Introduction to rings: ring types, ring sizing and ring loading W. D. Grover TRLabs & University of Alberta © Wayne D. Grover 2002, 2003 E E Module 7

E E Module 7 © Wayne D. Grover 2002, Unidirectional Path-switched Ring...Principle of operation Two main types of “survivable ring”....(1) UPSR

E E Module 7 © Wayne D. Grover 2002, Unidirectional Path-switched Ring... Unidirectional - because in normal operation all working demand flows in one direction only. i.e.,A sends to B clockwise, B also sends to A clockwise Path-switched - because in restoration each receiver selects an alternate end-to-end path through ring, regardless of where actual break occurred. Two main types of “survivable ring”....(1) UPSR

E E Module 7 © Wayne D. Grover 2002, Protection fibre Working fibre  Tail-end Switch UPSR Animation...

E E Module 7 © Wayne D. Grover 2002, 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. “ UPSR...line capacity requirement

E E Module 7 © Wayne D. Grover 2002, Can be thought of as a number of virtual 1+1 APS set-ups sharing a single set of high-speed transmission systems to obtain “economy of scale”. Economy of scale arises since one OC-96 (say) optical Tx / Rx pair, is a lot less expensive than 96 OC-1 Tx / Rx ! UPSRs are inherently 2-fibre structures. Primary use is in “access” applications. - distances are not great - under pure “hubbed” demand pattern UPSR is as efficient as BLSR. UPSR need not “revert” after protection switching. UPSR switching decisions are independent on a tributary-by-tributary basis: - switching on one channel has no effect on other channels. The “access” demand pattern Notes on the UPSR

E E Module 7 © Wayne D. Grover 2002, Bi-directional Line-switched Ring...Principle of operation (“4-fibre” BLSR illustrated) Two main types of “survivable ring”....(2) BLSR

E E Module 7 © Wayne D. Grover 2002, Bi-directional Line-switched Ring...Principle of operation (“4-fibre” BLSR illustrated) Bi-directional - because in normal operation working demand flows travel in opposite directions over the same route through the ring Line-switched - because in restoration the composite optical line transmission signal is switched to the other direction around the ring (on the other fibre pair) specifically around the failed section. Note implication: Protection fibre capacity must equal the largest-working capacity cross-section of any span on the ring. “ 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. “ Two main types of “survivable ring”....(2) BLSR

E E Module 7 © Wayne D. Grover 2002, Protection fibres Working fibres Loop-back  (4 fibre) BLSR Animation...

E E Module 7 © Wayne D. Grover 2002, fibre 1 fibre 2 Each fibre has its tributary channels arranged in two groups - Working - Protection The set of 4 channel groups on two fibres then acts logically just like a 4-fibre BLSR For the same demand pattern the required line rate is doubled Ex: OC-48 2 BLSR: fibre 1 (cw) fibre 2 (ccw) - Channels 1-24 Working - Channels 1-24 Working - Channels Protection - Channels Protection “loopback” BLSR can also be in a “2-fibre” variant

E E Module 7 © Wayne D. Grover 2002, Start by considering how BLSR demand routing differs from UPSR.... Protection fibres Working fibres Protection fibre Working fibre l1l1 l1l1 UPSR: every demand pair circumnavigates ring BLSR: demand pair can be routed over shortest path. Not all spans “see” any given demand pair  opportunity for “bandwidth reuse” BLSR...line capacity requirement to serve its demands

E E Module 7 © Wayne D. Grover 2002, Concept of “bandwidth re-use” in a BLSR.... Demand 1-3Demand 1-4 Demand Time Slot #1 Demand 1-4 Demand 3-4 Demand 1-3 The example shows one timeslot (or “channel”) being reused on 4 spans to serve three different demand pairs. Q. what demand pattern lends itself to perfect bandwidth re-use ? Now note: the blue demand (1-3) could equally well have gone on route as (since same distance used). If so, what would effect be on required line-rate capacity ? Implication: BLSR line-rate requirement depends on how the set of demands it is to serve are loaded into it ! BLSR...Bandwidth re-use improves BLSR efficiency

Introduction to the ring sizing and loading problems... W. D. Grover TRLabs & University of Alberta © Wayne D. Grover 2002, 2003

E E Module 7 © Wayne D. Grover 2002, A heuristic algorithm for BLSR ring loading (Wu ‘92) Rank all demands in descending order 2. Map any adjacent-node demands into the ring (and remove from list) 3. Repeat In descending order: - map next largest demand into ring over its shortest route - map the same demand into the ring over the complementary route - choose the route that produces: where w i is the accumulation of demands crossing span i. - if each route produces the same {max w i } choose the shorter route - if both routes are equal, alternate this route choice with that at the next similar “tie”. Until all demands are served BLSR … Line capacity dependence on internal demand routing

