E E 681 - Module 17

W. D. Grover

TRLabs & University of Alberta

© Wayne D. Grover 2002, 2003

ATM VP-based (or MPLS path) ATM VP-based (or MPLS path) Restoration with Controlled Over-Restoration with Controlled Over-

subscription of Restoration Capacitysubscription of Restoration Capacity

based on work by Y. Zheng, W.D. Grover

E E 681 - Module 17 © Wayne D. Grover 2002, 2003 2

Differences between STM & ATM or MPLS

• STM (Synchronous Transfer Mode)– Transfer mode traditionally used for the Transport Layer– Example: SONET/SDH– Switching is done on OC-n level carrier signals (“switching the containers”)– Demands are served using reserved time slots (synchronous)

• ATM – Technology developed for the B-ISDN network– Supports multiple Classes of Service (with guaranteed Quality of Service)– Allows more efficient use of bandwidth by exploiting the statistical nature

of data traffic– Switching is done on Virtual Paths Connections (VPC or VP)– Traffic is packetized and transmitted asynchronously

• MPLS – IETF Technology for realizing connection-oriented functions similar to

ATM VP / VC constructs in an IP-based network– Uses label-switching concepts pioneered by ATM.

E E 681 - Module 17 © Wayne D. Grover 2002, 2003 3

STM & ATM Differences from a Restoration Viewpoint

• Demand decomposition:

– STM: Demand bundles are broken apart for restoration

– ATM: VPs must have single equal-sized backup VP

• Bandwidth replacement:

– STM: “Perfect bandwidth replacement” is required

– ATM: Some flow convergence overloads (oversubscription of bandwidth) can be

tolerated during restoration

Impact on capacity requirement

Impact on capacity requirement

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STM/ATM Comparison: Restoration Granularity

• STM: “fine grain” rerouting

• ATM: single backup VP

A Z

A Z

10 STS1

5 3 1

22

54

1

Before failure

Possible STM restoration pattern

A Z

VP BW = 10 STS1

A Z

Backup VP BW =10 STS1

ATM Backup VP on single route only

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STM/ATM Comparison: Bandwidth Replacement

• STM:

B

D

ZA1

3

B

D

ZA

1

34

• ATM (or MPLS):

1

33

4/3: 33% restoration-induced flow convergence overload

ATM: Restoration flow convergence

technically possible(at the expenses of a

higher packet loss probability)

STM: Perfect bandwidth

replacement

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ATM Capacity Design: KST Algorithm

• An algorithm proposed in the literature (KST algorithm*) can be used to do the capacity design of an ATM network for restoration of any failure of a single VP

• Each VP has a pre-assigned disjoint backup VP for which capacity is reserved

• Until a failure arises on the working VP, no traffic is sent on the backup VP (Corresponding bandwidth can be used for working traffic). Backup VPs are therefore referred to as “Zero bandwidth backup VPs”

* R. Kawamura, K-I Sato, I. Tokizawa, “Self-healing ATM networks based on virtual path concept,” IEEE Journal on Selected Areas in Communications, vol. 12, no. 1, January 1994.

ATM “pipe”

Virtual Path

Virtual PathZero Bandwidth backup VPs

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The Problem of Backup Flow Over-Subscription

• The KST algorithm does not consider the fact that a physical failure can simultaneously affect several VPs

• When multiple VP’s fail simultaneously, restoration overload can be experienced on some spans:– Restoration over subscription factor on span j for failure of span i:

* R. Kawamura, K-I Sato, I. Tokizawa, “Self-healing ATM networks based on virtual path concept,” IEEE Journal on Selected Areas in Communications, vol. 12, no. 1, January 1994.

j,i

# working VP's - # "stub release" VP's + # Backup VP's activatedX

# working VP's + # spare VP's

j j,i j,ij,i

j j

W -Rs +RrX

W +S

:j,iRr Total bandwidth of failed VP's from span i

for which span j is on the backup route

:j,iRs Total bandwidth of VP's which disappear

:jW Total working VP bandwidth

over span j

:jS Total spare bandwidth allocation

• Over-subscription factors are not controlled in the design process and therefore can potentially reach high values

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Uncontrolled Over-subscription

• With KST algorithm the resulting over-subscription factors are not controlled in the design process

• As a result, worst case over-subscription factors can be very high

Source of Data: Y. Zheng, W. Grover, M. MacGregor, “Broadband network design with controlled exploitation of flow convergence overloads in ATM VP-based restoration,” Proc. Canadian Conference on Broadband Research (CCBR’98).

