Computer Networks and 1SDN Systems 26 (1093) 95 108 95 North-Holland Dimensioning of an ATM switch with shared buffer and threshold priority * J.F. Meyer ~', S. Montagna b, R. Paglino ~ " Department of Electrical Engineering and Computer Seience, 71re University o( Michigan, ,,inn Arbor, MI. USA t, Central Research Laboratories, haltel. Milano, ltaly Abstract A number of recent studies have addressed the use of priority mechanisms in Asynchronous Transfer Mode (ATM) switches. This investigation concerns the performance evaluation and dimensioning of a shared-buffer switching element with a threshold priority mechanism (partial buffer sharing). It assumes that incoming ATM ceils are distinguished by a space priority assignment, i.e., loss of a high priority cell should be less likely than loss of a low priority cell. The evaluation method is analytic, based on an approximate discrete-time, finite-state Markov model of a switch and its incoming traffic. The development focuses on the formulation of steady-state loss probabilities for each priority class. Evaluation of delay measures for each class is also supported by the model; results concerning the latter are illustrated without development. The analysis of loss probabilities is then used to dimension the buffer capacity and threshold level such that required maximum cell loss probabilities are just satisfied for each cell type. Moreover, when so dimensioned with respect to relatively stringent loss requirements, i.e., probabilities of 10 10 and 10 s for high and low priority cells, respectively, we find that both loss performance and resource utilization are appreciably improved over a comparable switch without such a mechanism. Keywords: Asynchronous transfer mode: Switching element; Partial buffer sharing; Loss performance 1. Introduction Broadband Integrated Services Digital Net- works (B-ISDNs) are intended to provide a vari- ety of different teleservices, all on a single "uni- versal" network. Moreover, such services can have widely differing Quality of Service (QOS) re- quirements which, at the cell (packet) level, trans- late to differences in permissible cell losses and cell transfer delays. Accordingly, B-ISDN archi- tectures, notably those employing Asynchronous Transfer Mode (ATM) techniques, should be able to accommodate such differences without appre- * This work was partially carried out within the framework o1 the Telecommunication Project of the Italian National Re- search Council (CNR). Correspondence to: J.F. Meyer, Department of Electrical En- gineering and Computer Science, The University of Michigan. Ann Arbor. MI. USA. ciable reductions in resource utilization or signifi- cant increases in network complexity. Generally, two types of priorities are consid- ered in a teletraffic context: time priority and ,space priority. In a time priority system, cells are distinguished according to transfer delay require- ments, where the higher the priority, the lower the average delay. Space priority (also referred as "loss" or "semantic" priority; see [2], for exam- ple) distinguishes cells according to loss require- ments, where the higher the priority, the lower the intended loss probability. Historically, time priority schemes have received greater attention but, with the advent of strict loss requirements associated with certain vital services,, concern with space priorities has begun to increase. Specifically, with regard to various ATM switching and multiplexing elements, a number of recent studies [1,4-10] have evaluated space pri- ority mechanisms (in certain cases, time priorities 0169-7552/93/$06.00 (L') 1993 Elsevier Science Publishers B.V. All rights reserved