Abstract
This paper studies a single server M/G/1 stochastic clearing queue operating in a 3-phase environment, where the time length of the first and third phase are assumed to follow exponential distributions, and the time length of the second phase is a constant value. At the completion of phase 1, the system moves to phase 2, and after a fixed time length, the system turns to phase 3. At the end of phase 3, all present customers in the system are forced to leave the system, then the system moves to phase 1 and restarts a new service cycle. Using the supplementary variable technique, we obtain the distribution for the stationary queue at an arbitrary epoch. We also derive the sojourn time distribution and the length of the server’s working time in a cycle.
1 Introduction
In most of our queueing models, the capacity of the facility is assumed to be largely enough. But in the life situation, things are not the same, the capacity of the queue is finite in general, so the probability that some waiting customers cannot be accommodated is negligibly small. Besides, the waiting customers that can’t be served are not always willing to retry for its next service, instead, they usually abandon the system. So, it is realistic to consider that all present customers are removed at the visit points of the facility. Such systems are referred to as stochastic clearing systems.
Stochastic clearing systems have important applications in health center and banks. Since the introduction of this concept, there are amount of papers were studied. Some classical papers can be referred to [1-4]. Furthermore, Kyriakidis and Dimitrakos[5] considered a Markov decision model and derived the optimal control of a compound immigration process. In [6], Boudali and Economou considered a Markovan queue from an economic viewpoint, where the arrivals stop when the system is under repair. Later, they[7] extended the model to that the new arrivals continue to arrive when the system is in the repair period. Yechiali[8] considered an M/M/c queue, and obtained some performance measures.
However, queues with deterministic time length is also an interesting aspect. Such system has wide application in analyzing the performance of various real life traffic situations as well as industrial queues. Jeet Singh[9] studied an M/G/1 queueing model with a vacation of fixed duration, using the probability argument, they gained the probability generating function of the units present in the system and some other performance measures. Madan[10] discussed the steady state behavior of an arbitrary service time queue with deterministic server vacation. In his paper, what he has considered is that the customers arrive at the system with uniform arrival rates. Later, he[11] considered an M/G/1 queuing model with time homogeneous and deterministic repair times.
The model in consideration can be used in real life situations, such as the application in bank account. In the bank, after a period of service time before noon, the bank will close for a deterministic time, then continue to serve in the afternoon. This is the same as the model we studied in this paper. And to the best of our knowledge, this model hasn’t been studied before. In this paper, we give the stationary queue size distribution at an arbitrary epoch. Following the idea presented by Jiang et al.[12], which provided an approach to analyze the sojourn time distribution and the length of the server’s working time in a cycle, we also derive these performance measures in our paper.
The rest of this paper is organized as follows: Section 2 is the model description. Using supplementary variable technique, Section 3 and Section 4 gives the stationary queue size distribution at an arbitrary epoch. Some performance measures are presented in Section 5. At last, Section 6 is the conclusion.
2 Model Description
Consider a single server M/G/1 queue operating in a 3-phase environment. Under environment i, i = 1, 2, 3, the Poisson arrival rate is λi, and the service times in phase 1 and 3 follow a general (arbitrary) distribution with mean
3 Stationary Queue Size Distribution
In this section, we use the method of supplementary variable technique to derive the stationary queue size distribution at an arbitrary epoch. It is worth notice that as long as θ3 > 0, the system in consideration is stable. So the system can be analyzed in steady state.
Let µi(x)dx be the conditional probability (hazard rate) of completion of a service during the interval (x, x + dx) with elapsed time x in phase 1 and 3, respectively. So we have
where i = 1,3.
At an arbitrary time t, the system can be described by {L(t), J(t), B–(t)}, where L(t) denotes the number of customers in the system at time t, J(t) denotes the phase in which the system operates at time t, and if L(t) ≥ 1 and J(t) = i,
Next, we introduce the following definitions.
Let Pi,n(x, t)dx, i = 1, 3, n ≥ 1 be the probability that there are n customers in the system (including the one being served) at time t when the server is busy in serving the customer with elapsed service time lying between x and x + dx in phase i. Let Qi,0(t), i = 1,3 be the probability that there is no customer in phase i and the server is idle at time t, Vn(t), n ≥ 0 be the probability that there are n customers in the system and the system in phase 2. Last, let
be the probability of r, r ≥ 0 arrivals during phase 2.
In steady state, we define the following limiting probabilities
Using the probabilities above and by the approach of supplementary variable technique, we can easily get the steady state equations for the system:
4 The Steady State Solution
First, we define the following equations:
Solving Equation (1), we have
According to Equation (4), we derive
Multiplying Equation (5) by zn and summation of n ≥ 1, we have
Lemma 1
The equation
Proof First, we define
Because both f(z) and g(z) are analytic for |z| = 1 + ε. Hence, by Rouche’s theorem, f(z) and f(z) – g(z) have the same number of roots in |z| < 1+ ε. Therefore, as ε tends to zero, it yields that f(z) – g(z) has only one root in |z| < 1, i.e., The equation
where γ is the unique root of
where
The equation (12) can be simplified as
From (3), (7) and (13), we have
By the result of Lemma 1, we also have
where α is the unique root of
Then the expression of P1(z) and P3(z) can be obtained by
that is,
With the help of the normalizing condition
and
We gain
Once P1(z), P3(z), V (z), Q1,0 and Q3,0 are obtained, the PGF of the stationary queue size distribution can be derived by
5 Performance Measures
In this section, we will give some important performance measures of the system. Firstly, let L be the stationary random variable for the number of customers in the system. Then the expected number of customers in the system is
where
Secondly, we will give the working time of the server in a cycle. We consider two types of cases:
Case 1: No customer arrival occurs in phase 1.
