A Simulation Study of Space Priorities in a Shared Memory ATM Switch

We study an ATM switch architecture in which the queues for all the switch output ports share space flexibly in a common buffer. Using a computer simulation of this switch under bursty traffic, we investigate various ways to manage space priorities in the shared memory. Our findings support one particular strategy which we call “Selective Pushout.” In this scheme, an arriving cell that finds the shared memory full overwrites a cell with priority less than or equal to itself from the longest output queue in the buffer (even if the arriving cell will be joining a different output queue). We simulated Selective Pushout as well as several simpler pushout and threshold schemes under a variety of load conditions. For each load pattern we studied, the Selective Pushout scheme performed at least as well and usually much better than its competitors. Selective Pushout offered a low overall cell loss rate, with very low losses for the high priority cells.