IEEE TRANSACTIONS ON RELIABILITY, VOL. 43, NO. 2, 1994 JUNE
-
327
An Algorithm for Computing the Reliability
of Weighted-k-out-of-n Systems
Jer-Shyan Wu
Rong-Jaye Chen, Member
IEEE
National Chiao Tung University, Hsinchu National Chiao Tung University, Hsinchu
Key Words
-
Weighted-k-out-of-n system, System reliabili- ty, Algorithm.Reader Aids -
General purpose: Report a new algorithm
Special math needed for explanations: Probability theory Special math needed to use results: Same
Result useful to: Reliability analysts & theoreticians
Summary & Conclusions
-
This paper constructs a new k- out-of-n model, viz, a weighted-k-out-of-n system, and proposes an O(n-k) algorithm for computing its reliability.1. INTRODUCTION
The k-out-of-n systems were extensively studied in [ 1-71. This paper proposes a new, more general model: a weighted- k-out-of-n:G(F) system which has n components, each with its own positive integer weight (total system weight = w ) , such that the system is good (failed) iff the total weight of good (fail- ed) components is at least k. The reliability of the weighted-k- out-of-n:G system is the complement of the unreliability of a weighted- (w -k+ 1)-out-of-n:F system. Without loss of generality, we discuss the weighted-k-out-of-n:G system only. The k-out-of-n:G system is a special case of the weighted-k- out-of-n:G system wherein the weight of each component is 1. An efficient algorithm is given to evaluate the reliability of the weighted-k-out-of-n:G system. The time complexity of this algorithm is 0 (n e k).
2. MODEL
Assumptions
1. Each component and the system is either good or failed. 2. All n component states are mutually s-independent. 3. Each component has its own positive integer weight. 4. The system is good iff the total weight of good com- ponents is at least k.
Notation n k
number of components in a system
minimum total weight of all good components which
makes the system good
wi weight of component i
pi,qi Pr{component i is [good, failed]}; p i + q i = 1 R ( i J ) reliability of the weighted-j-out-of-i:G system. Other, standard notation is given in “Information for Readers
& Authors” at the rear of each issue.
3. ALGORITHM
To derive R(n,k), we need to construct the table with
R ( i j ) , for i = O , l ,
...,
n, a n d j = 0,1,2,...,
k. Initially,(1)
R ( i , O ) = 1, for i = O , l ,
...,
n;R ( 0 j ) = 0, f o r j = 1 , 2
,...,
k. (2) Considering R (iJ) for i = 0, 1,..
.
,n, and j<
0, it is obvious that R ( i j ) = 1. So, we propose a recursive formula to generate each R(iJ), for i = l ,...,
n, a n d j = 1 , 2 ,..., k:+
q i . R ( i - l , j ) , ifj-wi L 0;+
q i . R ( i - l j ) , otherwise.(3) R ( i j ) =
Now, the final algorithm for R(n,k) is:
1. By (1) & (2), construct column #1 and row #1 in the 2. By (3), construct row #2, row #3,
... , row # ( n + l )
4Because the size of the table is ( n
+
1 ) ( k+
1 ) , the time com- plexity and space complexity are 0 (n . k ) .table.
in that order; the R(n,k) is eventually derived.
4. EXAMPLE
Consider a weighted-5-out-of-3:G system with weights: 2, 6, 4.
By (l), get column #1 wherein,
R(0,O) = R(1,O) = R(2,O) = R(3,O) = 1;
and by (2), get row #1 wherein,
R ( 0 , l ) = R(0,2) = R ( 0 , 3 ) = R(0,4) = R ( 0 , 5 ) = 0.
Therefore, by (3), derive rows #2, #3, #4 in table 1 as follows: 0018-9529/94/$4.00 01994 IEEE
328
TABLE 1
Weighted6-out-of-3:G system
[Columns 1&2 are the same; columns 3&4 are the same]
i\j 0 1 & 2 3 & 4 5
0 1 0 0 0
1 1 PI 0 0
2 1 P2+92P1 P2 P2
3 1 i-A P3 + 43P2 i-E
REFERENCES
R.E. Barlow, K.D. Heidtmann, “Computing k-out-of-n system reliabili- ty”, IEEE Trans. Reliability, vol R-33, 1984 Oct, pp 322-323. S.P. Jain, K. Gopal, “Recursive algorithm for reliability evaluation of k-out-of-n:G system”, ZEEE Trans. Reliability, vol R-34, 1985 Jun, pp P.W. McGrady, “The availability of a k-out-of-n:G network”, ZEEE Trans. Reliability, vol R-34, 1985 Dec, pp 451-452.
H. Pham, S.J. Upadhyaya, “The efficiency of computing the reliability of k-out-of-n systems”, ZEEE Trans. Reliability, vol 37, 1988 Dec, pp
S . Rai, A.K. Sarje, E.V. Prasad, A. Kumar, “Two recursive algorithm
for computing the reliability of k-out-of-n systems”, ZEEE Trans. Reliabili- ty, vol R-36, 1987 Jun, pp 261-265.
T. Risse, “On the evaluation of the reliability of k-out-of-n systems”, IEEE Trans. Reliability, vol R-36, 1987 Oct, pp 433-435.
A.K. Sarje, E.V. Prasad, “An efficient non-recursive algorithm for com- puting the reliability of k-out-of-n systems”, ZEEE Tram. Reliability, vol 38, 1989 Jun, pp 234-235.
1 4 4 146.
521-523.
AUTHORS
Jer-Shyan Wu; Dept. of Computer Science and Information Engineering; Na- tional Chiao Tung University; Hsincbu 30050 TAIWAN - R.O. CHINA. Internet (e-mail): jswu@algol .csie.nctu.edu . tw
Jer-Shyan Wu was born in Taipei, Taiwan in 1967. He received his BS (1989) in Computer Science from National Taiwan University, and MS (1991) in Computer Science from National Chiao Tung University. His research in- terests include reliability analysis, queueing theory, parallel computing, inter- connected network, tower of Hanoi problem and graph algorithms. Dr. Rong-Jaye Chen, Professor; Dept. of Computer Science and Infomation Engineering; National Chiao Tung University; Hsinchu 30050 TAIWAN - R.O. CHINA.
Rong-Jaye Chen (M’90) was born in Taiwan in 1952. He received his BS (1977) in Mathematics from National Tsing-Hua University, Taiwan and PhD (1987) in Computer Science from University of Wisconsin-Madison. Dr. Chen is Associate Professor in the Department of Computer Science and In- formation Engineering in National Chiao Tung University. He is a member of IEEE. His research interests include reliability theory, algorithms, math- ematical programming, computer networking.
Manuscript TR92-172 received 1992 September 17; revised 1993 April 5.
