Lecturer: Olga Galinina

Transcription

1 Lecturer: Olga Galinina

2 Outline Motivation Modulated models; Continuous Markov models Markov modulated models; Batch Markovian arrival process; Markovian arrival process; Markov modulated arrival process; Switched Markov modulated arrival process. Discrete Markov models Discrete Markov modulated models; Discrete-time batch Markovian arrival process; Discrete-time Markovian arrival process; Markov modulated Bernoulli process; Switched Markov modulated Bernoulli process; Discrete-time switched Poisson process; Discrete-time switched Bernoulli process; Fitting parameters example.

3 Why non-renewal models? Classic renewal processes: strictly stationary; completely uncorrelated; single arrivals. In general inadequate to capture traffic properties Real traffic: may not be strictly stationary; may be correlated; multiple arrivals are allowed. Note! We have to be close to reality! 3

4 Modulated models Basic concept: one process X i (t) determines the state S(t); states are associated with other processes. Figure 1: Graphical representation of general modulated process. What is important about this process: values of {S(t), t {T}} are not observable; values of {X i (t), t {T}}, i = 1,,, M are observable. 4

5 Modulated models In general: no restrictions on the choice of {S(t), t {T}} ; no restrictions on the choice of {X i (t), t {T}}, i = 1,,, M. Why these processes are non-renewal: value of the process depends on the state; in special case there is autocorrelation. We are interested in special case: modulating process is Markovian; processes in states are renewal. These are called Markov modulated processes 5

6 Markov modulated processes We distinguish between: continuous-time model interarrival times; discrete-time model number of arrivals in slots. Classification is related to measurements: continuous interarrival times time-consuming; discrete number of arrivals in slots easier to do. Figure : Discrete and continuous measurements. 6

7 Continuous-time time Markov models 7

8 Batch Markovian AP (BMAP) To define BMAP we start considering Poisson process assuming: no arrivals occurred prior to t=0; we consider it in terms of arrivals prior to time t: N(t); rate of the process is assumed to be λ. {N(t),t 0}, N(t) {0,1, } can be considered as pure birth process: states N(t) {0,1, } denote number of arrivals; exponential sojourn time in state i; after that process jumps to state i+1 resulting in exponential interarrival time. Figure 3: Interpretation of Poisson arrival process as pure birth process. 8

9 Batch Markovian AP (BMAP) Infinitesimal generator of {N(t), t 0}: d 0 d d0 d1... Q =, 0 0 d0... d1 = λ, d0 = d1 = λ. If the process is batch Poisson we have: d0 d1 d d3 0 d0 d1 d Q =, 0 0 d0 d1 d = λ p, i = 1,,...; d = d = λ. i i 0 i i=1= 1 Here p i is the probability that size of the batch is i. 9

10 Batch Markovian AP (BMAP) Batch Markovian arrival process: extension of the batch Poisson: interarrival times are no longer exponential; Markovian structure is still preserved. Consider process {N(t),J(t), t 0}, N(t) {0,1, }, J(t) {0,1,,M}: ( 0) ( 1) ( ) ( 3) 0 D ( 0 ) D ( 1 ) D ( ) 0 0 D( 0) D( 1) D D D D Q =. D(k), k = 0, 1,... are M M matrices; D(0): negative diagonal elements and non-negative off-diagonal; D(k), k = 1,,... are non-negative: D = i=0= 0 ( ) D i. 10

11 Batch Markovian AP (BMAP) Important notes about BMAP: most general analytically tractable continuous-time Markov modulated process; allows arbitrary distribution of the interarrival times; ACF: sum of exponential terms. Constructive interpretation of BMAP: continuous-time Markov chain {S(t), t R}, S t {1,,, M}: sojourn time is exponentially distributed with parameters λ i, i=1,,, M; when MC changes its state from i to j a batch of arrivals is generated. Denote probabilities of transitions with k arrivals as: p ij (k), i=1,,, M. we assume p ii (0)=0, i=1,,. 11

12 Batch Markovian AP (BMAP) How to characterize D-BMAP: number of states, M; intensitiesout of states λ i, i=1,,..., M; probabilitiesp ij (k), i, j=1,,..., M, k = 0, 1, How to have this information in compact form: matrices D(k) where each element is: ( ) = λ,, = 1,,...,, = 1,,... d k p i j M k ij i ij matrix D(0) each element of which is λi pij, i j dij =, λ i, i = j where for infinitesimal generator D the following holds: D D( i). = i=1 1

