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21.5 THE SIGNALING TRANSPORT (SIGTRAN) PROTOCOLS 701Figure 21.5-3. Common message header for the adaptation Layer.Figure 21.5-4. Variable-length parameter format for the adaptation layer.ASP (such as a MGC or an AS). The protocol stack and the signaling architecture areshown in Fig. 21.5-5: the Q.921 protocol is terminated at the SG, and the Q.931application runs in the ASP. IUA inserts an additional header in its messagesimmediately after the common header (Fig, 21.5-6). The header carries the InterfaceIdentifier, which identifies the physical SCN interface, and the Data Link ConnectionIdentifier (DLCI), which contains the SAPI and TEI of the LAPD interface.Q.931 messages are carried in Data messages of the QPTM class, inserted as ProtocolData parameters (Fig. 21.5-7). For a complete list of IUA messages and theirformats the reader is referred to [19].21.5.4 V5.2 User Adaptation Protocol (V5UA)This Adaptation Layer module supports V5.2 access interfaces on the SCN side andprovides backhauling of V5.2 messages between a SG and an ASP (such as a MGCor an AS). The protocol stack and the signaling architecture are shown inFigure 21.5-5. IUA signaling architecture. (From RFC 3057 [17].)