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

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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

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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
PhilosophyProtect the working capacity
directly and it doesnt matter what theservice
type is
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• Rings... Fast,
• but not capacity - efficient

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Two main types of survivable ring....(1) UPSR
Unidirectional Path-switched Ring...Principle of
operation
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UPSR Animation...
Working fibre
1
Tail-end Switch
5
2
Protection fibre
3
4
l1
7
UPSR (OPPR)...line capacity requirement
A -gt 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 -gt 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.
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(4 fiber) BLSR(or OPSR)
Working fibres
1
Loop-back
5
2
Protection fibres
3
4
l1
Loop-back
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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 -gt B
A
B -gt 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.
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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
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• Mesh... Capacity - efficient ,
• but (traditionally argued to be) slower,
• and have been hampered by DCS / OCX port costs

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Concept of a span- (link-) restorable mesh network
(28 nodes, 31 spans)
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Basics of Mesh-restorable networks
(28 nodes, 31 spans)
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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
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Mesh networks require less capacity as graph
connectivity increases
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Now we also have p-cycles ..

• p-cycles ..
• Fast,
• and
• capacity efficient ....

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Background - ideas of mesh preconfiguration
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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
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Optimal Spare capacity design with p-cycles
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Optimal Spare capacity design - Typical Results

• Excess Sparing Spare Capacity compared to
  Optimal Span-Restorable Mesh

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Corroborating Results COST239 European Study
Network
Copenhagen

• 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

London
Berlin
Amsterdam
Brussels
Luxembourg
Prague
Zurich
Paris
Vienna
Milan
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Corroborating Results...
See Schupke et al ICC 2002
Schupke found p-cycle WDM designs could have as
little as 34redundancy for 100span
restorability
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Understanding why p-cycles are so efficient...
Spare
p-Cyclewith 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
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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
22) 0.636
Example
Limiting case p-cycle redundancy N / ( N 2 -
2N)
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The Unique Position p-Cycles Occupy
Path rest, SBPP
p -cycles BLSR speedmesh efficiency
Speed
Span (link)rest.
200 ms
BLSR
50 ms
UPSR
100
50
200
Redundancy
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ADM-like nodal device for p-cycle networking
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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

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Another recent development --gt Enhanced Rings

• ..and how they differ from p-cycles

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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
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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 becomesc
Now, redundancy 2 / 1 200
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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
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Another recent approach to reduce undesirable
span overlaps in ring-based network design ...

• Oriented cycle double-covers

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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 nodethere is no problem
Even-degree node Odd degree node
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Bi-directional Cycle Covers

• Now consider the same problem of covering at an
  odd-degree nodec

At an odd degree nodeno bi-directional ring
cover exists that does not involve a span
overlap
Even-degree node Odd degree node
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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
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So are Oriented Cycle Covers the same as p-cycles
?

• Nobecause 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.

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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