13 Batch Markovian AP (BMAP) Matrices D(0), D(1),..., D(k),... take the form: λ 1 p1 ( 0 ) λ1 p13 ( 0 ) λ1... p1m ( 0 ) λ1 p1( 0) λ λ p3 ( 0 ) λ... pm ( 0) λ D( 0 ) =, pm1( 0) λm pm ( 0) λm pm3( 0 ) λm... λ M p 11( 1) λ 1 p 1 ( 1) λ 1 p 13( 01 ) λ 1... p 1 M ( 1) λ 1 p1( 1) λ p ( 1) λ p3 ( 1 ) λ... pm ( 1) λ D( 1 ) =, pm1( 1) λm pm ( 1) λm pm3( 1 ) λm... pmm ( 1) λ M ( ) D k ( ) ( ) ( 01 )... M ( ) ( ) ( ) ( 1 )... ( ) p 11 k λ 1 p 1 k λ 1 p 13 λ 1 p 1 k λ 1 p1 k λ p k λ p3 λ pm k λ = ( ) ( ) ( )... pm k λm pm k λm pm λm pmm ( k) λ M 13

14 Markovian arrival process: MAP MAP is a special case of BMAP: only single arrivals are allowed. MAP process {W(t), t R} is defined as: d 1 = λ p 1, i, j = 1,,..., M ; k = 0,1. d ( ) ( ) ( ) λi ij ( ) ij i ij ij p 1, i j 1 =. λ i, i = j D = D(0) + D(1). Mean is given by: [ ] 1 ( ( 0 )) 1 E W = = π D e λ λ is the overall intensity of the process. 14

15 Markov modulated arrival process: MMPP MMPPis a special case of MAP: arrivals are allowed when state changes from i to i, i=1,,,m. MMPP process is defined as: λ i pij ( 1 ), i = j d ij ( 1 ) =, 0, i j D = D(0) + D(1). In matrix form: d ij ( ) ( ) λi pij 1, i j 0 =. λi, i = j ( ) ( ) ( ) D 0 = D Λ, D 1 = Λ, D k = 0, k 1. Λ = diag ( λ are rates of Poisson process in 1, λ,..., λm ) = diag ( λ ), λi states i = 1,,, M; λ, D full description is then given by ( ) 15

16 Markov modulated arrival process: MMPP Characteristics of MMPP: steady-state vector π = (π 1, π,..., π M ) of modulating CTMC: π D = 0, π e = 1. mean arrival rate is given by: θ T λ = π ( ) ( 1 ) kd k e = π D e = πλ e = πλ. k = 1 if CTMC is irreducible aperiodic, autocovariance function is given by: N 1 1 i CW ( τ ) = E W ( t) W ( t + τ ) = λδ ( τ ) + ϕ0 + ϕ ie γ τ, ( ) = 1, τ = 0. ( ) = 0, τ 0 δ τ δ τ γ, i= 0, 1,, N 1 are N 1 eigenvaluesof CTMC given that ; i γ 0 = 0 if CTMC is irreducible aperiodic all eigenvalues and real; i= 1 16

17 Markov modulated arrival process: MMPP Distributions of the MMPP: PF number of arrivals is a weighted sum of Poisson distributions: N k i π iλi e p Pr { W ( t) k} λ k = = =, k = 0,1,..., k i= 1! π i is the steady state probability of CTMC is state i; PDF of interarrival times is a weighted sum of exponentials: N iw pw ( w) = π iλi e λ, i= 1 this is known as hyperexponential distribution; recall that C > 1 and C = 1 in a limiting case. What we can model with MMPP: empirical distributions with high variability: C > 1 provides simple check of MMPP suitability! ACF exhibiting (sum of) exponential decay. 17

18 Switched Markov modulated arrival process: SMMPP SMMPP is a special case of MMPP: modulating Markov chain has only states.. SMMPP is given by: λ1 0 r, 1 r Λ = D =, 0 λ r r D ( ) 0 = D Λ. Steady-state probabilities of modulating CTMC are: π λ r λ r =, π = λ1r + λr1 λ1r + λr1 π is the vector containing these probabilities. 18

