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702 SIGNALING FOR VoIPFigure 21.5-6. IUA message header.Figure 21.5-7. Protocol Data parameter for IUA Data messages.Fig. 21.5-8: the LAPV5 protocol is terminated at the SG, and V5.2 L3 applicationsreside in the ASP. V5UA inserts an additional header, immediately after thecommon header, which is virtually the same as the IUA additional header(Fig. 21.5-6). The only difference is that the unused field is used to carry the envelopefunction address (EFA), to identify the V5.2 L3 protocols carried inside theLAPV5 PDU. The messages carrying V5.2 L3 PDUs have the same format asthe corresponding messages for IUA. For a complete list of V5UA messages andtheir formats the reader is referred to [20].21.5.5 SS7 MTP2 User Adaptation Layer Protocol (M2UA)This Adaptation Layer module supports MTP2 User communication between anSS7 signaling point in the SCN and an ASP (such as a MGC or an AS) via a signalinggateway. It provides backhauling of MTP3 messages between the SG and theASP. The protocol stack and the signaling architecture are shown in Fig. 21.5-9.Figure 21.5-8. V5UA signaling architecture. (From RFC 3807 [18].)