E E 681 - Module 13 © Wayne D. Grover 2002, 2003 1

p-Cycles

Jens Myrup PedersenAalborg University

© Wayne D. Grover 2002, 2003

E E 681 - Module 13

( Version for book website Dec. 2003)

E E 681 - Module 13 © Wayne D. Grover 2002, 2003 2

Background and Motivation

“ Ring “A. 50 msec restoration timesB. Complex network planning

and growthC. High installed capacity for

demand-servedD. Simple, low-cost ADMsE. Hard to accommodate

multiple service classes F. Ring-constrained routing

“Mesh”G. Up to 1.5 sec restoration

timesH. Simple, exact capacity

planning solutions

I. well under 100% redundancy

J. Relatively expensive DCS/OXCK. Easy / efficient to design for

multiple service classesL. Shortest-path routing

“ Shopping list” : A, D, H, I, L (and K) please...keep

the rest

E E 681 - Module 13 © Wayne D. Grover 2002, 2003 3

Relative Characteristics of Known Schemes

Restoration Times

Capacity Redundancy

1+1 APS, Rings

p-CyclesMesh Span Restoration

Shared Backup Path Protection (SBPP)

True Mesh Path Restoration

100% redundancy: “the dividing line”

E E 681 - Module 13 © Wayne D. Grover 2002, 2003 4

p-Cycles

E E 681 - Module 13 © Wayne D. Grover 2002, 2003 5

p-Cycles - an “on-cycle” failure

Reaction to an “on-cycle” failure is logically identical to a unit-capacity

BLSR loopback reaction

loopback

loopback

“on-cycle” spans have both working and spare capacity like a BLSR

E E 681 - Module 13 © Wayne D. Grover 2002, 2003 6

p-Cycles - a “straddling span” failure

Reaction to a straddling span failure is to switch failed signals onto two protection pathsformed from the related p-cycle

Break-in

Break-in

Straddling spans have two protected working signal units and haveno spare capacity

E E 681 - Module 13 © Wayne D. Grover 2002, 2003 7

A lot !

Re-consider the example:

It consumes 13 unit-hops of spare capacity

It protects one working signal on 13 spans and two working on 9 spans

i.e., spare / working ratio = 13 / (13*1 + 9*2 )

= 42%

How much difference can this make ?

A fully-loaded Hamiltonian p-cycle reaches the redundancy limit, 1/(d-1)

x2x2

x2 x2

x2

x2

x2

x2

x2

E E 681 - Module 13 © Wayne D. Grover 2002, 2003 8

Working span capacities arising from one unit of demand on each node-pair:

Example of a whole p-cycle network design

Total working capacity:

158 units8

1

5

6

9

4

9

4

4

10

73 2

13

11

10

7

6

14

5

7

6

7

E E 681 - Module 13 © Wayne D. Grover 2002, 2003 9

Design Solution: 53.8 % overall redundancy

A 1B 1C 1D 2E 2

Total protection capacity: 85 unitsRedundancy: 53.8%Optimal configuration dynamically computable or self-organized

p-Cycle Copies

Total: 7

E E 681 - Module 13 © Wayne D. Grover 2002, 2003 10

Summary: 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 based 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– -> mesh capacity efficiency

• Only two nodes do any real-time switching for restoration – protection capacity is fully pre-connected– switching actions are known prior to failure– -> BLSR speed

• “pre-configured protection cycles” p - cycles

E E 681 - Module 13 © Wayne D. Grover 2002, 2003 11

p-Cycle Capacity Design

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

E E 681 - Module 13 © Wayne D. Grover 2002, 2003 16

Optimal 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

E E 681 - Module 13 © Wayne D. Grover 2002, 2003 17

Understanding why p-cycles are so efficient...

9 Spares cover 9 Workers

9 Spares

cover 29 working channels

on 19 spans

Spare

Working Coverage

UPSR or

BLSR

p-Cycle…with same

spare capacity

“the clam-shell diagram”

E E 681 - Module 13 © Wayne D. Grover 2002, 2003 18

Further comparing p-cycles to rings

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A Priori p-Cycle Efficiency: AE(p)

• AE (p) measures a cycle’s potential to provide protection relationships for working channels

,

,, ,

,1

2

p i

p iS p C pi S

i C pi S X

X S SAE p

c S

Xp,i = 1 if on cycleXp,i = 2 if straddler

SS,p = 3

SC,p = 9

AE(p) = 1.67

SS,p = 4

SC,p = 10

AE(p) = 1.80

SC,p = # on cycleSS,p = # straddlersci = unit cost of i

E E 681 - Module 13 © Wayne D. Grover 2002, 2003 20

Demand-weighted p-Cycle Efficiency: Ew(p)

• Ew(p) measures a cycle’s actual efficiency in providing protection relationships for uncovered working channels

,

,

1

min( , )

p i

i p ii S

wi

i S X

w XE p

c

11

33

22

3322

1144

1122

22

2244

AE(p) = 1.67

Ew(p) = 3.78

11

33

22

3322

33

22

1122

22

2244

AE(p) = 1.67

Ew(p) = 3.67

Xp,i = 1 if on cycleXp,i = 2 if straddler

wi = working on ici = unit cost of i

E E 681 - Module 13 © Wayne D. Grover 2002, 2003 21

Self-organization of the p-cycles ...

