All-optical packet switching has been intensively investigated in recent years as an alternative to static, crossconnect based networks. Several switch architectures have been proposed, all of them using buffers made of fibre delay lines. The paper addresses the basic concepts of packet switching in the optical domain and describes an analytical approach to evaluate the end-to-end performance of networks employing slotted (fixed length) optical packets. Thus, for a given topology and traffic matrix, the end-to-end cell loss ratio is computed assuming an uncorrelated traffic. A network dimensioning procedure relying on this approach is also presented.