701, page 8, (Ref. 4).] 14.4 SIGNALING SYSTEM STRUCTURE Figure 14.3, which illustrates the basic structure of SS No. 7, shows two parts to the system: the message transfer part (MTP) and the user parts. These are three user parts: tele- phone user part (TUP), data user part (DUP), and the ISDN user part (ISUP). Figures 14.2 and 14.3 shows OSI layers 1, 2, and 3, which make up the MTP. The paragraphs below describe the functions of each of these layers from a system viewpoint. Layer 1 defines the physical, electrical, and functional characteristics of the signaling data link and the means to access it. In the digital network environment the 64-kbps digital path is the normal basic connectivity. The signaling link may be accessed by means of a switching function that provides the capability of automatic reconfiguration of signaling links. Layer 2 carries out the signaling link function. It defines the functions and procedures for the transfer of signaling messages over one individual signaling data link. A signaling message is transferred over the signaling link in variable-length signal units. A signal unit consists of transfer control information in addition to the information content of the signaling message. The signaling link functions include: Delimitation of a signal unit by means of flags. Flag imitation prevention by bit stuffing. Error detection by means of check bits included in each signal unit. Error control by retransmission and signal unit sequence control by means of explicit sequence numbers in each signal unit and explicit continuous acknowledgments. Signaling link failure detection by means of signal unit error monitoring, and sig- naling link recovery by means of special procedures. 14.4 SIGNALING SYSTEM STRUCTURE Layer 3, signaling network functions, in principle, defines such transport functions and procedures that are common to and independent of individual signaling links. There are two categories of functions in layer 3: 1. Signaling Message-Handling Functions. During message transfer, thes