Uncontrolled peaks

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ATM Design with Controlled Over-Subscription

• IP1: Minimum Spare Capacity with Design Limit on Maximum Restoration Over-Subscription

– Objective:

– Subject to:

1) Sparing is sufficient to keep restoration over-subscription below the design limit for all failures

2) Backup VPs are sufficient to meet the target restoration for all working VPs

3) Only one backup VP can be used for each working VP, i.e. VP flows are not split

Minimize j jj

c S

S

, ( , ) ,j i tolX X i j S i j

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ATM Design with Controlled Over-Subscription

• IP2: Minimum Over-Subscription for given Spare Capacity allocation

– Objective:

– Subject to:

1) Backup VPs are sufficient to meet the target restoration for all working VPs

2) Only one backup VP can be used for each working VP, i.e. VP flows are not split

,( , )Minimize max j ij i

i j

X

S

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Results of IP1 with Xtol = 1.0

• Comparative Spare Capacity Requirements

Limiting Xj,i to 1.0 means: No over

subscription allowed (equivalent to STM)

The consequence is: Higher spare capacity

requirements to guarantee full

bandwidth replacement available for each

physical failure

The KST Algorithm has low spare capacity requirements but has no control over the

maximum Xj,i.

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IP1 Results: Spare Capacity Requirements vs. Xtol

Very high reduction of spare capacity

requirement is observed when the maximum

over-subscription ratio is increased

The question then is: “What over-subscription ratio can we allow without too much degradation of service during span-failure states?”

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Statistics of Xj,i for a given Xtol

• Xtol is only the design-limiting maximum over-subscription

• In a design with a worst case over-subscription Xtol, what is the experience of most spans that are not at the worst case?

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Relating Over-subscription to cell-level effects

• How much over-subscription might be tolerable without undue cell-level effects?

• Criterion:“Allow the worst case over-subscription ratio to be such that a pre-failure cell-loss-probability (CLP) of 10-9 would do degrade to more than 10-5, during the restored state”Note: even this worst-case outcome would occur only for span j, upon failure of span i, where Xj,i = Xtol is the design-limiting instance AND the failure occurs at the design busy traffic hour AND at the growth build-out horizon of the installed equipment.

Method:FIFO

B

(R1, 1, b1)

(R3, 3, b3)

(R2, 2, b2)

(RN, N, bN)

c-9 = equivalent bandwidth at which CLP=10-9 for buffer size B

Simulation Queuing Model

Simulation parameters: (n,R, , b)n: number of sourcesR: peak rate per source: utilization per sourceb: mean burst length

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Results of Equivalent Bandwidth Simulations

Simulation determines the equivalent bandwidth c-5 at which CLP=10-5

c-5 / c-9 corresponds to the over-subscription factor for which a CLP of 10-9 will degrade to 10-5.

Results show that in many case, an over-subscription of 5 to 10% would be acceptable based on the CLP increase criterionThis corresponds to a 10 to 20% reduction of spare capacity according to previous results (slide 12)

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Conclusion

• ATM is an alternative approach to transport networking that allows QoS control and a better exploitation of statistical nature of data traffic for higher capacity utilization

• However, we have seen that:– Capacity design for ATM transport networks needs to consider the

possibility of multiple VP failures resulting from single physical failures otherwise potentially high overload situations could be experienced during restoration

– 10-20 % capacity savings can still be obtained (relative to STM) when controlled limited over-subscription is allowed in the design process (based on a CLP limit of 10-5 when the original CLP is 10-9)

• These concepts also apply to more recent IP-based transport networks