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Reliability models are used to predict system failure rate based on component information.
Two most popular in North America models include MIL-HDBK-217 and Telcordia (formerly Bellcore) SR332, Issue 1.
MIL-HDBK-217 was the original standard for reliability. It was designed to provide reliability models for both commercial and defense systems.
The Telcordia reliability model was developed by AT&T based on MIL-HDBK-217 method, enhanced to take into account burn-in, field, and laboratory testing. It was geared towards commercial systems.
Flash SSD vendors quote MTBF computed using either Telcordia or MIL-HDBK-217 method.
It has to be noted that for Flash SSD the result may vary dramatically depending on environmental and operational assumptions. Changing operating temperature by 5°C may change the result by 20-25%. Changing environment from Ground Fixed Controlled to Ground Fixed Uncontrolled may change the results by 100%. The prediction outcome, as with any engineering model, is an approximation of reality. Arguably the best approach is to use the MTBF number as a relative comparison between various products ensuring that the assumptions made to calculate the MTBF number are the same.
For Flash SSD, it is also important to consider drive capacity as the number of components used on the board may vary dramatically. Typically, highly integrated designs such as Memkor Flash SSD's, that use ASIC solutions as opposed to those that use FPGA's/processors, will yield higher MTBF.
Telcordia method was developed with focus on telecommunication equipment and is widely accepted for MTBF calculation of commercial equipment.
MIL-HDBK-217 was developed as a more general, market agnostic method and is used for both commercial and military equipment.
The MIL-HDBK-217 method requires more detailed part definition. It typically yield more conservative MTBF assessment.
There are many publications available readily on the Internet that provide detailed comparison between the both approaches.
Ground, Fixed, Controlled GB, Pe = 1.0
Nearly zero environmental stress with optimum engineering operation and maintenance. Typical applications are central office, environmentally controlled vaults, environmentally controlled remote shelters, and environmentally controlled customer premise areas.
Ground, Fixed, Controlled GF, Pe = 2.0
Some environmental stress with limited maintenance. Typical applications are manholes, poles, remote terminals, customer premise areas subject to shock, vibration, temperature, or atmospheric variations.
Ground, Mobile (both vehicular mounted and portable) GM, Pe = 6.0
Conditions more severe than Gf , mostly for shock and vibration. More maintenance limited and susceptible to operator abuse. Typical applications are mobile telephone, portable operating equipment, and test equipment.
Airborne, Commercial AC, Pe = 10
Conditions more severe than for Gf, mostly for pressure, temperature, shock, and vibration. In addition, the application is more maintenance limited than for Gf . Typical applications are in the passenger compartment of commercial aircraft.
Space-based, Commercial SC, Pe = 15
Low earth orbit. Conditions as for AC, but with no maintenance. Typical applications are commercial communication satellites.
Ground, Benign, GB Non-mobile, temperature and humidity controlled environments readily accessible to maintenance; includes laboratory instruments and test equipment, medical electronic equipment, business and scientific computer complexes, and missiles and support equipment in ground silos.
Ground, Fixed, GF Moderately controlled environments such as installation in permanent racks with adequate cooling air and possible installation in unheated building; includes permanent installation of air traffic control radar and communications facilities.
Ground, Mobile, GM Equipment installed on wheeled or tracked vehicles and equipment manually transported; includes tactical missile ground support equipment, mobile communication equipment, tactical fire direction systems, handheld communications equipment, laser designations and range finders.
Naval, Sheltered, NS
Includes sheltered or below deck conditions on surface ships and equipment installed in submarines.
Naval, Unsheltered NU
Unprotected surface ship borne equipment exposed to weather conditions and equipment immersed in salt water. Includes sonar equipment and equipment installed on hydrofoil vessels.
Airborne, Inhabited, Cargo AIC
Typical conditions in cargo compartments, which can be occupied by an aircrew. Environment extremes of pressure, temperature, shock and vibration are minimal. Examples include long mission aircraft such as the C130, C5, B52 and C141. This category also applies to inhabited areas in lower performance smaller aircraft as the T38.
Airborne, Inhabited, Fighter AIF
Same as AIC but installed on high performance aircraft such as fighters and interceptors. Examples include the F15, F16, F111, F/A18 and A10 aircraft.
Airborne, Uninhabited, Cargo AUC
Environmentally uncontrolled areas, which cannot be inhabited by an aircraft, crew during flight. Environmental extremes of pressure, temperature and shock may be severe. Examples include uninhabited areas pf long mission aircraft such as the C130, C5, B52 and C141. This category also applies to uninhabited areas pf lower performance smaller aircraft such as the T38.
Airborne, Uninhabited, Fighter AUF
Same as AUC but installed on high performance aircraft such as fighters and interceptors. Examples include the F15, F16, F111, and A10 aircraft.
Airborne, Rotary Winged, ARW
Equipment installed on helicopters. Applies to both internally and externally mounted equipment such as laser designators, fire control systems, and communications equipment.
Space, Flight, SF
Earth orbital. Approaches benign ground conditions. Vehicles neither under powered flight nor in atmospheric reentry; include satellites and shuttles.
Missile, Flight, MF
Conditions related to powered flight or air breathing missiles, cruise missiles, and missiles in unpowered free flight.
Missile, Launch, ML
Severe conditions related to missile launch (air, ground, and sea), space vehicle boost into orbit, and vehicle re-entry and landing by parachute. Also applies to solid rocket motor propulsion powered flight, and torpedo and missile launch from submarines.
Cannon, Launch, CL
Extremely severe conditions related to canon launching of 155mm and 5 inch guided projectiles. Conditions apply to the projectile from launch to target impact.