19 Switched Markov modulated arrival process: SMMPP CDF F(x) = Pr{X x} of interarrival times is given by: ( D Λ) x 1 u1x u x F ( x) = 1 π e ( Λ D) Λ e = 1 qe + ( 1 q) e, e = ( 1,1 ),0 < q < 1. Probability density function (PDF) is: A u x ( ) ( ) 1 u x f x qu e q u e q = + 1,0 < < 1. 1 Autocorrelation function is given by: ( 1 1 )( + 1 [ + 1] ) ( ) [ ] K X k = E X E X X k E X k = ( ) ( 1 ( ) ) k + 1 k π D D eπ = Λ Λ Λ Λ ( Λ D) Λ e = Aσ, k = 1,,... where A and σ are given by: ( ) r r ( + ) ( + + ) λ1 λ 1 λ1λ =, σ =. λ1r λr1 λ1λ λ1r λ r λ 1 1λ + λ1r + λr1 19

20 Switched Markov modulated arrival process: SMMPP What is important about SMMPP: distribution is hyperexponential; ACF decays exponentially. What we may capture by SMMPP: interarrival distributions with: monotone decreasing behavior; coefficient of variation: C > 1. q 1 q + [ ] ( [ ] σ X E X E X ) u1 u C = = = 1 1. ( E [ X ]) ( E [ X ]) q 1 q + u1 u exponentially decaying ACF only! 0

21 Switched Markov modulated arrival process: SMMPP Figure 7: Possible distributions of SMMPP. 1

22 Switched Markov modulated arrival process: SMMPP Figure 8: Possible normalized ACFs of SMMPP.

23 Discrete Markov modulated models 3

24 Discrete Markov modulated models What is special about such models: modulated process is discrete Markov in nature transition probability matrix is in the form: d d d M d d d, 1, 1,,...,. M 1 M Q = d ji = j = M i= 1 d d d M1 M MM What is interesting: usually easier to deal with; may have arbitrary distribution; may have complex ACF structure: sum of geometrical terms. 4

25 Discrete Markov modulated models Discrete processes: time is divided into intervals (slotted); durations of intervals are the same t; some arrivals may occur in each interval. Figure 11: Illustration of discreteness of the arrival process. Note the following: interpretation of discrete processes can be considered as approximations of real arrivals; sometimes this is a natural way. we are going to work with number representation; intervals representation is also possible. 5

26 Discrete-time time batch Markovian arrival process Basic characteristics: most general analytically tractable discrete Markov modulated process; allows arbitrary distribution of the number of arrivals in a slot; ACF: sum of geometrical terms. Assume time axis is slotted; the slot duration is constant and given by t = (t i+1 t i ); discrete-time homogenous aperiodic, irreducible MC {S(n), n=0, 1,...}: state spaces(n) { 1,,..., M } ; transition probability matrix D. {W(n), n=0, 1, } is D-BMAP with MMC is {S(n), n=0, 1,...} if: value of {W(n), n=0, 1, } is function of the current state of {S(n), n=0, 1,...} 6

27 Discrete-time time batch Markovian arrival process How to completely define D-BMAP: matrices D(k), k = 0, 1, : state change from ito j, i,j = 0, 1,... ; arrival of k customers. Example: d ij (0): transition from state ito state j without any arrivals; d ij (k): transition from state ito state j with a batch arrival of size k. In general for d ij (k) we have: d ij (k) = Pr{W(n) = k, S(n) = j S(n 1) = i}, k= 0, 1,... Note the following: for pair (i, j), d ij (k), k = 0, 1,... are called conditional probability functions: M j= 1 k= 0 ( ) d k = 1, j = 0,1,..., M. ij For different pairs (i, j) d ij (k) are allowed to be different. 7

28 Discrete-time time batch Markovian arrival process Figure 1: Illustration of the D-BMAP. 8

29 Discrete-time time batch Markovian arrival process π = π π π ( 1,,..., M) Let be the vector of stationary probabilities of {S(n)}: i ( n) π = lim π, i = 1,,... n We can find π = π, π,..., π using: i ( 1 M) π D = π. π e = 1 easiest way to compute: D i and take any row where i is large (> 1000): Example of how to compute π : D = = Hence, π = , ( ) 1000, D. 9

30 Discrete-time time batch Markovian arrival process Using the mean arrival rate in the slot is: E W = kd k e, e = 1,...,1. π [ ] ( ) ( ) π k= 1 The variance of D-BMAP is: D W R 0 k D k e E W. ( ) [ ] = ( ) = ( ) [ ] W π k=1= 1 Let R W (i), i0, 1,... be the ACF of the D-BMAP: ( ) ( ) ( ) [ ] ( ) W ( ) [ ] RW i = E W n W n + 1 E W, i 0, R 0 = D W. The ACFof D-BMAP is: R i = kd k D kd k e E W, i 0. W π k= 1 k= 1 ( ) i 1 ( ) ( ) ( ) [ ] 30

