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Unformatted text preview: Probability and Statistics with Reliability, Queuing and Computer Science Applications Second edition by K.S. Trivedi Publisher-John Wiley & Sons Chapter 8 (Part 4) :Continuous Time Markov Chain Performability Modeling Dept. of Electrical & Computer engineering Duke University Email:kst@ee.duke.edu URL: www.ee.duke.edu/~kst Copyright © 2003 by K.S. Trivedi 1 Outline Why performability modeling? Erlang loss performability model Modeling cellular systems with failure Multiprocessor Performability Conclusion Copyright © 2003 by K.S. Trivedi 2 Outline Why performability modeling? Erlang loss performability model Modeling cellular systems with failure Multiprocessor Performability Conclusion Copyright © 2003 by K.S. Trivedi 3 Wireless “ilities” besides performance for a specified operational time Performability measures of the network’s ability to perform designated functions Reliability at any given instant Availability performance under failures Survivability R.A.S.-ability concerns grow. High-R.A.S. not only a selling point for equipment vendors and service providers. But, regulatory outage report required by FCC for public switched telephone networks (PSTN) may soon apply to wireless. Copyright © 2003 by K.S. Trivedi 4 Causes of Service Degradation Limited Resources Equipment failures Software failures Planned outages (e.g. upgrade) Human-errors in operation Resource full Resource loss Long waiting-time Time-out Service blocking Service Interruption Loss of information Copyright © 2003 by K.S. Trivedi 5 The Need of Performability Modeling New technologies, services & standards need new modeling methodologies Pure performance modeling: too optimistic! Outage-and-recovery behavior not considered Pure availability modeling: too conservative! Different levels of performance not considered Copyright © 2003 by K.S. Trivedi 6 Measures To Be Evaluated Dependability Reliability: R(t), System MTTF Availability: Steady-state, Transient Downtime Security, safety “Does it work, and for how long?'' Performance Throughput, Blocking Probability, Response Time “Given that it works, how well does it work?'' Copyright © 2003 by K.S. Trivedi 7 Measures To Be Evaluated (Contd.) Composite Performance and Dependability “How much work will be done(lost) in a given interval including the effects of failure/repair/contention?'' Need Techniques and Tools That Can Evaluate Performance, Dependability and Their Combinations Copyright © 2003 by K.S. Trivedi 8 Outline Why performability modeling? Erlang loss performability model Modeling cellular systems with failure Hierarchical model for APS in TDMA Multiprocessor Performability Conclusion Copyright © 2003 by K.S. Trivedi 9 Erlang Loss Pure Performance Model Telephone switching system : n channels Call arrival process is assumed to be Poissonian with rate λ Call holding times exponentially distributed with rate µ A new call is accepted if at least one idle channel is available, otherwise it is blocked. Copyright © 2003 by K.S. Trivedi 10 Erlang Loss CTMC Model State index is the number of channels in use Let π j be the steady state probability for the Continuous Time Markov Chain Blocking Probability: Pb = π n n Expected number of calls in system: E[ N ] = ∑ jπ j j =0 Desired measures of the form: n E[M ] = ∑rjπ j j =0 Copyright © 2003 by K.S. Trivedi 11 Sharpe Textual Input File : • • • • * * * * Code for the Pure Performance Model note the use of loop in the specification of CTMC This allows size of the CTMC to be variable use repeated pattern of transitions for conciseness bind lambda 49 mu 0.35 end markov perf(n) loop i,0,n-1 $(i) $(i+1) lambda $(i+1) $(i) (i+1)*mu end Copyright © 2003 by K.S. Trivedi end 12 Sharpe code : * Pb : Steady-state call blocking probability func Pb(n) prob(perf,$(n);n) * The value of n (number of channels) varied from 4 to 100 loop nb,4,100,10 expr Pb(nb) end end Copyright © 2003 by K.S. Trivedi 13 Blocking probability vs. Number of channels Number of channels Copyright © 2003 by K.S. Trivedi 14 Availability model Availability Analysis: (Telephone Switching system with n channels ) Wish to compute Steady-state system Unavailability : U Steady-state system Availability : A Instantaneous system Availability : A(t) Downtime : downtime (in minutes per year) The times to channel failure and repair are exponentially distributed with mean 1/ γ and 1/τ , respectively. γ=1/MTTF: Failure rate of channel τ=1/MTTR: Repair rate of channel Copyright © 2003 by K.S. Trivedi 15 Erlang Loss Pure Availability Model Let π j be the steady state probability for the CTMC Steady state unavailability: Expected number of non-failed channels: Desired measures of the form: A = π0 n E[ N ] = ∑ jπ j j =0 n E[M ] = ∑rjπ j j =0 Copyright © 2003 by K.S. Trivedi 16 Sharpe code : bind MTTF 1000 MTTR 24 end markov avail(n) loop i,n,1,-1 $(i) $(i-1) i/MTTF $(i-1) $(i) 1/MTTR end end * Initial probability, assume that n channels are up initially $(n) 1 end Copyright © 2003 by K.S. Trivedi 17 Sharpe code : func func func func U(n) prob(avail,0;n) A(n) 1-U(n) At(t,n) 1-tvalue(t;avail,0;n) downtime(n) 60*8760*U(n) loop nb,4,20,1 expr U(nb),A(nb),downtime(nb) end format 8 bind N 4 loop t,1,291,10 expr At(t,N) end end Copyright © 2003 by K.S. Trivedi 18 Downtime vs. Number of channels Number of channels Copyright © 2003 by K.S. Trivedi 19 Instantaneous Availability vs. Number of channels time Copyright © 2003 by K.S. Trivedi 20 Erlang loss composite model A telephone switching system : n channels The call arrival process is assumed to be Poissonian with rate λ , the call holding times are exponentially distributed with rate µ The times to channel failure and repair are exponentially distributed with mean 1 / γ and 1 / τ, respectively The composite model is then a homogeneous CTMC Copyright © 2003 by K.S. Trivedi 21 Erlang loss composite model The state (i, j ) denotes i non-failed channels and j ongoing calls in the system CTMC with (n+1)(n+2)/2 states Total call blocking probability: n −1 Tb = A + π n ,n + ∑ π i ,i i =1 Example of expected reward rate in steady state State diagram for the Erlang loss composite model Copyright © 2003 by K.S. Trivedi 22 Total call blocking probability T b = A + π n ,n + Blocked due to unavailability Blocked due to buffer full Copyright © 2003 by K.S. Trivedi n −1 ∑π i =1 i ,i Blocked due to buffer full in degraded levels 23 Problems in composite performability model Largeness: Number of states in the Markov model is rather large Tolerance: Automatically generate Markov reward model starting with an SRN (stochastic reward net) Avoidance: Use a two-level hierarchical model Stiffness: Transition rates in the Markov model range over many orders of magnitude Tolerance: Use stiffly stable methods of num. Solution Avoidance: Use a two-level hierarchical model Potential solution to both problems is a hierarchical performability model Copyright © 2003 by K.S. Trivedi 24 Erlang loss hierarchical model The hierarchical performability model provides an approximation of the exact composite model Each state of pure availability model keeps track of the number of non-failed channels. Each state of the performance model represents the number of talking channels in the system Upper availability model Lower performance model Call blocking probability is computed from pure performance model and supplied as reward rates to the availability model states Copyright © 2003 by K.S. Trivedi 25 Erlang loss model (contd.) The steady-state system