A new buffer management scheme for hierarchical shared memory switches

We study a multistage hierarchical asynchronous transfer mode (ATM) switch in which each switching element has its own local cell buffer memory that is shared among all its output ports. We propose a novel buffer management technique called Delayed Pushout that combines a pushout mechanism (for sharing memory efficiently among queues within the same switching element) and a backpressure mechanism (for sharing memory across switch stages). The backpressure component has a threshold to restrict the amount of sharing between stages. A synergy emerges when pushout, backpressure, and this threshold are all employed together. Using a computer simulation of the switch under symmetric but bursty traffic, we study Delayed Pushout as well as several simpler pushout and backpressure schemes under a wide range of loads. At every load level, we find that the Delayed Pushout scheme has a lower cell loss rate than its competitors. Finally, we show how Delayed Pushout can be extended to share buffer space between traffic classes with different space priorities.