Difference between revisions of "System availability"
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| ||= mean time to detect fault + mean time to fix fault | | ||= mean time to detect fault + mean time to fix fault | ||
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| + | |||
| + | ==HW failure== | ||
| + | <br> | ||
| + | '''MTBF''' | ||
| + | *Probability of HW faults calculated using MTBF figures | ||
| + | *MTBF ≠ System Availability | ||
| + | <br> | ||
| + | '''MTBF calculations''' | ||
| + | *Emperical method | ||
| + | *MIL-HDBK-217 | ||
| + | *Telcordia | ||
| + | <br> | ||
| + | '''Emperical methods''' | ||
| + | *Based on statistics from the field | ||
| + | <br> | ||
| + | '''MIL-HDBK-217 and Telcordia''' | ||
| + | *All component entered in database with set environmental condition | ||
| + | *Provides usually lower MTBF figure than emperical methods | ||
| + | **Does include real usage conditions | ||
| + | **Use worst case environmental conditions | ||
| + | <br> | ||
| + | '''*More components gives higher MTBF''' | ||
| + | <br> | ||
| + | '''*MTBF and single points of failure''' | ||
| + | <br> | ||
| + | ==Other failures== | ||
| + | '''Software fault''' | ||
| + | *Automatic watch dog functions | ||
| + | *Automatic recovery functions | ||
| + | *Maturity of system | ||
| + | *Structured software design and test | ||
| + | <br> | ||
| + | '''Vandalism and Extreme conditions''' | ||
| + | *Robustness to vandalism and extreme conditions | ||
| + | *IP and IK class | ||
| + | *IP security functions to hinder denial of service attacks (DOS) | ||
| + | <br> | ||
| + | '''Power outage''' | ||
| + | *UPS and redundant power supplier | ||
[[Category:AlphaCom E System]] | [[Category:AlphaCom E System]] | ||
Revision as of 14:09, 4 August 2009
System availability
System Availability
- The percentage of time that the system can perform its intended function
System Availability = 1 – System Downtime
Downtime per year
| Availability | Nines | Downtime | |
| 90% | 1 | 36.5 | days/year |
| 99% | 2 | 3.65 | DAys/year |
| 99.9% | 3 | 8.78 | hours/year |
| 99.99% | 4 | 52 | minutes/year |
| 99.999% | 5 | 5 | minutes/year |
System Downtime
Many events causes system downtime:
- HW fault
- Software fault
- Vandalism
- Extreme conditions (fire, flooding etc)
- Power outage
- IP network failure
- Planned system maintenance
System Downtime = ∑ P * S * MTTR
| P | = Probability of event taken place |
| S | = Severity of event |
| = Percentage of service affected by fault | |
| MTTR | = Mean Time To Repair |
| = mean time to detect fault + mean time to fix fault |
HW failure
MTBF
- Probability of HW faults calculated using MTBF figures
- MTBF ≠ System Availability
MTBF calculations
- Emperical method
- MIL-HDBK-217
- Telcordia
Emperical methods
- Based on statistics from the field
MIL-HDBK-217 and Telcordia
- All component entered in database with set environmental condition
- Provides usually lower MTBF figure than emperical methods
- Does include real usage conditions
- Use worst case environmental conditions
*More components gives higher MTBF
*MTBF and single points of failure
Other failures
Software fault
- Automatic watch dog functions
- Automatic recovery functions
- Maturity of system
- Structured software design and test
Vandalism and Extreme conditions
- Robustness to vandalism and extreme conditions
- IP and IK class
- IP security functions to hinder denial of service attacks (DOS)
Power outage
- UPS and redundant power supplier