Semi-Markov Processes

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A taxi alternates between three locations. When it reaches location 11 it is equally likely to go next to either location 22 or 33. When it reaches location 22 it will next go to 11 with probability 1/31/3 and to 33 with probability 2/32/3. From location 33 it always goes to 11. The mean time between locations ii and jj are t12=20t_{12} = 20, t13=30t_{13} = 30, t23=30t_{23} = 30. Assume exponential service and assume tij=tjit_{ij} = t_{ji}.

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(a)

Find the (limiting) probability that the taxi's most recent stop was at location ii, i=1,2,3i = 1, 2, 3.

(b)

What is the (limiting) probability that the taxi is heading for location 22?

(c)

What fraction of time is the taxi traveling from location 22 to location 33?

Remark.
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Note for (c): Upon arrival at a location the taxi immediately departs.

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(a)

Assume that the Markov process in Exercise 7.5 is changed in the following way: whenever the process enters state 00, the time spent before leaving state 00 is now a uniformly distributed rv, taking values from 00 to 2/λ2/\lambda. All other transitions remain the same. For this new process, determine whether the successive epochs of entry to state 00 form renewal epochs, whether the successive epochs of exit from state 00 form renewal epochs, and whether the successive entries to any other given state ii form renewal epochs.

(b)

For each ii, find both the time-average interval and the time-average number of overall state transitions between successive visits to ii.

(c)

Is this modified process a Markov process in the sense that P{X(t)=i∣X(τ)=j,X(s)=k}=P{X(t)=i∣X(τ)=j}\mathbb {P}\left\{ X(t) = i \mid X(\tau ) = j, X(s) = k\right\} = \mathbb {P}\left\{ X(t) = i \mid X(\tau ) = j\right\} for all 0<s<τ<t0 < s < \tau < t and all i,j,ki, j, k? Explain.

Consider an M/G/1 queueing system with Poisson arrivals of rate λ\lambda and expected service time E[X]\mathbb {E}\left[X\right]. Let ρ=λE[X]\rho = \lambda \mathbb {E}\left[X\right] and assume ρ<1\rho < 1. Consider a semi-Markov process model of the M/G/1 queueing system in which transitions occur on departures from the queueing system and the state is the number of customers immediately following a departure.

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(a)

Suppose a colleague has calculated the steady-state probabilities {pi}\left\{ p_{i}\right\} of being in state ii for each i≥0i \geq 0. For each i≥0i \geq 0, find the steady-state probability π\pi of state ii in the embedded Markov chain. Give your solution as a function of ρ\rho, πi\pi_{i}, and p0p_{0}.

(b)

Calculate p0p_{0} as a function of ρ\rho.

(c)

Find πi\pi_{i} as a function of ρ\rho and pip_{i}.

(d)

Is pip_{i} the same as the steady-state probability that the queueing system contains ii customers at a given time? Explain carefully.

Consider an M/G/1 queue in which the arrival rate is λ\lambda and the service time distribution is uniform (0,2W)(0, 2W) with λW<1\lambda W < 1. Define a semi-Markov chain following the framework for the M/G/1 queue in Section 7.8.1.

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(a)

Find P0jP_{0j} for j≥0j \geq 0.

(b)

Find PijP_{ij} for i>0i > 0; j≥i−1j \geq i-1.

Consider a semi-Markov process for which the embedded Markov chain is irreducible and positive recurrent. Assume that the distribution of inter-renewal intervals for one state jj is arithmetic with span dd. Show that the distribution of inter-renewal intervals for all states is arithmetic with the same span.

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References

[Gal13] Gallager R.G. Stochastic Processes: Theory for Applications. Cambridge University Press, 2013.