A generic system description can contain several thousands options. Example of an option is the choice between different languages (English, French etc) in commands and printouts. Options in different products can be interdependent, it may be necessary to select options by a person (such as a salesman) who is not familiar with the system description on the detail level. It is therefore important that the relation between interdependent options are formally described, so that the options can be selected in a consistent manner, and preferably supported by tools.

SDL contains constructs to formally describe options in a generic system description. Such a system description is parameterized by the use of external synonyms, which can be regarded as some sort of formal parameters. An external synonym is defined by

<external synonym> ::=

SYNONYM <external synonym name>

<predefined sort> := EXTERNAL

/* Description of allowed values and their meaning, using SDL syntax for comment*/

Synonym is a name for a value. The keyword EXTERNAL (which is replaced by a value for an ordinary synonym) indicates that the value for the external synonym should be supplied outside the system (as an actual parameter) when a subset of the generic system description is selected. Allowed values and their meaning can be given as a comment.

An external synonym is defined (only once of course) at a suitable place of the system description. It can be used in any place, where it is visible, to express an option. In this way, interdependent options in the system description will be automatically interrelated. The person who makes the selection, only has to supply values for the external synonyms, he need not know where and how these synonyms are used in the system description.

An external synonym is used preferably in boolean expressions in select statements and transition options, but can be used in any place where an ordinary synonym is allowed.

Example, see figure 33. In this figure the system diagram for system Alfa is given, containing three blocks B1, B2 and B3. Block B1 and the channels connected to it are optional, dependent on the values of the external synonyms p and extension.

Figure 33 Example of a generic system

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