|
MTBF: For hardware components, MTBF information can be
obtained from hardware manufactures data sheets. If
the hardware has been developed in house, the
hardware group would provide MTBF information for
the board.
MTTR: Estimates for hardware are based on
the degree to which the system will be monitored by
operators.
The ability of an item to be in a state to
perform a required function under given conditions
at a given instant of time or during a given time
interval, assuming that the required external
resources are provided.
At its simplest level..
Availability = Uptime / (Downtime
+ Uptime)
The time units are generally hours and the time
base is 1 year . There are 8760 hours in one year.
|
Availability % |
= |
Actual Availability/Agreed Availability * 100%
|
|
Actual Availability |
= |
Size of measurement
interval – Downtime
|
|
Downtime |
= |
Time to repair |
Availability of the module is the percentage of
time when system is operational. Availability of a
hardware/software module can be obtained by the
formula given below.
A =
MTBF/MTBF + MTTR
Availability is typically specified in nines
notation. For example 3-nines availability
corresponds to 99.9% availability. A 5-nines
availability corresponds to 99.999% availability.
Downtime per year is a more intuitive way of
understanding the availability. The table below
compares the availability and the corresponding
downtime.
|
Availability |
Downtime |
| 90% (1-nine) |
36.5 days/year |
| 99% (2-nines) |
3.65 days/year |
| 99.9% (3-nines) |
8.76 hours/year |
| 99.99% (4-nines) |
52 minutes/year |
| 99.999% (5-nines) |
5 minutes/year |
| 99.9999% (6-nines) |
31 seconds/year ! |
|
Component |
MTBF |
MTTR |
Availability |
Downtime |
|
Input
Transducer |
100,000 hours |
2
hours |
99.998% |
10.51
minutes/year |
|
Signal
Processor Hardware |
10,000
hours |
2
hours |
99.98% |
1.75
hours/year |
|
Signal
Processor Software |
2190
hours |
5
minute |
99.9962% |
20
minutes/year |
|
Output
Transducer |
100,000 hours |
2
hours |
99.998% |
10.51
minutes/year |
|
