wavelength channels can either be used for demand routing or connected into p-cycles for protection Copenhagen London Amsterdam Berlin Paris Brussels Luxembourg Prague Vienna Zurich Milan

Wayne D. Grover ICOCN ‘02 Singapore 22 Corroborating Results... See: Schupke et al… ICC 2002 Schupke found p-cycle WDM designs could have as little as 34% redundancy for 100% span restorability

Wayne D. Grover ICOCN ‘02 Singapore 23 Understanding why p-cycles are so efficient... 9 Spares cover 9 Workers 9 Spares cover 29 working on 19 spans Spare Working Coverage UPSR or BLSR p-Cycle …with same spare capacity “the clam-shell diagram”

Wayne D. Grover ICOCN ‘02 Singapore 24 Efficiency of p-Cycles (Logical) Redundancy = 2 * no. of straddling spans + 1* no. on-cycle spans no. spans on cycle 7 spans on-cycle, 2 straddlers : 7 / ( 7 + 2*2) = Example: Limiting case: p-cycle redundancy = N / ( N 2 - 2N)

Wayne D. Grover ICOCN ‘02 Singapore 25 The Unique Position p-Cycles Occupy Redundancy Speed “50 ms” 100 %50 %200 % Path rest, SBPP Span (link) rest. UPSR 200 ms p -cycles: BLSR speed mesh efficiency BLSR

Wayne D. Grover ICOCN ‘02 Singapore 26 ADM-like nodal device for p-cycle networking

Wayne D. Grover ICOCN ‘02 Singapore 27 Summary of 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 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 Only two nodes do any real-time switching for restoration –protection capacity is fully preconnected –switching actions are known prior to failure

..and how they differ from p-cycles Another recent development: --> “Enhanced Rings”

Wayne D. Grover ICOCN ‘02 Singapore 29 To understand “enhanced rings..”consider If the fill level of the two “working fibers” at the span overlap is 50% each then the overall LA- SLC arrangement is 300% redundant ! i.e., (total protection + unused working) _________________________ used working

Wayne D. Grover ICOCN ‘02 Singapore 30 “Enhanced” rings... Idea is to allow the two “facing” rings to share switched access to a single common protection span. So, the cross-sectional view becomes:c Now, redundancy = 2 / 1 = 200%

Wayne D. Grover ICOCN ‘02 Singapore 31 Is an enhanced ring the same as a p-cycle ?... No, because there is still a requirement for at least a matching amount of working and protection capacity on every span. In other words protection is still only provided and used in the “on- cycle” ring-like type of protection reaction. In contrast if the same problem is addressed with p-cycles, the troublesome span can be treated as: Or... no protection fibers at all on straddling span: redundancy = 1 / 1 = 100% no need to equip two working fibers if load does not require protection: redundancy ~ 0%

Oriented cycle double-covers Another recent approach to reduce undesirable span overlaps in ring-based network design...

Wayne D. Grover ICOCN ‘02 Singapore 33 Bi-directional Cycle Covers Even-degree nodeOdd degree node Consider the problem of “covering” all spans at a node with conventional bi-directional rings, without causing a span overlap... At an even degree node… there is no problem

Wayne D. Grover ICOCN ‘02 Singapore 34 Bi-directional Cycle Covers Even-degree nodeOdd degree node Now consider the same problem of covering at an odd-degree nodec At an odd degree node… no bi-directional ring cover exists that does not involve a span overlap

Wayne D. Grover ICOCN ‘02 Singapore 35 But with Unidirectional (Oriented) Cycle Covers Even-degree nodeOdd degree node …you can always cover both even and odd nodes without the equivalent of a ring span overlap... examples of undirectional ring covers... Equivalent to the bidirectional cover The unidirectional ring cover avoids any double-coverage ! (A mirror image set provides bidirectional W,P)

Wayne D. Grover ICOCN ‘02 Singapore 36 So are Oriented Cycle Covers the same as p-cycles ? No…because they still only protect in an on-cycle way. The result is to get to ring-protection at exactly the 100% redundancy lower limit. In an optimum oriented cycle cover every span will have exactly matching working and protection fibers. P-cycles involve spans that have 2 working and zero protection fibers, which will never be found in an oriented cycle cover.

Wayne D. Grover ICOCN ‘02 Singapore 37 Summary p-Cycles offer a promising new option for efficient realization of network protection –are preconfigured structures –use simple BLSR-like realtime switching –but are mesh-like in capacity efficiency Other recent advances can be superficially confused with p-cycles: –enhanced rings reduce ring network redundancy by sharing protection capacity between adjacent rings –oriented cycle (double) covers adopt a undirectional graph cycle- covering approach to avoid span overlaps Neither involves straddling spans; spans with working but no spare capacity –Both aim to approach their lower limits of 100% redundancy from well above 100% –p-cycles are well below 100% redundancy