The complexity and dynamics of the Internet is driving the demand for scalable and effective network control. This paper proposes a collaborative on-line simulation architecture to provide pro-active and automated control functions for networks. The general model includes autonomous on-line simulators which continuously monitor/model the network conditions and execute a search in the parameter state space for better settings of the protocol parameters. The protocol parameters are then tuned by the on-line simulation system. We describe the building blocks of this architecture and investigate the implementation challenges in the areas of network modeling, on-line simulation and parameter search. We also discuss the applicability of this system and present the simulation and test results of a preliminary implementation.

In this paper we describe QNAT, a software tool developed at Indian Institute of Technology, Kanpur, India, for the analysis and simulation of queueing networks. Arbitrary configurations of open or closed networks of multi-server queues with infinite or finite capacity, fork-join queues with or without synchronization queues can be analyzed or simulated using QNAT. Queueing Networks with multiple classes of customers mall be specified with each class being a closed or an open class independently if there are finite capacity queues in the system, the type of blocking mechanism-transfer, repetitive service or rejection, can also be specified. For the applications where the accuracy of the results is important, an option to simulate the network is also provided. QNAT has proved to be a useful tool for the design of telecommunication systems, computer networks, modeling of industrial systems, design of banking systems, teaching courses and research on queueing theory etc. QNAT is a user-friendly analysis tool, developed with a Windows based Graphical User Interface (GUI). Mathematica forms the computing platform for QNAT due to its ability to perform symbolic computation.

In this paper we describe an approximate method for the analysis of an open network of finite capacity queues. Finite buffer capacities at the nodes introduce blocking of jobs that finish service at a node and find that the destination node is full at that time. When this happens, we assume that the blocked job is held at the server of the queue where it just completed service, blocking that server until the destination can accommodate it. This is called transfer blocking or blocking after service. We also assume that an external arrival that finds a full queue is lost. We consider an open queueing network with inter-arrival times of external arrivals and service times at each queue having a generalized exponential (GE) distribution. Queues can have finite buffers. To solve this system we augment the network by adding a "holding node" for every stream that can be blocked to hold the blocked jobs during the period corresponding to them blocking the server. The mean time spent in the holding node will be equal to that spent while being blocked. Also, to account for the increased service time of a blocked job as seen by customers behind it in the queue, the service times of these customers need to be increased. We thus use an iterative procedure to converge on to the parameters of the GE distributions of the inter-arrival and service time distributions. Results from our analysis are compared against simulations and they compare very well.