unavailability A (u) The blocking probability with i nonBlocked due to failed channels =π (l ) i Total blocking probability (u) A + Pb (n)π (u) n n−1 + ∑ Pb (i)π i( u ) buffer full Blocked due to buffer full in degraded levels i =1 Blocked due to unavailability Copyright © 2003 by K.S. Trivedi 26 Erlang loss model (cont’d) Compare the exact total blocking probability with approximate result Advantages of the hierarchical Avoid largeness Avoid stiffness More intuitive No significant loss in accuracy Total blocking probability in the Erlang loss model Copyright © 2003 by K.S. Trivedi 27 Total blocking probability Has three summands Loss due to unavailability (pure availability model will capture this) Loss when all channels are busy (pure performance model will capture this) Loss with some channels busy and others down (degraded performance levels) Performability models captures all three types of losses Higher level, lower level model or both can be based on analytic/simulation/measurements Copyright © 2003 by K.S. Trivedi 28 Performability Evaluation (1) Two steps The construction of a suitable model The solution of the model Two approaches are used Combine performance and availability into a single monolithic model Hierarchical model where lower level performance model supplies reward rates to the upper level availability model Copyright © 2003 by K.S. Trivedi 29 Performability Evaluation (2) Measures of performability [Haver01] Expected steady-state reward rate (we have only used this measure in the section) Expected reward rate at given time Expected accumulated reward in a given interval Distribution of accumulate reward Expected task completion time Distribution of task completion time Copyright © 2003 by K.S. Trivedi 30 Outline Why performability modeling? Markov reward models Erlang loss performability model Modeling cellular systems with failure Multiprocessor Performability Conclusion Copyright © 2003 by K.S. Trivedi 31 Handoffs in wireless cellular networks Handoff: When an MS moves across a cell boundary, the channel in the old BS is released and an idle channel is required in the new BS Hard handoff: the old radio link is broken before the new radio link is established (AMPS, GSM, DECT, DAMPS, and PHS) Copyright © 2003 by K.S. Trivedi 32 Wireless Cellular System Traffic in a cell New Calls Common Channel Pool Call completion Handoff Calls Handoff out From neighboring cells To neighboring cells A Cell Copyright © 2003 by K.S. Trivedi 33 Performance Measures: Loss formulas or probabilities When a new call (NC) is attempted in an cell covered by a base station (BS), the NC is connected if an idle channel is available in the cell. Otherwise, the call is blocked If an idle channel exists in the target cell, the handoff call (HC) continues nearly transparently to the user. Otherwise, the HC is dropped Loss Formulas New call blocking probability, Pb : Percentage of new calls rejected Handoff call dropping probability, Pd : Percentage of calls forcefully terminated while crossing cells Copyright © 2003 by K.S. Trivedi 34 Guard Channel Scheme Handoff dropping less desirable than new call blocking! Handoff call has Higher Priority: Guard Channel Scheme GCS: g channels are reserved for handoff calls. g trade-off between Pb Copyright © 2003 by K.S. Trivedi & Pd 35 Assumptions Poisson arrival stream of new calls λ1 Poisson stream of handoff arrivals λ2 Limited number of channels: n Exponentially distributed completion time of ongoing calls µ1 Exponentially distributed cell departure time of ongoing calls µ2 Copyright © 2003 by K.S. Trivedi 36 Modeling for cellular network with hard handoff G. Haring, R. Marie, R. Puigjaner and K. S. Trivedi, Loss formulae and their optimization for cellular networks, IEEE Trans. on