Case 2: Customers’ arrival occur in phase 1.
We first define the following random variables. Let Ai, i = 1, 3 be the inter-arrival times of the customer in phase i, Di, i = 1, 3 the duration times of phase i, Rk, k = 1, 2 denote the length of working times of the server in case k. We also let pk, k = 1, 2 denote the probability of case i. Then we have
First, we give a lemma as follows.
Lemma 2
(see Lemma 2 in [12]) Let X, Y, Z be the random variables, and Z = min(X,Y), where X follows a general (arbitrary) distribution with mean
In case 1, we consider two subcases: No customer arrives in phase 2 and customers’ arrival occur in phase 2. Let F denote the length of working times of the server in phase 3 under the condition that no customer arrives in phase 2, Fr the length of the working times in phase 3 under the condition that customers’ arrival occur in phase 2, and Bi,k denote the length of busy period which start by k customers. We first consider the case no customer arrives in phase 2:
where
T1 = (D3|D3 < B3,1), T2 = (B3,1|D3 > B3,1). Then
Then
and
that is,
Simplifying the above equation, we have
where
Next, we consider the case customers’ arrival occur in phase 2. Let Fr denote the length of working times of the server in phase 3 under the condition that customers’ arrival occur in phase 2.
where Y1 = (D3|D3 < B3,r), Y2 = (B3,r|D3 > B3,r). Then,
that is,
where
So in case 1,
In case 2, we also consider two subcases, one of which is the busy period ends before the completion of phase 1, and the other subcase is that phase 1 completed before the busy period ends. Let R2,1 denote the working times under the condition that the busy period ends before the completion of phase 1, R2,2 denote the working times under the condition that phase 1 completed before the busy period ends, and R the total working times of server in a cycle. Let K denote the number of existing customers at the end of the phase 1, with the result of Kim et al.[14], K(z), the PGF of K, is given by
Hence, we obtain
where
Finally, considering a tagged customer, we will derive the LST of the stationary sojourn time distribution of an arbitrary customer, where the sojourn time is the period from the time he enters the system to the time he leaves by either the service completion or the end of the thrid phase. Denote W and W∗ (s) be the stationary sojourn time and its LST. In order to obtain W∗ (s), we consider the following cases:
Case 1: The tagged customer arrives in state (0, 1).
Case 2: The tagged customer arrives in state (n, 1), n ≥ 1.
Case 3: The tagged customer enters the system in state (0, 2).
Case 4: The tagged customer enters the system in state (n, 2), n ≥ 1.
Case 5: The tagged customer arrives in state (0, 3).
Case 6: The tagged customer arrives in state (n, 3), n ≥ 1.
Let Ti be the random variable of the duration of time when the system resides in phase i (i = 1, 3), and let Bh,i, h ≥ 1 be the service time of the customer at position h when the tagged customer arrives in phase i(i = 1,2,3). We also assume that when the customer doesn’t finish his service in phase 1, he will restart his service in phase 3, instead of continuing the service from the stopping point.
In case 1, on the tagged customer’s arrival, he receives the service immediately and leaves the system by either the service completion or the end of phase 3. Let W0,1 be the sojourn time in this case. Define
Using lemma 2, we have
In case 2, let Wn,1 be the sojourn time, and let Sn,i, i = 1, 2, 3 denote the unfinished work after the arrival epoch of the tagged customer, then
where
we have
According to Corollary 2.2.1 in the paper [13] of Kerner, which gives a method to compute the LST of the remaining service time at the arrival instant of a customer, given that he finds n customers present in the system.
with the initial condition
Define
then
In case 3, let W0,2 be the sojourn time. Define
then
In case 4, let Wn,2 be the sojourn time. Define
then
We can similarly compute the sojourn time in case 5, that is,
In case 6, let Wn,3 be the sojourn time. Then similarly to case 2, we have that
Define
using Lemma 2, we have that
Then the LST of the stationary sojourn time distribution of an arbitrary customer can be written as
The mean sojourn time of an arbitrary customer is gained by
6 Conclusion
This paper studied a single server M/G/1 queue operating in a 3-phase environment, where the first and third phase is working phase, and the second phase is a determinstic-time phase without service. Using supplementary variable technique, we derived the probability generating function of the stationary queue size at an arbitrary epoch. The mean queue length and the server’s working time in a cycle are also obtained. Furthermore, we derived the LST of the stationary sojourn time distribution of an arbitrary customer.
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