1. Design of Networks based on multiple rings
ECE Module 10
Design of Networks based on multiple rings
W. D. Grover
TRLabs & University of Alberta
© Wayne D. Grover 2002, 2003, 2004

2. The multi-ring design problem
Given:
- a two-connected (or bi-connected) graph
- a set of “ring technologies” and costs. e.g OC-192 4/BLSR, OC-48 UPSR, etc...including 1+1 APS
- a set of demands to be served.
- a subset of node locations where demands may transit from ring-to-ring
Determine:
- the number, size, type and placement of all rings
- the location of glass-throughs (and ADM terminals) on all rings
- the end-to-end routing of each demand
For minimal cost of all rings placed, including costs associated with demands transiting from ring to ring.

3. Key concepts, issues, and principles in multi-ring design
Concept (each follows in more detail):
graph coverage:
Balance
Capture
Span elimination
Dual-ring interconnect
transit sites
glass-throughs
....a set of rings that covers every edge of the graph. This is one class of ring network.
....in a BLSR, how well are the wi quantities “balanced” ? (since the largest of them dictates the protection capacity).
....to what extent does a given ring tend to serve demands that both originate and terminate in the same ring.
....a multi-ring design may not “cover” all graph edges, if the working demands can take non-shortest path routes.
....for the highest service availability, some demands may employ geographically redundant duplicate inter-ring transfers
....not all nodes may be sites where demands can switch rings.
....each ring needs ADMs where demands add / drop, but not elsewhere ( ~> Express rings etc.).

4. “Graph coverage” and the concept of span elimination
a set of rings that uses or overlies all edges of the physical facilities graph is called a “ring cover”.
“Coverage-based” design is a special (simpler) case of multi-ring design.
More generally the aim is to protect all demands, not necessarily to “cover all spans.”
“span eliminations”
example
a single ring design that may also serve all demands… Q. what is implied?
a three ring “cover”

5. “Graph coverage” and concept of span elimination
how the routing of demands varies to accommodate “span eliminations”
to “eliminate” span (2-5)
coverage

6. “Graph coverage” and concept of span elimination
problem statements
coverage
design with span eliminations
“ protect every transmission span of the network by including it in a ring “
“ serve all demands in a protected manner end to end across the network “
- leads to solutions using only subset of available graph edges
- leads to built in presumption of coverage -type designs
- over wide design ranges eliminations reduce # rings, reduce total cost.
- working demands follow shortest paths independent of later ring choices
- However, span eliminations can be taken too far. (ring capacity requirements grow due to excessive re-routing)
- coverage formulations can form starting point for designs with span eliminations.
- more complex, but realistic planning problem.
- routing policy no longer shortest paths, but coupled to ring selections and loading.
- simpler, idealized “academic” problem
(more later...algorithms for finding good span eliminations)

7. Dual-ring interconnect (DRI)-1
“drop-and-continue” method for BLSRs (also called a “matched nodes arrangement”- Nortel)
Building 1
Building 2

8. Dual-ring interconnect (DRI)-2
“drop-and-continue” method for BLSRs
Drop
Add
normal “drop”,
cross-office wiring,
then “adds” into r2
Drop
Add

9. Dual-ring interconnect (DRI)-3
“drop-and-continue” method for BLSRs
the primary gateway node has a 1+1 receive selection setup here.
protected by BLSR line-loopback reaction in r1
protected by 1+1 APS inter-ring setup
protected by BLSR line-loopback reaction in r2

10. Dual-ring interconnect (DRI)-4
“drop-and-continue” method applied to interconnect UPSRs
each ADM in a UPSR already has 1+1 intra-ring receive selection setup here.
just add dual geographically diverse transitions and suppress normal signal splitting in receiving ADMs for ring 2.

11. Dual-ring interconnect (DRI)-5
Explicit dual-feeding ...”df”
explicit dual feeding uses up more intra-ring line capacity, but in some cases uses even less drop-and-continue capacity (inter-ring transition nodes need not be adjacent)
this will be part of a later optimal routing policy using mixtures of “mn” and “df” inter-ring transitions along an end-to-end path.

12. Concept of a Ring Connectivity Graph (RCG)
RCG is a transformation of the graph that represents the opportunities to transition from ring to ring.
example: with ring-set given, r1 is connected to r2 through only one node.
For DRI routing, only the RCG edges with 2 or more parallel arcs are available for routing

13. Concepts of Balance, Capture (and routing)-1
Routing Efficiency: a measure of how well demands are routed through the network (i.e., average hops/demand).
Good (intrinsic) routing
Poor (instrinsic) routing (but may increase ring balance or capture)

14. Concepts of Balance, Capture (and routing)-2
Balance (Capacity) Efficiency: a measure of how evenly the working span capacities are matched in the rings (i.e., average fill).
Poor Balance (but could have good capture) ...
Good Balance 10 12 3 5
OC-12
OC-12 8

15. Concepts of Balance, Capture (and routing)-3
Capture Efficiency: a measure of how well the rings contain the demands that they carry, in terms ideally of delivering them from source to sink within the same ring. (i.e., relevance is to the cost of inter-ring transitions).
Good Capture
Poor Capture
from source
source
L = 1
T = 2
L = 2
T = 0
sink
for demands shown:
to sink
L is the number of spans on the ring, in the path of the demand
T is the number of transitions on or off of the ring (excluding those at source / sinks)

16. SCIP: A “pure coverage” ring design model
SCIP = “span coverage IP”
Assumes:
working demands already routed -> i.e., wi values are given
rings have ADMs in every node (no glass-throughs)
demands can transition between rings at any node
there are no costs for inter-ring transitions
Let:

17. SCIP: idealized “pure coverage” design
minimize cost of all rings placed, of each modular size
ensure sum of all modules overlying any span meets or exceeds its working capacity
decisions are to place only a whole number of rings of each modular capacity and layout

18. Comments on SCIP SCIP contains several idealizations but:
- is optimally solvable for small-medium sized problems.
- can be used as a starting point for improvement heuristics.
- identifies ring choices with very high “balance” efficiency.
- can be viewed as a upper bound on expected design cost when routing, glass-throughs, span-eliminations and transition costs are also optimized.
- can be used as a “surrogate” for ring network design cost in comparative studies (where relative, not absolute costs matter as the figure of merit).
Class Question:
Given the idealizations in SCIP, are its designs more realistic / accurate for:
(a) metropolitan-scale network design
(b) long-haul network design ?
Answer: In the long-haul design context ADM node, glass-through, and transition costs are usually less important than transmission capacity efficiency, which correlates strongly to shortest path routing and balance-optimized ring choices.