Expected-Reliability Analysis for WirelessExpected-Reliability Analysis for Wireless CORBA with Imperfect Components CORBA with Imperfect Components

Xinyu Chen and Michael R. LyuXinyu Chen and Michael R. Lyu

Department of Computer Science and EngineeringDepartment of Computer Science and EngineeringThe Chinese Univ. of Hong KongThe Chinese Univ. of Hong Kong

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Wireless CORBA ArchitectureWireless CORBA Architecture

Cell

Access Bridge

Mobile Host

Static Host

Radio Link

Wired Link

Wired Network

Handoff: a mechanism for a Mobile Host to seamlessly

change a connection from one Access Bridge to another

Home Location Agent

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OutlineOutline

• BackgroundBackground

• Definitions and assumptionsDefinitions and assumptions

• Expected-reliability analysis for different Expected-reliability analysis for different communication schemes communication schemes

• ConclusionsConclusions

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ReliabilityReliability

• TT – a random variable representing the – a random variable representing the lifetime of a componentlifetime of a component

• f(t)f(t) – the probability density function of – the probability density function of TT

• R(t) R(t) – the reliability function of the – the reliability function of the componentcomponent

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Mean Time to Failure (MTTF)Mean Time to Failure (MTTF)

• Mean Time to Failure (MTTF)Mean Time to Failure (MTTF)• the expected value of the lifetime Tthe expected value of the lifetime T

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Two-Terminal Reliability Two-Terminal Reliability in Wired Networksin Wired Networks

• Assumption:Assumption:• Nodes or links experience failuresNodes or links experience failures

• The probability that there exists an operating The probability that there exists an operating path from a source node to a target nodepath from a source node to a target node

S

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T

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Why Expected-ReliabilityWhy Expected-Reliability

• Terminal mobility introduces handoffTerminal mobility introduces handoff

• Handoff causes the change of Handoff causes the change of numbernumber and and typetype of engaged communication of engaged communication components, then results in different components, then results in different system statessystem states

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Expected-ReliabilityExpected-Reliability

• Two-terminal expected-reliability at time Two-terminal expected-reliability at time tt

• QQss(t)(t)• the probability of the system in state the probability of the system in state ss at time at time tt

• RRss(t)(t)• the reliability of the system in state the reliability of the system in state ss at time at time tt

• Mean Time to FailureMean Time to Failure

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AssumptionsAssumptions

• There will always be a reliable path in There will always be a reliable path in the wired networkthe wired network

• The wireless link failure is negligibleThe wireless link failure is negligible

• All the four components, AB, MS, SH, All the four components, AB, MS, SH, and HLA, of wireless CORBA are and HLA, of wireless CORBA are failure-prone and fail independently failure-prone and fail independently

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The Reliability of the System in State The Reliability of the System in State ss at Time at Time tt

• RRss(t)(t)

• n(s) – the number of engaged components in system n(s) – the number of engaged components in system state sstate s

• RRii(t) – the reliability of the i(t) – the reliability of the ithth component component• c – the type of a componentc – the type of a component

• mh, ab, sh, or hlamh, ab, sh, or hla

• kkcc(s) – the number of component c in state s (s) – the number of component c in state s

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Assumptions (cont’d)Assumptions (cont’d)

• The failure parameters for the four The failure parameters for the four components, MH, AB, SH, and HLA, components, MH, AB, SH, and HLA, are constant, which are are constant, which are , , , , , and , and , , respectivelyrespectively

• The MH’s sojourn time with an AB The MH’s sojourn time with an AB and the handoff completion time are and the handoff completion time are exponentially distributed with exponentially distributed with parameters parameters and and , respectively, respectively

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Four Communication SchemesFour Communication Schemes

• Static Host to Static Host (SS)Static Host to Static Host (SS)• a traditional communication schemea traditional communication scheme

• Mobile Host to Static Host (MS)Mobile Host to Static Host (MS)

• Static Host to Mobile Host (SM)Static Host to Mobile Host (SM)

• Mobile Host to Mobile Host (MM)Mobile Host to Mobile Host (MM)

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The MS SchemeThe MS Scheme

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The System State ProbabilityThe System State Probability

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Expected-Reliability of the MS SchemeExpected-Reliability of the MS Scheme

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Two-Terminal MTTF of the MS SchemeTwo-Terminal MTTF of the MS Scheme

Handoff rateHandoff completion rate

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•Mobile Interoperable Object Reference (MIOR)•The LOCATION_FORWARD message

The SM SchemeThe SM Scheme

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Expected-Reliability of the SM SchemeExpected-Reliability of the SM Scheme

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Two-Terminal MTTF of the MS SchemeTwo-Terminal MTTF of the MS Scheme

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Time-Dependent Reliability ImportanceTime-Dependent Reliability Importance

• It measures the contribution of It measures the contribution of component-reliability to the system component-reliability to the system expected-reliabilityexpected-reliability

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Reliability-Importance of the SM SchemeReliability-Importance of the SM Scheme

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The MM SchemeThe MM Scheme

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The MM Scheme (cont’d)The MM Scheme (cont’d)

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The MM Scheme (cont’d)The MM Scheme (cont’d)

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General Two-Terminal MTTFGeneral Two-Terminal MTTF

• nnmm MHs and n MHs and nss SHs SHs

• Each MH or SH has the same Each MH or SH has the same probability to initiate a communicationprobability to initiate a communication

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General Two-Terminal MTTF General Two-Terminal MTTF (cont’d)(cont’d)

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ConclusionsConclusions

• Define the expected-reliability to embody the mobility Define the expected-reliability to embody the mobility characteristic introduced by handoffcharacteristic introduced by handoff

• Observe:Observe:• The failure parameters of MH, AB, and SH behave similarly on The failure parameters of MH, AB, and SH behave similarly on

the MTTF; however, the failure parameter of HLA takes little the MTTF; however, the failure parameter of HLA takes little effect on the MTTFeffect on the MTTF

• If the handoff happens frequently, we should improve the If the handoff happens frequently, we should improve the performance of the handoff completion and location forwarding performance of the handoff completion and location forwarding mechanismmechanism

• The general two-terminal MTTF increases with the number of The general two-terminal MTTF increases with the number of SHs but decreases with the number of MHs. SHs but decreases with the number of MHs.

• Identify the reliability importance of each component Identify the reliability importance of each component with respect to the expected-reliabilitywith respect to the expected-reliability