me of these aspects were already considered when the OSI RM defined its transport layer. Here we will only comment on the first bullet in the list, starting with how SS7 deals with it. From the outset, only a few MTP L3 call-handling applications were foreseen (only six are defined, e.g., TUP and ISUP). As time went by, it became clear that more suitable services would be needed and that more applications would appear than could be accommodated by the APi. Since the range of the MTP L3-APi could not be increased, MTP L3 was instead extended with another layer (the SCCP) that could support a larger range of APi. This resulted in an SCCP/MTP L3 switching stratum, that consists of two layers, each defining its own APi, as shown in Fig. 3.28. Socket routes Other Other SIs SSNs calledSSN, {calledSSN,callingSSN,... {SI=SCCP, DPC, OPC, SLS T-ROs MTP L3 Signaling links T-RE(SCCP-user): REi->DPC,calledSSN T-AP(SCCP): hostedNWi->cailingSSN MTP L2 Fig. 3.28 Socket routes in SS7. SI=service indicator; SSN=subsystem number It is important not to look at SCCP as a common part of a hosting network (hosted in MTP L3). SCCP handles no T-RE, which means that there is no such thing as an SCCP logical network. The only logical network is the combined SCCP/MTP L3 logical network, defined by a two-layer stratum (similar as we view the TCP/IP stratum in IP-based networks). Again we have to learn some new terms for the same things: The APi of SCCP is called subsystem number (SSN), the sending side in SCCP is a calling party, the receiving side a called party. SCCP applies the layer-ele- ment APi method, which implies that routes might be used between hosted nodes that are assigned different SSNs. Let's also repeat the MTP L3 terminology: the APi of MTP L3 is called service indicator (SI). Network addresses are signalling point codes (SPC). A destina- tion network address is therefore a destination point code (DPC) and an origi- nating network address is an originating point code (OPC). MTP L3 applies the 3 Node Structures layer APi meth