31 Discrete-time time batch Markovian arrival process The mean process of D-BMAP: {G(n), n=0, 1,...} with G(n) = G i : M j= 1 k= 1 ( ) G = kd k, i = 1,,..., M. i G = G,G,..., G M ( ) 1 ij is the mean vector of D-BMAP. The mean input rate in the slot is given by: M [ ] = π = [ ] E G G E W k= 1 The variance of the mean process of D-BMAP is given by: i i. M ( ) [ ] = ( ) = π [ ] D G R 0 G E G. G i i k= 1 The ACF of the mean process is given by: ( ) i 1 ( ) R ( ) ( [ ]) G i = π kd k D kd k e E G, i 0. k= 1 k= 1 31

32 Discrete-time time batch Markovian arrival process Advantages of using D-BMAP: quite general process; analytically tractable. Shortcomings of using D-BMAP: really hard to parameterize: we have to estimate matrices D(k), k = 0, 1, ; M M k max parameters. We usually use a special case of D-BMAP: D-MAP: discrete-time Markovian arrival process; D-MMBP: discrete-time Markov modulated batch process; D-SBP: discrete-time switched batch process; D-SPP: discrete-time switched Poisson process; D-SBP: discrete-time switched Bernoulli process. 3

33 Discrete-time time Markovian arrival process D-MAP is a special case of D-BMAP: only single arrival in a slot is possible D(0) and D(1)! Mean is given by: E W = πd 1 e, [ ] ( ) The variance of D-MAP is: The ACF of D-BMAP is: D W = R 0 = πd 1 e- E W, ( ) [ ] ( ) ( ) [ ] W 1 R ( i ) π D( 1) D i W = e- E[ W ], i 0. ( ) Note the following: D-MAP reduces versatility of D-BMAP in terms of batch arrivals; D-MAP still have different conditional PFs for each different pair of states (i, j)! 33

34 Markov modulated Bernoulli process MMBPis a special case of D-BMAP: conditional PFs depends on the current state only. Recall for D-BMAP we had: { } ( ) ( ) ( ) ( ) d ( k) = d ( k), j l. d k = Pr W n = k, S n = j S n 1 = i, k = 0,1,..., i, j = 0,1,..., M. ij in general, ij il too many conditional PFs to determine; these conditional PFs depends on pair of states (ij). ForMMBPwe have: ij ( ) = ( ),. d k d k j l il in overall, we only have M conditional PFs; now, it does not matter to which state transition occurs; ACF, meanand variancecan be obtained using the same expression as for D-BMAP 34

35 Markov modulated Bernoulli process When it does not matter to which state transition occurs we may define: M { } ( ) ( ) ( ) ( ) a k = Pr W n = k, S n = j S n 1 = i, k = 0,1,..., i, j = 0,1,..., M. i j= 1 a i (k), k = 0, 1,..., i = 1,,... are conditional PFs of arrivals; these conditional PFs depend on the state from which transition occurs. For a i (k), k = 0, 1,..., i = 1,,...: k=0 i ( ) = 1,, = 0,1,...,. a k i j M 35

36 Switched D-MMBP Special case of D-MMBP: only two states of the MMC. Assume that transition probability matrix of MMC has the following form: 1 α α D =. β 1 β Steady-state distribution expressed in terms of α and β: ACFof the mean process is: β α π1 =, π1 =. α + β α + β i G ( ) = [ ] λ, = 0,1,... R i D G i D[G] is the variance of the mean process; λ is single non-unit eigenvalueof the MMC: MMC has only two states: λ 0 = 0, 0 λ 1 < 1. 36

37 Switched D-MMBP We can express ACF of the mean process in terms of G G i i R ( ) 1 ( 1 ) [ ]( 1 ), 0,1,... G i = αβ α β D G α β i α + β = = λ = (1 α β); G 1 and G are means in states 1 and, respectively. Normalized ACF of the mean process is then: Note! ( i ) [ ] KG i KG ( i) =, i 0,1,... D G = λ = we have no simple relation between R G (i) and R W (i) except for: ( ) ( ) ( ) ( ) R i = R i, i = 0,1,..., R 0 = R 0 + x, W G W G (when conditional PFs are close to Poisson, x is close to E[W]). process may produce fair approximation for geometrically decaying ACF; conditional PFs are allowed to be arbitrary. 37

