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 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. 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 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 average delay experienced by packets.