Networks of infinite-server queues with nonstationary Poisson input

In this paper the authors focus on networks of infinite-server queues with nonhomogeneous Poisson arrival processes. They start by introducing a more general Poisson-arrival-location model in which arrivals move independently through a general state space according to a location stochastic process after arriving according to a nonhomogeneous Poisson process. The usual open network of infinite-server queues, which is also known as a linear population process or a linear stochastic compartmental model, arises in the special case of a finite state space. The mathematical foundation is a Poisson-random-measure representation, which can be obtained by stochastic integration. It implies a time-dependent product-form result: For appropriate initial conditions, the queue lengths (numbers of customers in disjoint subsets of the state space) at any time are independent Poisson random variables. Even though there is no dependence among the queue lengths at each time, there is important dependence among the queue lengths at different times. The authors show that the joint distribution is multivariate Poisson, and calculate the covariances. A unified framework for constructing stochastic processes of interest is provided by stochastically integrating various functionals of the location process with respect to the Poisson arrival process. They use this approach to study the flows in the queueing network; e.g., the authors show that the aggregate arrival and departure processes at a given queue (to and from other queues as well as outside the network) are generalized Poisson processes (without necessarily having a rate or unit jumps) if and only if no customer can visit that queue more than once. They also characterize the aggregate arrival and departure processes when customers can visit the queues more frequently. In addition to obtaining structural results, the authors use the stochastic integrals to obtain explicit expressions for time-dependent means and covariances. They do this in two ways. First, the authors decompose the entire network into a superposition of independent networks with fixed deterministic routes. Second, they make Markov assumptions, initially for the evolution of the routes and finally for the entire location process. For Markov routing among the queues, the aggregate arrival rates are obtained as the solution to a system of input equations, which have a unique solution under appropriate qualifications, but not in general. Linear ordinary differential equations characterize the time-dependent means and covariances in the totally Markovian case.