%T Bottom-up beats top-down hands down %A Mirco Tribastone %L eprints2555 %O 6th Workshop on Process Algebra and Stochastically Timed Activities (PASTA) held in Edindurgh (UK), July 2007 %B Proceedings of the 6th Workshop on Process Algebra and Stochastically Timed Activities (PASTA) %D 2007 %I AESOP %C Edinburgh (UK) %P 97-104 %X In PEPA, the calculation of the transitions enabled by a process accounts for a large part of the time for the state space exploration of the underlying Markov chain. Unlike other approaches based on recursion, we present a new technique that is iterative—it traverses the process’ binary tree from the sequential components at the leaves up to the root. Empirical results shows that this algorithm is faster than a similar implementation employing recursion in Java. Finally, a study on the user-perceived performance compares our algorithm with those of other existing tools (ipc/Hydra and the PEPA Workbench).