Vehicular Technology, 50(3), 664-673, May 2001 Copyright © 2003 by K.S. Trivedi 37 Markov chain model of wireless hard handoff n(t): the number of busy channels at time t Steady-state probability Copyright © 2003 by K.S. Trivedi 38 Loss formulas for wireless network with hard handoff Dropping probability for handoff: Blocking probability of new calls: Notation: if we set g=0, the above expressions reduces to the classical Erlang-B loss formula Copyright © 2003 by K.S. Trivedi 39 Computational aspects Overflow and underflow might occur if n is large Numerically stable methods of computation are required Recursive computation of dropping probability for wireless networks Recursive computation of the blocking probability For loss formula calculator, see webpage: http://www.ee.duke.edu/~kst/wireless.html Copyright © 2003 by K.S. Trivedi 40 Optimization problems Optimal Number of Guard Channels O1: Given n, A, and A1, Pd 0 determine the optimal integer value of g so as to minimize Pb ( g ) such that Pd ( g ) ≤ Pd 0 Optimal Number of Channels O2: Given A , A1, Pb 0 , Pd 0 determine the optimal integer values of n and g so as to Pb ( n , g ) ≤ Pb 0 minimize n such that Pd ( n , g ) ≤ Pd 0. Copyright © 2003 by K.S. Trivedi 41 Fixed-Point Iteration Handoff arrival rate will be a function of new call arrival rate and call completion rates Handoff arrival rate will have to be computed from handoff-out throughput Assuming that all cells are statistically identical, handoff out throughput from a cell equals the handoff arrival rate o the cell Copyright © 2003 by K.S. Trivedi 42 Loss Formulas—Fixed Point Iteration A fixed point iteration scheme is applied to determine the Handoff Call arrival rates: The arrival rate of HCs=the actual throughput of handed out calls from the cell λ2 We have theoretically proven: the given fixed point iteration is exists and is unique A solution by successive substitution converges fairly rapidly in practice A good initial value is suggested in the paper Copyright © 2003 by K.S. Trivedi 43 Modeling cellular systems with failure and repair (1) The object under study is a typical cellular wireless system The service area is divided into multiple cells There are n channels in the channel pool of a BS Hard handoff. g channels are reserved exclusively for handoff calls Let λ 1 be the rate of Poisson arrival stream of new calls and λ 2 be the rate of Poisson stream of handoff arrivals Let µ1 be the rate that an ongoing call completes service and µ 2 be the rate that the mobile engaged in the call departs the cell The times to channel failure and repair are exponentially distributed with mean 1 / γ and 1 / τ , respectively. Copyright © 2003 by K.S. Trivedi 44 Modeling cellular systems with failure and repair (2) Upper availability model (same as that in Erlang loss model) The steady state unavailability A (u) Copyright © 2003 by K.S. Trivedi 45 Modeling cellular systems with failure and repair (3) Lower performance model Copyright © 2003 by K.S. Trivedi 46 Modeling cellular systems with failure and repair (4) Solve the Markov Chain, we get pure performance indices The dropping probability Pd(l ) (i) (l ) The blocking probability P (i) b Copyright © 2003 by K.S. Trivedi 47 Modeling cellular systems with failure and repair (5) Loss probability (now call blocking or handoff call dropping) is computed from pure performance model and supplied as reward rates to the availability model states Total dropping probability Buffer full n−1 Td = A +π P (n) + ∑(πi(u) P(l ) (i)) d (u) (l ) n d i=1 Unavailability Total blocking probability g Degraded buffer full n−1 Tb = A + ∑π +π P (n) + ∑(πi(u) P(l ) (i)) b i=1 ( u) i ( u) ( l ) n b i=g+1 Copyright © 2003 by K.S. Trivedi 48 Assumptions: λ1 : The arrival rate of new calls λ2 : The arrival rate of hand-off calls into the cell µ1 : Service completion rate of on going calls (new or hand-off) µ2 : Service rate of hand-off outgoing calls from the cell n : Total number of channels g : Number of guard channels Copyright © 2003 by K.S. Trivedi 49 Pure Performance (traffic) model: Markov Model: State index indicates the number of channels in use Steady-state call blocking probability (Pb) Steady-state call dropping probability (Pd) λ1 + λ2 0 1 ( µ1 + µ 2 ) ... n-g-1 λ1 + λ2 (C − g )( µ1 + µ 2 ) Pd = π C Pb = π C − g + ... + π C n-g λ2 n-g+1 ... n-1 (C + 1 − g )( µ1 + µ 2 ) λ2 n C ( µ1 + µ 2 ) Call blocking probability Copyright © 2003 by K.S. Trivedi 50 Sharpe code for Markov Model * Code for the Pure Performance Model bind lambda1 49 lambda2 21 * mu + mu2 = 1 mu1 mu2 g3 end 0.35 0.65 Copyright 2003 by K.S. Trivedi 51 Sharpe code (contd.) markov perf(n) loop i,0,n-g-1 $(i) $(i+1) lambda1+lambda2 $(i+1) $(i) (i+1)*(mu1+mu2) end loop i,n-g,n-1 $(i) $(i+1) lambda2 $(i+1) $(i) (i+1)*(mu1+mu2) end end end Copyright © 2003 by K.S. Trivedi 52 Sharpe code (contd.) * Pd : Steady-state call dropping probability func Pd(n) prob(perf,$(n);n) * Pb : Steady-state call blocking probability func Pb(n) sum(i,n-g,n,prob(perf,$(i);n)) * The value of n (number of channels) taken from g+1 (4) to 100 loop nb,g+1,100,10 expr Pd(nb), Pb(nb) end end Copyright © 2003 by K.S. Trivedi 53 Blocking probability vs. Number of channels Number of channels Copyright © 2003 by K.S. Trivedi 54 Dropping probability vs. Number of channels Number of channels Copyright © 2003 by K.S. Trivedi 55 Hierarchical Approximation (Performability model) Top level is the Availability Model Bottom level is a sequence of Performance Models Copyright © 2003 by K.S. Trivedi 56 λ1 + λ2 0 1 ... n-g-1 ( µ1 + µ 2 ) C / MTTF n λ1 + λ2 n-g (C − g )( µ1 + µ 2 ) n-g+1 ... n-1 (C + 1 − g )( µ1 + µ 2 ) (C − 1) / MTTF n-1 1 / MTTR λ2 n-2 λ2 n C ( µ1 + µ 2 ) 1 / MTTF ... 1 / MTTR Copyright © 2003 by K.S. Trivedi 1 0 1 / MTTR 57 Sharpe code: total call dropping probability * function to use to define the reward rates for the measure * the total call dropping probability * Reward function used for k>g func RewDbig(k) prob(big,$(k);k) * Reward function used for k<=g func RewDsmall(k) prob(small,$(k);k) * k is the number of non-failed channels Copyright © 2003 by K.S. Trivedi 58 Sharpe code : total call block probability * function to use to define the reward rates for measure * the total call blocking probability * The function RewB1 will be called only when k>g func RewBbig(k) \ sum(j,k-g,k, prob(big,$(j);k)) Name of the state Number of channels Copyright © 2003 by K.S. Trivedi 59 Sharpe code : initialize bind lambda1 49 lambda2 21 * mu + mu2 = 1 mu1 0.35 mu2 0.65 g3 MTTF 1000 MTTR 24 end Copyright © 2003 by K.S. Trivedi 60 Sharpe code : model setup * model called when k>g markov big(k) loop i,0,k-g-1 $(i) $(i+1) lambda1+lambda2 $(i+1) $(i) (i+1)*(mu1+mu2) end loop i,k-g,k-1 $(i) $(i+1) lambda2 $(i+1) $(i) (i+1)*(mu1+mu2) end end end Copyright © 2003 by K.S. Trivedi 61 Sharpe code : model setup * model called when k<=g markov small(k) loop i,0,k $(i) $(i+1) lambda2 $(i+1) $(i) (i+1)*(mu1+mu2) end end end Copyright © 2003 by K.S. Trivedi 62 Sharpe code: total call dropping probability markov hier loop i,n,1,-1 $(i) $(i-1) i/MTTF $(i-1) $(i) 1/MTTR end reward loop i,0,g $(i) RewDsmall(i) end loop i,g+1,n $(i) RewDbig(i) end Copyright © 2003 by K.S. Trivedi 63 Sharpe code: total call dropping probability (contd.) * Initial probability $(n) 1 end var Td exrss(hier) loop nb,4,60,10 bind n nb expr Td end Copyright © 2003 by K.S. Trivedi 64 Sharpe code: total call block probability markov hier1() loop i,n,1,-1 $(i) $(i-1) i/MTTF $(i-1) $(i) 1/MTTR end reward loop i,g+1,n $(i) RewBbig(i) end * Since the number of non-failed channels is less or equal g * then all new calls are blocked loop i,0,g $(i) 1 end end Copyright © 2003 by K.S. Trivedi 65 Sharpe code: total call block probability (contd.) * Initial probability $(n) 1 end var Tb exrss(hier1) loop nb1,4,60,10 bind n nb1 expr Tb end Copyright © 2003 by K.S. Trivedi 66 Hierarchical Performability model: Outputs: Total call blocking probability for the approximate model: Tb_a Total call dropping probability for the approximate model: Td_a Copyright © 2003 by K.S. Trivedi 67 Total blocking probability: Exact vs. Approximate model Copyright © 2003 by K.S. Trivedi 68 Total dropping probability: Exact vs. Approximate model Copyright © 2003 by K.S. Trivedi 69 Outline Why performability modeling? Markov reward models Erlang loss performability model Modeling cellular systems with failure Multiprocessor Performability Conclusion Copyright © 2003 by K.S. Trivedi 70 A Performability Example Consider a Multiprocessor System How Many Processors Should It Have? Vary the # procs; each with the same capacity Objectives System Availability System Performance Composite Measures of Performance & Availability Copyright © 2003 by K.S. Trivedi 71 Availability Benefits Of Multiprocessor Higher Availability/ Reliability/ MTTF Lower Blocking Probability (Due to system down) Note: These are potential benefits A simple reliability block diagram or fault tree can be used for computing reliability/availability/MTTF In order to capture realistic behavior, we use CTMC Copyright © 2003 by K.S. Trivedi 72 System Descriptions And Parameters n Processors, at least 1 needed for System to be UP Each Processor Fails at Rate γ Each Processor is Repaired at Rate τ Coverage Probability c Copyright © 2003 by K.S. Trivedi 73 System Descriptions And Parameters (Contd.) Average Reconfiguration Delay After a Covered Failure 1/δ Ave. Reboot Delay After an Uncovered Failure 1/β Model System Availability Using a Markov Chain Copyright © 2003 by K.S. Trivedi 74 Multiprocessor Availability Model ( n − 1)γ c nγ c Dn n τ n γ (1 − c ) Bn δ Dn-1 δ τ n-1 β Bn-1 n-2 ............... β 1 γ τ 0 ( n − 1)γ (1 − c ) Copyright © 2003 by K.S. Trivedi 75 Multiprocessor Availability Model (Contd.) Compute the steady state probability πj for each state j System unavailability = n n j =2 j =2 1 − ∑ j =1π j = π 0 + ∑ π D j + ∑ π B j n Copyright © 2003 by K.S. Trivedi 76 Downtime Calculation vs. no. of Processors with Mean Delay as parameter. Copyright © 2003 by K.S. Trivedi 77 Downtime Calculation vs. no. of Processors with c as parameter. Copyright © 2003 by K.S. Trivedi 78 LESSONS To Realize Availability Benefits of Multiprocessing Coverage Must be Near-Perfect Reconfiguration Delay Must be Very Small OR Most of the Other Processors Must Be Able to Carry Out Useful Work While 1 Fault is Being Handled. Must Consider Different Levels of (Degradable) Performance Copyright © 2003 by K.S. Trivedi 79 Performance Benefits Of Multiprocessing Higher Throughput Lower Blocking Probability Lower Response Time Lower Prob. of Missing Deadlines Note: These are potential benefits Copyright © 2003 by K.S. Trivedi 80 Performance Model Use a Finite Buffer Queuing Model To Determine Prob. that the Task is rejected due to buffer full Task Arrival Rate λ Task Service Rate µ Number in the system b Throughput = Tb(i) with i Processors Buffer Full Prob. = qb(i) with i Processors Copyright © 2003 by K.S. Trivedi 81 Performance model Queuing model M/M/i/b 1 . . . i b Copyright © 2003 by K.S. Trivedi 82 Performance Model (Contd.) Compute the steady state probability πj for each state j Throughput with i Processors # ( serving) µ , # (serving) + # (proc) T (i ) = ∑ r π , r = b j j j 0, otherwise j∈Ω =i Buffer Full Prob. = qb(i) with