E E Module 7 © Wayne D. Grover 2002, Example of the heuristic BLSR loading algorithm.... Demands (sorted in decreasing order): AC 10 EB8 EA 6 * ED 6 * DB 5 DC4 * EC4 BC 3 * AB 2 * D A B C E step 1 : Place adjacent- node demands: * denotes demand between adjacent nodes w i = 6 w i = 4 w i = 3 w i = 2 D A B C E BLSR...Example of Wu’s heuristic loading algorithm

E E Module 7 © Wayne D. Grover 2002, Remaining demands (sorted): AC 10 EB8 DB 5 EC4 D A B C E step 2 : Consider routing of the AC demand: w i = 6 w i = 4 w i = 13 w i = = 12 D A B C E w i = 16 w i = 14 w i = 3 w i = 2  shorter route is preferred: map AC via route A-B-C (max w i = 13) BLSR... Example of the heuristic BLSR loading algorithm

E E Module 7 © Wayne D. Grover 2002, Remaining demands (sorted): EB8 DB 5 EC4 D A B C E step 3 : Consider routing of the EB demand: w i = 14 w i = 6 w i = 4 w i = 13 w i = 20 D A B C E w i = 6 w i = 14 w i = 12 w i = 21 w i = 12  shorter route is again preferred: map EB via route E-A-B (max w i = 20) BLSR... Example of the heuristic BLSR loading algorithm

E E Module 7 © Wayne D. Grover 2002, Remaining demands (sorted): DB 5 EC4 D A B C E step 4 : Consider routing of the DB demand: w i = 14 w i = 6 w i = 9 w i = 18 w i = 20  shorter route is again preferred: map DB via route D-C-B (max w i = 20) D A B C E w i = 19 w i = 11 w i = 4 w i = 13 w i = 25 BLSR... Example of the heuristic BLSR loading algorithm

E E Module 7 © Wayne D. Grover 2002, Remaining demands (sorted): EC4 D A B C E step 5 : Consider routing of the EC demand: w i = 18 w i = 6 w i = 9 w i = 22 w i = 24  shorter route is again preferred: map EC via route E-D-C (max w i = 20) D A B C E w i = 14 w i = 10 w i = 13 w i = 18 w i = 20 BLSR... Example of the heuristic BLSR loading algorithm

E E Module 7 © Wayne D. Grover 2002, Resultant ring loading and sizing plan: D A B C E w i = 14 w i = 10 w i = 13 w i = 18 w i = 20 Demand pair route AC 10ABC EB8EAB EA 6 *direct ED 6 *direct DB 5DCB DC4 *direct EC4EDC BC 3 *direct AB 2 *direct Resulting in these net span loadings: and thus requiring (in practise) an OC-24 4-fiber BLSR or OC-48 2-fiber BLSR or an “ideal” 4-fiber OC-20 BLSR possible project idea: implement Wu’s algorithm followed by a meta-heuristic search for improvement towards optimal BLSR... Example of the heuristic BLSR loading algorithm

E E Module 7 © Wayne D. Grover 2002, D A B C E w i = 14 w i = 10 w i = 13 w i = 18 w i = 20 consider the ring just designed... One measure of BLSR efficiency is: ~ capacity usefully serving demands ~ redundant protection capacity required here... or conversely the redundancy is % s i required to be 20 everywhere BLSR... Capacity efficiency / redundancy assessment

E E Module 7 © Wayne D. Grover 2002, D A B C E w i = 14 w i = 10 w i = 13 w i = 18 w i = 20 to serve the same set of demands, the UPSR would require the ring line rate to be : but the amount of demand-serving capacity of the BLSR loading still applies as the measure of useful service or utility: Therefore, the redundancy measure (“spare to working” ratio) for the UPSR can be formed as: Compare to UPSR...

E E Module 7 © Wayne D. Grover 2002, From preceding it is evident that BLSR demand-serving ability depends in general on the demand pattern. Some of the recognized tendencies in real demand patterns are: or “mesh” ideal case for BLSR perfect bw re-use BLSR much more efficient than UPSR no optimization required this is the general tendency in inter-city backbone network optimization of ring loading this is a fairly exact model for access ring applications BLSR efficiency = UPSR same basic “access” demand pattern but dual hubs employed for access survivability Effect of some “generic” demand patterns on BLSR

E E Module 7 © Wayne D. Grover 2002, systematic study of relative demand-serving ability of (2 fibre) BLSR to UPSR... (Tom Flanagan, IEEE Communications Magazine, June see web site “reading” for lecture 9) with perfect bw re-use BLSR gets proportionally better as ring size increases with perfect hubbing demand patterns, BLSR never has any advantage over UPSR in this range optimized BLSR loading (and ring selection) can give significant benefits over UPSR Total demand serving capability Effectiveness of BLSR relative to UPSR depending on demand pattern

