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

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


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


Protection fibre

Working fibre 1

2

3

4

5

UPSR Animation...UPSR Animation...

Tail-end Switch


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. “


Protection fibres

Working fibres

Loop-back

Loop-back

1

2

34

5

(4 fiber) BLSR…(or OPSR)(4 fiber) BLSR…(or OPSR)


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. “


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


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


Basics of Mesh-restorable networksBasics of Mesh-restorable networks

(28 nodes, 31 spans)

span cut

40% restoration70% restoration100% restoration


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”


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 “..


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?


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


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


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


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


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


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”


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)


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


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


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”


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


““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%


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 ...


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


Bi-directional Cycle CoversBi-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


But with But with UnidirectionalUnidirectional (Oriented) (Oriented) Cycle Covers Cycle Covers


…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)


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.


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

Documents

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