i Processors 1, # (buffer) = b − i q (i ) = ∑ r π , r = b j j j 0 , otherwise j∈Ω Copyright © 2003 by K.S. Trivedi 83 Combining Performance And Availability Attach a Reward rate ri, to State i of the Failure/Repair Markov Model We have a Markov Reward Model Compute the Expected Reward Rate in the Steady-State: Weighted sum of state probabilities Copyright © 2003 by K.S. Trivedi 84 Combining Performance And Availability (Contd.) Capacity-Oriented Availability is computed By: ri = Number of Up Processors in State i/n Throughput-Oriented Availability is computed By: ri = 0, if state i is a down state = Tb(i)/Tb(n) , otherwise Copyright © 2003 by K.S. Trivedi 85 Capacity and ThroughputOriented Availability Reward assignment: ri = 0, if i is a DOWN state ri = i/n, if i is UP (Capacity) ri = Tb(i)/Tb(n) , if i is UP (Throughput) Obtained measures: n n ∑ rπ + ∑ r i =1 i i i=2 Di n π D + ∑ rB π B + r0π 0 i i =2 i i Copyright © 2003 by K.S. Trivedi 86 Copyright © 2003 by K.S. Trivedi 87 Total Blocking Probability ri =1 (Contd.) if i is a down state ri = qb (i ) if i is an up state n −1 n n TBP = ∑ qb (i )π i +∑ π B + ∑ π D i =1 i =2 Copyright © 2003 by K.S. Trivedi i i =2 i +π0 88 TOTAL BLOCKING PROBABILITY Copyright © 2003 by K.S. Trivedi 89 Copyright © 2003 by K.S. Trivedi 90 Conclusions: Performability Example Optimal number of processors increases With The Task Arrival Rate With Smaller Buffer Space Copyright © 2003 by K.S. Trivedi 91 For A General System Model Obtain Prob. of Rejection due to System being down numerically: SHARPE, SPNP Obtain Prob. of Rejection due to System being full (in each configuration): Analytic Queuing Network Solver:SHARPE Discrete-event Simulation: Bones Stochastic Petri Net Model:SHARPE, SPNP Measurements Compute Total Blocking Probability as before Copyright © 2003 by K.S. Trivedi 92 Outline Why performability modeling? Markov Reward models Erlang loss performability model Modeling cellular systems with failure Hierarchical model for APS in TDMA Multiprocessor Performability SHARPE input files Conclusion Copyright © 2003 by K.S. Trivedi 93 Conclusions Performability: an integrated way to evaluate a real-world system Two approaches Composite models Hierarchical models CTMC and MRM models for performability study of a variety of wireless systems and multiprocessor system A tool for solution to models: SHARPE Copyright © 2003 by K.S. Trivedi 94 References 1. 6. Y. Ma, J. Han and K. S. Trivedi, Composite Performance & Availability Analysis of Wireless Communication Networks, IEEE Trans. on Vehicular Technology, 50(5): 1216-1223, Sept. 2001. G. Haring, R. Marie, R. Puigjaner and K. S. Trivedi, Loss formulae and their optimization for cellular networks, IEEE Trans. on Vehicular Technology, 50(3), 664-673, May 2001. K. S. Trivedi, Probability and Statistics with Reliability, Queuing, and Computer Science Applications, 2nd Edition, John Wiley, 2001 (especially Section 8.4.3). Y. Cao, H.-R. Sun and K. S. Trivedi, Performability Analysis of TDMA Cellular Systems, P&QNet2000, Japan, Nov., 2000. H.-R. Sun, Y. Cao, K. S. Trivedi and J. J. Han, Availability and performance evaluation for automatic protection switching in TDMA wireless system, PRDC’99, pp15--22, Dec., 1999 http://www.ee.duke.edu/~kst/wireless.html 7. B. Haverkort, R. Marie, G. Rubino, K. Trivedi, Performability Modeling, John Wiley, 2001 8. D. Selvamuthu, D. Logothetis, and K. S. Trivedi, Performance analysis of cellular networks with generally distributed handoff interarrival times, Proc. of SPECTS2002, July 2002. K. Trivedi, O. Ibe, A. Sathaye, R. Howe, Should I Add a Processor?, 23rd Annual Hawaii Conference on System Sciences, 1990. 2. 3. 4. 5. 9. Copyright © 2003 by K.S. Trivedi 95 ...
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