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A.4.1.9 Specifications and Recommended Practices Supporting Maintainability Belicore and EKE have developed specifications which support ma~ntam ability. As wad built- in test and testability, documentation of Me communications device is important to achieving a low mean time to repair (AIDE. Table A.4.7-! summarizes some applicable industry specifications. A.4.2 Reliability, Availability, and Maintainability Planning and Estimating Common reliability, availability, and maintainability terms, def~nidons, and calculations are present in Table A.4.2~. These definitions are adaptions from references I-10. In particular, Reference I., Chapter ~ I, is followed because it has smaighffonvard concepts very similar to Lose discussed in References 7 and 8. It should be noted that reliability, availability, failures, and maintainability modeling are statistically based and can be complex. Thus, modeling is most accurate when verified wad field data of subsolar systems and then applied in future modeling activities. Even Be highly regarded M~-HKBK 217F is crazed because, due to continuing rapid technical advances, marry of the parameters in the document are outdated. An important element of system design is He establishment of system availability goals and He allocation (usually based on modeling) to venous system elements including subsystems, equipment, components (where reqliiIed), mediums, etc. Availability of a system is a function of the Mean-T~me-Between-Failure ~3E:) and He Mean-T~me-To-Repair ~) of He elements of the system. The military, telecommunication, and electronic ~ndustnes have developed methodologies for calculating integrated system availability that uses MTBF, MTTH, and related parameters for system elements. As failure and repair involve uncertainty, many advanced sophisticated statistical techniques are available to model the many variations including component/equipment failure charactenstics, equipment and system architectures, alternative redundancy schemes, etc. We win present a generally accepted basic methodology that should provide adequate ITS system planning estimates. Table A.4.2~2 presents He number of hours in "n" years. L;wa~pt\ N=~3-51 · ~2F~R - n A~14
Table A.4.2-2 Hours in "n" years n years hours (n x 365 x 24) 8,760 17.520 1 2 3 4 26280 35,040 43,800 52,560 5 6 7 8 619320 70,080 78,840 87.600 9 12 10 11 96,360 105~120 A typical telecommunication industry goal for availability is 99.98% (or better), which equates to an uptime of 8758 hours per year (8760 x .999X) and a downtime (not available) of 2 hours per year. What constitutes uptime (or available time) can be defined according to operational requirements. For example, uptune can be defined as: I) AN elements of the system operational; 2) All critical elements up and an acceptable % of non-cntical elements up; and 3) A % or defined combination of non-cntical items up, etc. Up conditions for availability purposes could include pardal functionality. An rrS-related example would be Mat We backbone communication subsystem be critical and local loop communication links and signal controllers be non-cntical win independent subsystem goals as opposed to Integrate system goals. Table A.4.2-1 discusses serial and paraBe! system elements. Reference I, chapter ~ I, presents simplified approximating equations for estimating the composite MTBF, failure rate (F), Ml~, and repair rate (R). These equations are presented in Table A.4.2-3. The series element equations can be referred to as summations for approximations for more than two element; however, the paraRel element equations must be applied repetitively to two elements at a time. Fortunately, most TIs system redundancy typically involves two parapet elements. The parade] approximations calculate the estimates for overlapping (in dme) two element failures which is L;`Na~Wba~p~\ NCHRP3-51 · Phase2F'nalRepon A4~15
Table A.4.2-1 Common Reliability Terms, Parameters. and Calculations , Term/Parameter/Calculation Definiti _ - _ . Reliability The probability that a element for system) will successfully perform a required function, under required conditions, usually for a stated period of time. . _ Maintainability use of failure rate data to predict system MTTR and to logistic requirements to maintain system. Availability (AV) The average percent of time that a component, (uptime) equipment, subsystem, system, etc is AV = MTBF/(MTBF+MTTR) or satisfactorily performing its required function. = Rt(F+R) Unavailability he percent of time that a compliment equipment (down time, or DT) subsystem, system, etc is not satisfactorily DT = 1 - AV performing its required function failure rate, F The average number of failures per hour of a component, equipment, subsystem, system, etc. . Mean-Time-Between-Failures (MTBF) The average time between failures, usually MTBF = 1/failure rate = 1/ F for a expressed in hours of a component, equipment, a component, equipment, subsystem, subsystem, system, etc. system, etc. Repair rate, R The average number of repairs per hour of a component, equipment, subsystem, system, etc. . Mean-Tim+To-Repair (MTTR) The average time to return to service, usually (also down time of element) expressed in hours, for a component, MTTR = 1/R for a equipment, subsystem, system, etc. When a component, equipment, subsystem, replaced by a spare, the failed element may still system, etc. need repair but usually does not affect availability. Element VlllR is evaluated as part of spares inventory replacement and not system ~MTTR. Serial elements A configuration of component, equipment, subsystem, system, etc such that ~ one element Parallel elements subsystem, system, etc such that all parallel elements must fail for the system to be considered failed. This is essentially redundancy. Table A.4.2~3 Series Elements ~ F (failures/hour) Fs = F1 + F2 Fp = F. x F2 x (I /ITTR1 +MTTR2) I\/ITBF (hours/failure) MTBFs= 1/FS M~Fp = 1/Fp R (repairs/hr) Rs = (F1 x R1 + F2X R2) /Fs R. = R. · Rat . MTTR (hours/repair) MTTRS = 1/ Rs MTTRp = 1/Rp Probability of Unavailable Fs x MTTRS Fp x MTTRp L;`NcH~wha~n NCHRP3-51 · Phase2~malReport Al16
