Download

16.1 INTRODUCTION 499Figure 16.1-1. TCAP protocol stack.Peer-level communication between TC-users (ASEs) located in remote nodestakes the form of dialogs, which consist of an exchange of components with thesame Dialog ID. TCAP supports two kinds of dialogs:1. Unstructured dialog2. Structured dialogUnstructured dialog allows the sending of one or more related components—in aUNIDIRECTIONAL message—to the remote end without expecting a response. Itis used to implement class 4 operations.Structured dialog is a true dialog with a beginning, a continuation, and an end,involving multiple messages. It allows the transfer of information betweenTC-users in full-duplex mode and is used to carry out class 1 through 3 operations.This kind of dialog is managed via the Dialog ID in TC dialog-handling primitives.The Dialog ID maps into the Transaction ID in TR primitives (see below).Peer-level exchanges between component sublayers are called transactions.Transactions are carried out by the transaction sublayer as a service to theTR-user (i.e., the Component sublayer). Dialogs map one-to-one into transactions.Communication within a node between the TC-user, TC sublayers, SCCP, andMTP is done via primitives, namely, TC primitives, TR primitives, N-unitdataprimitives, and MTP primitives (Fig. 16.1-2) [4–8].Figure 16.1-2 shows the SS7 entities involved in a transaction between ASEsbelonging to two AEs located in node A and node B, respectively. The physicalmessage path traverses the TCAPs, SCCPs, and MTPs at the nodes, and thesignaling network, which transfers MTP’s message signal units (MSUs) betweenthe nodes.16.1.3 PrimitivesThere are two types of TC primitives (see Table 16.1-1):1. Dialog-handling primitives2. Component-handling primitives