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Understand p -Cycles, Enhanced Rings, and Oriented Cycle Covers

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## Understand p -Cycles, Enhanced Rings, and Oriented Cycle Covers

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### Understand p-Cycles, Enhanced Rings, and Oriented Cycle Covers

Wayne D. Grover

TRLabs and University of Alberta

Edmonton, AB, Canada

web site for related papers etc: http://www.ee.ualberta.ca/~grover/

ICOCN 2002, November 11-14, Singapore

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 orientedcycle-covering techniques

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 lightwavechannels on each span

Philosophy:Protect the working capacity directly and it doesn’t matter what theservice type is

Rings... Fast,

but not capacity - efficient

UPSR (OPPR)...line capacity requirement

A -> B

A

- Consider a bi-directional demand quantity between nodes A, B: dA,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 paircircumnavigates the entire ring.
- Hence in any cross section of the ring,we would find one unidirectional instanceof every demand flow between nodes of the ring.
- Therefore, the line capacity of the UPSRmust be:

E

B

B -> A

C

D

“ 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. “

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 -> B

A

B -> A

E

B

C

D

“ 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. “

A particular issue in multi-ring network design...

Example of 3 (of 7) rings from an optimal design for network shown

Ring 8

Ring span overlaps

Ring 6

Ring 7

Ideally, BLSR-basednetworks 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

30% restoration

70% restoration

100% restoration

Concept of a span- (link-) restorable mesh network(28 nodes, 31 spans)

40% restoration

100% restoration

70% restoration

Basics of Mesh-restorable networks(28 nodes, 31 spans)

Basics of Mesh-restorable networks

Spans where spare capacity was shared

over the two failurescenarios ? .....

This sharing efficiency increases with the degree of network connectivity

“nodal degree”

Protection using p-cycles

If span i fails,p-cycle j provides one unit of restoration capacity

i

j

If span i fails,p-cycle j provides two units of restoration capacity

j

i

Optimal Spare capacity design - Typical Results

- “Excess Sparing” = Spare Capacity compared to Optimal Span-Restorable Mesh

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

Berlin

Amsterdam

Brussels

Luxembourg

Prague

Zurich

Paris

Vienna

Milan

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

Understanding whyp-cycles are so efficient...

Spare

p-Cycle…with same spare capacity

UPSR or BLSR

Working Coverage

9 Spares cover 29 working on 19 spans

9 Spares cover 9 Workers

“the clam-shell diagram”

Efficiency of p-Cycles

(Logical) Redundancy =

2 * no. of straddling spans + 1* no. on-cycle spans

------------------------------------------------------------------

no. spans on cycle

Example:

7 spans on-cycle,

2 straddlers :

7 / ( 7 + 2*2) = 0.636

Limiting case: p-cycle redundancy = N / ( N 2 - 2N)

The Unique Position p-Cycles Occupy

Path rest, SBPP

p -cycles: BLSR speedmesh efficiency

Speed

Span (link)rest.

200 ms

BLSR

“50 ms”

UPSR

50 %

100 %

200 %

Redundancy

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-capacityp-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

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

“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%

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:

no protection fibers at all on straddling span:

redundancy = 1 / 1 = 100%

Or...

no need to equip two working fibers if load does not require protection:

redundancy ~ 0%

Another recent approach to reduce undesirable span overlaps in ring-based network design ...

Oriented cycle double-covers

Bi-directional Cycle Covers

- 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

Even-degree node Odd degree node

Bi-directional Cycle Covers

- 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

Even-degree node Odd degree node

But with Unidirectional (Oriented) Cycle Covers

…you can always cover both even and odd nodes without the equivalent of a ring span overlap...

examples of undirectional ring covers...

Even-degree node Odd degree node

(A mirror image set providesbidirectional W,P)

The unidirectional ring coveravoids any double-coverage !

Equivalent to the bidirectional cover

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.

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

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