An Upward Compatible Timed Extension to LOTOS

We propose a timed extension of LOTOS, denoted TLOTOS, which is upward compatible with the standard LOTOS. It means first that a timed behaviour expression which does not use any language extension will have the same semantics as in standard LOTOS. In addition, we have pushed this upward compatibility one step beyond by requiring and obtaining that all the familiar equivalence laws of standard LOTOS be preserved, e.g. B [] B ~ B, B [] stop ~ B. This is needed in order to keep the intuition of a standard LOTOS user unchanged. TLOTOS has been mainly inspired by other approaches such as Moller & Tofts’s TCCS (Temporal CCS), Hennessy & Regan’s TPL (Temporal Process Language) and Nicollin & Sifakis’s ATP (Algebra of Timed Processes). Our model is not strictly asynchronous (like standard LOTOS, CCS, CSP, ACP) nor strictly synchronous (like SCCS, CIRCAL, Meije). For compatibility and simplicity, we decided to keep the model asynchronous for a large part and, for dealing with time, to introduce a “synchronous part” by way of a new basic “synchronous” or timed action in the asynchronous semantic model. The design of our TLOTOS is presented together with other possible design alternatives which are all rejected according to our compatibility or expressive power requirements. The solution that we obtain is simple and satisfactory, and its operational semantics is presented in depth. Two examples are provided to illustrate the use of TLOTOS.