Connectionless communications using the Asynchronous Transfer Mode

For a large number of applications, there is a strong need for the provision of a connectionless service by the B-ISDN. However, the cell-based ATM, which is used for switching and multiplexing in this public telecommunication network, is a connection-oriented technique. The B-ISDN should therefore be extended with functionality to allow for connectionless communications. This dissertation addresses the design and analysis of such an extension. An architectural framework is presented, which places the protocols to be used in perspective. Two possible network architectures result from the functional decomposition of the connectionless service into cooperating protocol entities \ud and the underlying ATM service. In the first one, end-systems of the B-ISDN are interconnected by means of end-to-end ATM connections. In the second one, endsystems are connected to special entities in the B-ISDN, called Connectionless Servers (CLSs). The CLSs are interconnected by ATM connections, thus constituting a connectionless overlay network on top of ATM.\ud A number of different implementation architectures for a CLS are proposed, and analysed with respect to effectiveness, availability, scalability, and in particular, performance. The major distinction between these implementation architectures is the distribution of functionality over modules. Furthermore, two different modes of operation are identified for a CLS. In the message mode of operation, a packet is reassembled from the incoming cells before it is processed and forwarded. In streaming mode of operation, the first cell of a packet is immediately processed and forwarded upon arrival, while state information is maintained for the processing and forwarding of subsequent cells of the packet. A number of performance models are developed in this dissertation. An approximate model of a CLS is analysed to allow for comparison of the delay which is experienced by cells for different implementation architectures and modes of operation. If the bandwidth assigned to ATM connections between CLSs is relatively high, message mode of operation yields the lowest delay, otherwise streaming mode performs best. In order to support the dimensioning of a reassembly buffer in a CLS operating in message mode, another, more detailed model is developed and analysed. It allows the computation of the packet loss probability of a buffer, as a function of its size. An essential function for the provision of a connectionless service using ATM is connection management. This function instructs the signalling system of the B-ISDN to establish and release ATM connections as needed for the transfer of packets. A new mechanism is proposed that exploits the expected correlation\ud between subsequent packet arrivals to reduce the average bandwidth that needs to be reserved by the ATM network. A performance model is developed and analysed to determine the optimal control parameters of the new mechanism, and to evaluate its behaviour. It is shown that bandwidth reductions of up to 95% can be obtained, compared to conventional mechanisms, without affecting the \ud average delay experienced by packets

Design and evaluation of a connection management mechanism for an ATM-based connectionless service

The Asynchronous Transfer Mode (ATM) has been developed as a connection-oriented technique for the transfer of fixed-size cells over high-speed networks. Many applications, however, require a connectionless network service. In order to provide such a technique, one can built a connectionless service on top of the connection-oriented service. In doing so, the issue of connection management comes into play. In this paper we propose a new connection management mechanism that provides for low bandwidth usage (as compared to a permanent connection) and low delays (as compared to a connection-per-packet approach). We model the new mechanism under two workload scenarios: an ordinary Poisson process and an interrupted Poisson process. We use Markovian techniques as well as matrix-geometric methods to evaluate the new connection management mechanism. From the evaluations it turns out that the proposed mechanism is superior to older approaches (which can be seen as limiting cases)

Semantics and Verification of UML Activity Diagrams for Workflow Modelling

This thesis defines a formal semantics for UML activity diagrams that is suitable for workflow modelling. The semantics allows verification of functional requirements using model checking. Since a workflow specification prescribes how a workflow system behaves, the semantics is defined and motivated in terms of workflow systems. As workflow systems are reactive and coordinate activities, the defined semantics reflects these aspects. In fact, two formal semantics are defined, which are completely different. Both semantics are defined directly in terms of activity diagrams and not by a mapping of activity diagrams to some existing formal notation. The requirements-level semantics, based on the Statemate semantics of statecharts, assumes that workflow systems are infinitely fast w.r.t. their environment and react immediately to input events (this assumption is called the perfect synchrony hypothesis). The implementation-level semantics, based on the UML semantics of statecharts, does not make this assumption. Due to the perfect synchrony hypothesis, the requirements-level semantics is unrealistic, but easy to use for verification. On the other hand, the implementation-level semantics is realistic, but difficult to use for verification. A class of activity diagrams and a class of functional requirements is identified for which the outcome of the verification does not depend upon the particular semantics being used, i.e., both semantics give the same result. For such activity diagrams and such functional requirements, the requirements-level semantics is as realistic as the implementation-level semantics, even though the requirements-level semantics makes the perfect synchrony hypothesis. The requirements-level semantics has been implemented in a verification tool. The tool interfaces with a model checker by translating an activity diagram into an input for a model checker according to the requirements-level semantics. The model checker checks the desired functional requirement against the input model. If the model checker returns a counterexample, the tool translates this counterexample back into the activity diagram by highlighting a path corresponding to the counterexample. The tool supports verification of workflow models that have event-driven behaviour, data, real time, and loops. Only model checkers supporting strong fairness model checking turn out to be useful. The feasibility of the approach is demonstrated by using the tool to verify some real-life workflow models