An SDL system interacts with its environment only by means of signals (the term instance is again omitted in this section). This is possible only if a number of restrictions are imposed on the possible behaviour of the environment, which is basically unpredictable. We can imagine that there is an implicit channel connecting the system to its environment, considering the environment just another object, see figure 37.

We can consider that the system and the environment interact in accordance with a protocol (or interface) description. Since the system behaves according to its description, this protocol description must be the same as the system description. Hence, the environment is assumed to know the system description, and must obey the constraints given by it.

It is further assumed that there are processes in the environme1t that are capable to send and receive signals to/from the system. These processes must have unique addresses, different from the address of any process within the system.

Figure 37 Relation between the system and its environment

As a consequence, the user of SDL need not specify actions for all possible behaviour of the environment. Unspecified behaviour is simply not allowed (or. in other words, s ignored by the system). Since the behaviour is specific in the process descriptions, it can be a tedious task to find out in a comprehensive system description what behaviour is not allowed on the part of the environment In addition, the user of SDL generally makes some further assumptions on the expected behaviour of the environment It is therefore advisable to summarize the constraints and assumptions on the environment in a natural language as an introduction to the system description.

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