38 Discrete-time time switched Poisson process Special case of switched D-MMBP: conditional PFs are no longer arbitrary; in this particular case it is Poisson. D-SPP: 1, i = 0 R W ( i) = R G ( i) + E[ W] δ i, δ i =. 0, i = 1,,... [ ] = [ ] = π1 1 + π E W E G G G is the mean of SPP. Note! ACF has geometrical decay; distribution is a mixture of two Poisson distributions (not Poisson). 38

39 Discrete-time time switched Poisson process Figure 13: Possible behavior of the distribution of D-SPP. 39

40 Discrete-time time switched Poisson process What is required to parameterize D-SPP: transition probability matrix of MMC: α and β; means in states 1 and : G 1 and G ; recall that mean completely determine Poisson distribution. Special case of D-SPP is interrupted D-SPP: mean in state 1 is zero (no arrivals); mean in state is not zero. Characteristics of interrupted D-SPP: ACF still has geometrical decay; distribution is Poisson. 40

41 Discrete-time time switched Bernoulli process Special case of switched D-MMBP: conditional PFs are no longer arbitrary; in this particular case it is Bernoulli: only single parameter in each state: probability of arrival: { } ( ) ( ) ( ) ( ) d 1 = Pr W n = k, S n = 1 S n 1 = i = p, i, j = 1,. ij note that conditional PFs depend only on the current state. i D-SBP: setting p 1 =1, p =0 (or vice versa): W ( ) = ( ), = 0,1,..., R i R i i G (you can check it by inserting mean and variance in expression for ACF of D-BMAP). when both p 1 and p are not zero this property does not hold. 41

42 Fitting parameters example 4

43 Fitting parameters. Switched Markov modulated arrival process: SMMPP Parameters of PDF can be found using λ 1, λ, r 1, r : u1 x u x f x = qu e + 1 q u e,0 < q < 1. u q ( ) ( ) 1 λ + λ + r + r δ λ + λ +, u r + r + δ = =, λ r1 + λ1 r u =. λ r + λ r u u u u ( 1 1 )( 1 ) δ can be found as follows: 1 ( r r ) 4 r r. δ = λ λ Recall: parameters A and σof ACF can be found using λ 1, λ, r 1, r : A k K X ( k ) = Aσ, k = 1,,... ( ) r r ( + ) ( + + ) λ1 λ 1 λ1λ =, σ =. λ 1 1 1r λ r1 λ1λ r r λ1r λ r λ λ + λ + λ

44 Fitting parameters SMMPPis completely defined by λ 1, λ, r 1, r ; histogram and ACF of data are completely defined by u 1, u, σ and q; there is unique SMMPP capturing empirical data. Algorithm: estimate u 1, u, q from empirical pdf; estimate σ from empirical ACF; find λ 1, λ, r 1, r using the following: 1 λ1 = q ( 1 σ )( u1 u ) σ u1 u ( q ( 1 σ )( u1 u ) σ u1 u ) 4 σ u1u , λ = r 1 1 r = = u1u ( λ1 q ( u1 u ) u ) λ1u1 λ1q ( u1 u ) u ( u λ )( u λ ) λ λ 1 ( λ u )( λ + r u ) λ λ 1,., 44

45 Fitting two first moments and NACF Determine u 1, u, q as follows: hyperexponentialdistribution with balanced means q/u 1 = (1 q)/u ; probabilities q can be found as: 1 C 1 q = 1 +. C 1 + rates u1 and u are given by: u [ ] 1 ( q) E [ Y ] q 1 =, u =. E Y Determine σ by setting σ = K Y (1) or minimizing: ( m) ( ) m= m m 0 1 K Y σ γ = m K m 0 m= 1 Y K Y (i), i= 1,,... is the normalized ACF of empirical data; m 0 is the intervals for which K Y (m 0 )

46 Example of fitting Interarrival times and corresponding statistics Figure 9: Trace, histogram and NACF of interarrival times.. 46

47 Example of fitting Moments of data: E[Y] =.7, σ [Y] = 4.978, C[Y] = What are conclusions about data: looks like data are exponential C [Y ] 1; we can still use hyperexponentialdistribution to approximate exponential. use empirical moments estimate parameters of H (q, u 1, u ) as: q = 0.53, u 1 = 0.476, u = 0.4. use empirical NACF estimate σ as σ = K(1) = determine parameters of SMMPP λ 1, λ, r 1, r as: λ 1 = 0.45, λ = 0.170, r 1 =.557E 3, r =

48 Example of fitting Figure 10: Visual comparison of empirical characteristics and characteristics of the model. 48