t the peer DLC entity is already in the disconnected state, it shall enter the disconnected state. The DLC entity that issued the DISC command is now in the disconnected state and will notify its SU. 3.3 ANALYSIS MSCs FOR THE V.76 PROTOCOL In order to better understand the protocol, we have created five Message Sequence Charts (MSCs) (similar to UML Sequence Diagrams) illustrating the main behaviors. Such an approach is recommended, especially if the system is complex, before starting the SDL model. The MSC in Figure 3.4 is named xidl and shows two DLC entities A and B performing an XID transmission, as described in Section 3.2, initiated by DLC A. The MSC in Figure 3.5 shows two DLC entities A and B performing a DLC establishment (a connection), as described in Section 3.2.3, initiated by DLC A. The MSC in Figure 3.6 shows two DLC entities A and B performing data (I frames) transfer from A to B, as described in Section 3.2.4 - to simplify, we consider that the information to 3 A number in the L-DATA request indicates through which DLC the data must be transmitted. 4 More than one data link connection can run in parallel, numbered 0, 1 etc. The number identifying the connection to release is indicated in the L-RELEASE request. The V.76 Protocol Case Study 29 this line this line /* XID transmission. */ represents represents the Service the Service User side A User side B L_SETPARMreq XIDcmd L_SETPARMind L_SETPARMresp XIDresp SETPARMconf Figure 3.4 The MSC xid1: XID (eXchange IDentification /* DLC establishment. */ L_ESTABreq SABME L_ESTABind L_ESTABresp L_ESTABconf Figure 3.5 The MSC cnx1: DLC establishment data_ab1 /* Data transfer from A to B. */ L_DATAreq L_DATAind Figure 3.6 The MSC data_abl: data transfer from A to B transmit fits into a single I frame. This scenario can only occur after the DLC establishment represented in Figure 3.5. The MSC in Figure 3.7 is symmetric with Figure 3.6. The MSC in Figure 3.8 shows two DLC entities A and B performing a DLC release, as described in Section 3.2.5, initiated by D