Introduction to LOTOS

Lotos was developed to define implementation-independent formal standards of OSI services and protocols. ‘Lotos’ stands for Language Of Temporal Ordering Specification because it is used to model the order in which the events of a system occur. Lotos has two very clearly separated parts. The first part provides a behavioural model derived from process algebras, principally from CCS (Calculus of Communicating Systems, Milner (1989)) but also from CSP (Communicating Sequential Processes, Hoare (1985)). The second part of the language allows specifiers to describe abstract data types and values, and is based on the abstract data type language Act One (Ehrig and Mahr (1985)). These two aspects of Lotos guided the organisation of this chapter. Section 3.1 introduces basic concepts of the language. Section 3.2 presents the process algebra aspect of Lotos, called Basic LOTOS. Section 3.4 presents the Act One basis of data typing. Finally, Section 3.5 indicates how these aspects are combined as Full LOTOS.
The syntax and semantics of Lotos are defined in the relevant standard (ISO (1989l)). The definition has four main parts: the syntax, the static semantics, the algebraic semantics of data types, and the dynamic semantics of behaviour expressions. A description of the detailed semantics of Lotos is beyond the scope of this introductory chapter.