5.3 Signal refinement

Tutorial on SDL-88
Belina, Hogrefe (edits Reed)

This feature is not supported in SDL-2000.

The signal refinement mechanism has been introduced in SDL to "hide" low level signals for higher levels of abstraction, and to allow a top-down description of the behaviour of the system..

The signal refinement allows the user to partition signals into sub-signals resulting in a hierarchical structure, as in the case of blocks and sub-blocks. That is, inside a signal definition it is possible to define a set of new signals which are said to be refined signals or sub-signals of the enclosing signal definition.

The signal refinement is closely related to the block partitioning, because only those signals that are transported by a channel connected to a partitioned block can be refined. When such a signal is refined, it is replaced in the partitioned block by its sub-signals. The channel will automatically transport all the sub-signals of the signal, even if some of the sub-signals are flowing in the opposite direction (a unidirectional channel will then be implicitly bi-directional).

An example of the signal refinement is given in figure 32. This example represents a system in which a process in one block is sending text files to a process in another block. At the highest refinement level this is achieved by sending signals sf, each representing a text file. On the next refinement level we want to specify that such a text file consists of a number of records which are sent one by one (signal sr), and that the receiver has to acknowledge (signal nr) the reception of each record. The sender will send an end-of-file signal eof at the end of the transmission. In this example, at the highest refinement level. processes in Bl and B2 are communicating using the signal sf; on the next lower level, processes in B11 and B21 are communicating using the signals sr, nr and eof.

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Figure 32 Example of Signal refinement

Signal refinement is purely a static concept; there is no dynamic relation between a signal and its sub-signals. That means in figure 32 that the case is not allowed in which processes in B21 communicate with the signal sf, and processes P1 and P2 communicate with the sub-signals nr, sr and eof. That means also restrictions on the allowed consistent partitioning subsets.