e system types, block types, process types and service types. 3 This is not part of SDL. SDL tools generally provide extensions (placed in comments) to allow implementing an operator in C, with various options for every need. This is useful to interface the C generated from an SDL description with existing codes such as device drivers. Quick Tutorial on SDL 23 USE SS7pack; system SS7_test ss7ch (ss7) (ss7) Figure 2.29 The system SS7_test using the package SS7pack 2.5.2.1 Block type A block type allows you to create a kind of reusable component. The two blocks in Figure 2.29 have been replaced, as illustrated in Figure 2.30, by system SS7_test block type LE LE_1 LE LE_2 : LE (ss7) (ss7) (ss7) ss7ch (ss7) This is a (ss7) block type (ss7) Figure 2.30 The blocks LE_1 and LE_2 based on block type LE the block type LE, containing the previous block contents, and two block definitions LE_1 and LE_2 based on LE. You can see one gate peer in Figure 2.30: this gate is used to connect the signal route srl in LE to the channel ss7ch in SS7_test. block type LE process type LEp wait4ACM ready LE : LEp (inPDU) (inPDU) wait4ACM (voidPDU) ready This is a (ss7) process type ready inPDU PDU; (ss7) (ss7) (ss7) (ss7) (ss7) Figure 2.31 The process LE based on process type LEp 24 Validation of Communications Systems with SDL 2.5.2.2 Process type The process LE in Figure 2.29 has been replaced, as illustrated in Figure 2.31, by the process type LEp, containing the previous process contents, a process definition LE based on LEp. 2.5.3 Specialization In SDL, a type can inherit from another type: for example, a block type SS7 could inherit from a block type protocol, or a signal sigl could inherit from a signal sig2. In addition, the structure of a type or the transitions it contains can be redefined in the subtype. The V.76 Protocol Case Study 3.1 PRESENTATION The system used for the case study is a simplified version of the protocol described in the ITU-T V.76 Recommendation based on the Link Access Procedure for Modems (LAPM). This reco