understand p -cycles, enhanced rings, and oriented cycle covers
DESCRIPTION
Understand p -Cycles, Enhanced Rings, and Oriented Cycle Covers. Wayne D. Grover TR Labs 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 ? - PowerPoint PPT PresentationTRANSCRIPT
Understand Understand pp-Cycles, Enhanced Rings, and -Cycles, Enhanced Rings, and Oriented Cycle CoversOriented Cycle Covers
Wayne D. Grover Wayne D. Grover TRTRLabsLabs and University of Alberta and University of Alberta
Edmonton, AB, CanadaEdmonton, AB, Canada
web site for related papers etc: web site for related papers etc:
http://www.ee.ualberta.ca/~grover/
ICOCN 2002, ICOCN 2002, November 11-14, Singapore November 11-14, Singapore
Wayne D. Grover ICOCN ‘02 Singapore 2
OutlineOutline
• 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 contextThe 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 the
service type is
Rings... Fast,
but not capacity - efficient
Wayne D. Grover ICOCN ‘02 Singapore 6
Protection fibre
Working fibre 1
2
3
4
5
UPSR Animation...UPSR Animation...
Tail-end Switch
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UPSR (OPPR)...line capacity requirementUPSR (OPPR)...line capacity requirement
•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:
A
D
E B
C
A -> B
B -> A
UPSR iji j
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. “
Wayne D. Grover ICOCN ‘02 Singapore 8
Protection fibres
Working fibres
Loop-back
Loop-back
1
2
34
5
(4 fiber) BLSR…(or OPSR)(4 fiber) BLSR…(or OPSR)
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BLSR …(OPSR) line capacity requirementBLSR …(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
max k kBLSR ij cw ij ccw
i jk
c d 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. “
Wayne D. Grover ICOCN ‘02 Singapore 10
A particular issue in A particular issue in multi-multi-ring network design...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-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
Wayne D. Grover ICOCN ‘02 Singapore 12
Concept of a span- (link-) restorable mesh networkConcept of a span- (link-) restorable mesh network
(28 nodes, 31 spans)
30% restoration70% restoration100% restoration
span cut
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Basics of Mesh-restorable networksBasics of Mesh-restorable networks
(28 nodes, 31 spans)
span cut
40% restoration70% restoration100% restoration
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Basics of Mesh-restorable networksBasics 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 Mesh networks require less capacity as graph connectivity capacity as graph connectivity increasesincreases
Nodal Degree vs. Average Capacity(Network: CSELTNet, Gravity Demand, Least Usage Elimination)
0
100
200
300
400
500
600
700
800
900
2.0 2.5 3.0 3.5 4.0
Average Nodal Degree
Cap
acit
y R
equ
ired
w orking
spare
capacity ~ 3x factorin potential
networkcapacity
requirement
“ p-cycles “..
Fast,
and
capacity efficient ....
Now we also have “ p-cycles “..
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Background - ideas of mesh “preconfiguration”Background - ideas of mesh “preconfiguration”
X
Node X forfailure 1
Node X forfailure 3
Node X forfailure 2
Node X forfailure 4
Q. How could you ever have the spare capacity of a mesh network completely pre-connected in advance of any failure?
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Protection using Protection using pp-cycles-cycles
A p-cycle
A span on the cycle fails - 1 Restoration Path, BLSR-like
A span off the p-cycle fails - 2 Restoration Paths, Mesh-like
A. Form the spare capacity into a particular set of pre-connected cycles !
," 1 " case
i jx
," 2 " case
i jx
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
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Optimal Spare capacity design with Optimal Spare capacity design with pp-cycles-cycles
Step 1: Find set of elementary cycles of the network graph
Step 2: For each cycle, determine x i,j : the no. of restoration paths that cycle i contributes for failure j. x i,j
Step 3: Integer Program to select optimal p-cycle set:
Objective: minimize: total cost of spare capacity.
Subject to:
1. Restorability: All working links on each span have
(simultaneously feasible) access to one or more p-cycles.
2. Spare Capacity: All p-cycles placed are feasible within the
span spare capacities assigned
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Optimal Spare capacity design - Typical ResultsOptimal Spare capacity design - Typical Results
TestNetwork
Excesssparecapacity
# of unit-capacityp-cyclesformed
# ofdistinctcyclesused
1 9.09 % 5 52 3.07 % 88 103 0.0 % 250 104 2.38 % 2237 275 0.0 % 161 39
• “Excess Sparing” = Spare Capacity compared to Optimal Span-Restorable Mesh
i.e., “mesh-like” capacity
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Corroborating Results: Corroborating Results: COST239 European Study NetworkCOST239 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
10
1
56
783
9
0 2 4
Copenhagen
LondonAmsterdam Berlin
Paris
Brussels LuxembourgPrague
Vienna
Zurich
Milan
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Corroborating Results...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
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Understanding Understanding whywhy pp-cycles are so efficient...-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”
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Efficiency of Efficiency of pp-Cycles-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) = 0.636
Example:
Limiting case: p-cycle redundancy = N / ( N 2 - 2N)
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The Unique Position The Unique Position pp-Cycles Occupy-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
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ADM-like nodal device for ADM-like nodal device for pp-cycle networking-cycle networking
nodal redundancy =
spare 1
working 1
: 3 25%
R
k
example k R
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Summary of Important Features of Summary of Important Features of pp-Cycles-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”
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To understand “enhanced rings..”considerTo understand “enhanced rings..”consider
Ring 1
Ring 2
AA
Cross-sectional view at A-A
Ring 1 Ring 2
W W P Pworking-fill
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...Enhanced” rings...
Cross-section view at A-A with enhanced rings
Ring 1 Ring 2
W W P
Shared protectionchannel
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%
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Is an enhanced ring the same as a Is an enhanced ring the same as a pp-cycle ?...-cycle ?...
Cross-section view at A-A with p-cycle
Straddling span 1 Straddling span 2
W W
Cross-section view at A-A with p-cycle
Straddling span 1 Straddling span 2
W (not equipped)
•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 ...
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Bi-directional Cycle CoversBi-directional Cycle Covers
Even-degree node Odd 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
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Bi-directional Cycle CoversBi-directional Cycle Covers
Even-degree node Odd 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
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But with But with UnidirectionalUnidirectional (Oriented) (Oriented) Cycle Covers Cycle Covers
Even-degree node Odd 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 coveravoids any double-coverage !
(A mirror image set providesbidirectional W,P)
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So are So are OrientedOriented Cycle Covers the same as Cycle Covers the same as pp-cycles ?-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.
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SummarySummary
• 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