\title{Timed Automata with Urgent Transitions} 

\author{Roberto Barbuti \and Luca Tesei}

\begin{abstract} In this paper we propose an extension to the
formalism of timed automata by allowing urgent transitions.  A urgent
transition is a transition which must be taken within a fixed time
interval from its enabling time.  We give a set of rules formally
describing the behaviour of urgent transitions and we show that, from
a language theoretic point of view, the addition of urgency does not
improve the expressive power of timed automata. However, from a
specification point of view, the use of urgent transitions is crucial,
especially in modular specification of systems.  
\end{abstract}

{\bf Keywords:} real-time systems, timed automata, modular
specification, parallel composition.