E E Module 7 © Wayne D. Grover 2002, D A B C E D A B C E Under single-hub pattern. The ring is “sized” by the cross-section of demands accumulating in spans next to the hub. If no. nodes is odd, half the demands appear in each such span. ~ UPSR like, but for a factor of 1/2 (If no. nodes is even, the best we can do is stay at same sizing principle, by splitting the flow where needed.) Under adjacent-node pattern we see perfect bw re-use. The more nodes, the more demands are served with the same line rate of the ring. BLSR relative to UPSR depending on demand pattern

E E Module 7 © Wayne D. Grover 2002, Ring “capacity” - generally means the optical line rate capacity of the ring but context matters: - is it the line-rate of an actual given ring system ? - or is someone speaking of the capacity required to serve some demands ? also convention: - usually the working capacity is referred to, with understanding for BLSR that the protection capacity is identical. e.g. “OC-48 4-BLSR” really represents two complete OC-48 bi-directional transmission systems Ring “size” should be avoided unless explicitly clarified... - does it mean the number of spans / nodes on the ring ? (“circumferential size”) or - does it mean the line capacity of the ring? Avoiding some confusions in working with rings

E E Module 7 © Wayne D. Grover 2002, SONET rings operate at OC-n line rates and the STS-1 tributaries are the “channels” The nodes of a ring are equipment called “Add-Drop Multiplexers” (ADMs) SONET rings may have a maximum of 16 active nodes, plus “glass-through” sites “Glass-throughs” are just nodes transited by the ring, but where no ADM is present “Glass-throughs” may be simply fibre splices or a regenerator point (“pass throughs”) Demand splitting refers to whether or not the total demand exchanged between two nodes has to be kept together on the same route of a ring or can be ‘split’ Time slot interchange (TSI) refers to whether the ADMs have the ability to cross- connect timeslot contents (assign a new time slot to a demand on the next span) More recent Optical rings have a DWDM optical line signal and add / drop single wavelengths or wave-bands - the logical “channel” is a wavelength ( ) or waveband - UPSR OPPR (Optical Path Protection Ring) - BLSR OSPR (Optical Shared Protection Ring) - ADM OADM - TSI (Time slot interchange) conversion Some other info about rings...

E E Module 7 © Wayne D. Grover 2002, Ring “Sizing” - CONTEXT: A number of demand pairs are to be served by a BLSR - QUESTION IS: What is the minimum line rate BLSR required? demands that must be served Required BLSR line capcity line rate = f (demands, routing in ring) Q. What is it that has to be optimally decided to minimize the required line rate ? i.e. (What do we have control over here?) A. for each demand: cw, or ccw ? BLSR related optimization problems (1)

E E Module 7 © Wayne D. Grover 2002, Ring “Loading” - CONTEXT: A number of demand pairs are to be served, but not necessarily all in same ring. i.e., there is a “pool” of outstanding demands to consider for selection into a given ring. - QUESTION IS: What is the maximum number of these demands that a BLSR with given capacity can serve? or... (alternate goal) Which set of demands (and routings) achieves greatest utilization of ring capacity? pool of demands needing to be served ? which demands to pick ? fixed ring capacity BLSR related optimization problems (2)

E E Module 7 © Wayne D. Grover 2002, …. Many published papers on either ring sizing or loading problems are called ring “loading” problems without distinction. - One has to study each paper to see if it is really addressing a sizing or a loading problem. ( Aside: “A word to the wise” )

E E Module 7 © Wayne D. Grover 2002, Ring “Sizing” : General Optimum design formulation Inputs (“parameters”) Outputs (“variables”) BLSR related optimization problems

E E Module 7 © Wayne D. Grover 2002, Understanding how the 1 / 0 parameters or variables encode the problem knowledge D A B C E D A B C E case (a) demand EC is considered for clockwise routing i =1 i =5 i =4 i =3 i =2 i =1 i =5 i =4 i =3 i =2 case (b) demand EC is considered for counter-clockwise routing ( + ) ( - ) BLSR related optimization problem formulations

E E Module 7 © Wayne D. Grover 2002, Ring “Sizing” : General Optimum design formulation minimize the required ring capacity keep sum of all demands crossing a span under the capacity every demand has to be routed either cw or ccw, but not both routing decisions are binary (cw or ccw) BLSR related optimization problem formulations

E E Module 7 © Wayne D. Grover 2002, Ring “Loading” : General Optimum design formulation Inputs (“parameters”) Outputs (“variables”) BLSR related optimization problem formulations

E E Module 7 © Wayne D. Grover 2002, Ring “Loading” : General Optimum design formulation maximize the number of demand pairs wholly served or, maximize total demand volume served keep the sum of all flows crossing a span under the line capacity you can refuse any demand, or to select it and route it cw or ccw, but not both decisions are binary BLSR related optimization problem formulations