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Reliability and Recovery
Assessment Methodology and Criteria
The availability of a subsystem is described by the following equation:
where MTBF represents Mean Time Between Failures and MTTR represents Mean Time to
Recover/Repair. MTTR is the time to diagnose, respond, and resume service.
This equation is also sometimes presented in industry literature as the following:
where MTTF is defined as Mean Time to Failure, and equates to (MTBF - MTTR).
Using the above equation, the estimated average annual minutes of downtime experienced due
to a subsystem failure can be expressed as:
AnnualDowntimeMinutes
(Based on assuming 365.25 days in a year, There are 365.25
year).
For projecting a enterprise's total system availability, the sum of annual downtime contributed
from all subsystems is calculated, and then the system availability is estimated by using this
sum.
TotalSystemAvailability
Note that this formula will result in a crude approximation of availability, as it does not account
for the groups of subsystems whose downtimes overlap.
Definition of Critical Outages and Downtime
The industry recognized criteria used for defining failure, outages and the downtime minutes
experienced is based on the Telecordia (Bellcore) GR-512 Reliability Model requirements for
telecommunications equipment.
256 Avaya Application Solutions IP Telephony Deployment Guide
MTBF MTTR
--------------------------------------------- -
Availability
=
--------------------------------------------- -
Availability
=
MTTF
=
1 Availability
–
th
i
SubsystemContributedMinuteDowntime
i
----------------------------------------------------------------------------------------------------------------------------------- -
=
1
–
–
MTBF
MTTF
+
MTTR
525960minutes / year
24
60 = 525960 minutes in a
525960Minutes / Year
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