• p-cycles certainly could be centrally computed and configured. – based on the preceding formulation

However, an interesting option is to consider if the network can adaptively and continually self-organize - a near-optimal set of p-cycles within itself, - for whatever demand pattern and capacity

configuration it currently finds.

E E 681 - Module 13 © Wayne D. Grover 2002, 2003 22

Self-organization of the p-cycles

• Based on an extension / adaptation of SHN™ distributed mesh restoration algorithm– “DCPC” = distributed cycle pre-configuration protocol

• Operates continually in background– Non-real time phase self-organizes p-cycles

– Real time phase is essentially BLSR switching

– p-cycles in continual self-test while in “storage”

• Centralized “oversight” but not low-level control– Method is autonomous, adaptive

• Networks actual state on the ground is the database

E E 681 - Module 13 © Wayne D. Grover 2002, 2003 23

Key concepts of DCPC protocol

• Node roles:– Cycler node state , Tandem node state

• DCPC implemented as event-driven Finite State Machine

(FSM)

• Nodal interactions are (directly) only between adjacent nodes– Indirectly between all nodes (organic self-organization)– via “statelets” on carrier / optical signal overheads

• Three main steps / time-scales / processes– Each nodes act individually, “exploring” network from its standpoint as

cycler node.– All nodes indirectly compare results – Globally best p-cycle is created

E E 681 - Module 13 © Wayne D. Grover 2002, 2003 24

Overview of DCPC protocol

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How DCPC discovers “best p-cycles” (2)

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How DCPC discovers “best p-cycles” (1)

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DCPC Performance studies

E E 681 - Module 13 © Wayne D. Grover 2002, 2003 28

Illustrating the Real time phase

E E 681 - Module 13 © Wayne D. Grover 2002, 2003 29

Adapting p-cycles to the IP-layer …

E E 681 - Module 13 © Wayne D. Grover 2002, 2003 30

IP Network Restoration

• IP Networks are already “Restorable”• Restoration occurs when the Routing protocol updates

the Routing Tables• This update can take a Minute or more - Packets are lost

until this happens

• Speed-up of IP Restoration is needed• Not losing packets would be great too• Also some control over capacity / congestion impacts

needed

• p-cycles proposed as “fast” part of a fast + slow strategy that retains normal OSPF-type routing table re-convergence

E E 681 - Module 13 © Wayne D. Grover 2002, 2003 38

Operation of IP-layer p-cycles

Failed Link

Router

Data De-Encapsulation

Data Encapsulation

Router

p-cycle

(a) On-Cycle Failure (1 restoration Path)

(b) Straddling Failure

(2 Restoration paths)

E E 681 - Module 13 © Wayne D. Grover 2002, 2003 43

• Node Encircling p-Cycles. Each Node has a p-Cycle dedicated to its failure

• For each Node, a p-Cycle is chosen which includes all logically “Adjacent” Nodes but not the Protected Node

Router Failure Restoration using“Node-Encircling” p-Cycles

Node-Encircling p-

cycle

Other Nodes

Encircled Node

E E 681 - Module 13 © Wayne D. Grover 2002, 2003 44

p-Cycles are Virtual Circuits/Protection Structures which can redirect Packets around Failures– Plain IP is Connectionless but p-Cycles can be realized with

MPLS, IP Tunneling/Static Routes

Router Restoration using“Node-Encircling” p-Cycles

Node Failure

E E 681 - Module 13 © Wayne D. Grover 2002, 2003 49

Concluding Comments

• p-cycles offer new approaches to both WDM and IP-layer transport

– “ mesh-like efficiency with ring-like speed ”

• Capacity-planning theory

– for 100% span restoration in WDM / Sonet with mesh sparing

– for controlled worst-case over-subscription in IP-layer

• “Node-encircling” p-cycles

– fast integrated restoration against either router or link-failures

• Nortel has implemented span-restoration via IP p-cycles

– ~ 10 msec restoration time, no packet loss in their experiments

• Ongoing studies:

• Integrated planning of composite node / link restoration p-cycles

• Availability analysis of p-cycles