 horizontally-partitioned network system. An evolution in that direc- tion of public networks is already on-going, sometimes denoted the "next gener- ation network" (NGN) and already applied, e.g., in the UMTS network system ("3G"). 3.4.3.3 Switched Routes in SS7 In SS7, the switched routes are realized by MTP L3 which, as route identification is concerned, is an X.25 network enhanced by the layer-APi method (see Fig. 3.26). MTP L3 manag. protocols SCCP, ISUP, TUP, etc. (MTP L3) MTP L3 switched routes SI=2..15 Signaling link set (SLset) SI=0,1,8 Signaling link (SL) {destSl, DPC, OPC, SLS,...) T-ROs MTP L3 MTP L2 Signaling data links T-RE(MTP L3 user): REi->DPC Circuit-switching X. & V. T-ROs(MTP L3): DPC-SLset (=set of SLs) layer element layer elements T-RE(MTP L3): SL-SDL T-RE(MTP L2): SDL-REdef Fig. 3.26 The MTP L3-switching logical node 3 Node Structures Hosted nodes in MTP L3 are of two classes: nodes which perform "signaling network management" (a control layer) and nodes of logical networks (primarily the PSTN/ISDN call-handling layer) that use the MTP L3 datagram service. MTP L3 offers a single type of switched routes: datagram routes, which are addressed by hosted nodes by MTP L3 network addresses, called signalling point code (SPC). Thus, hosted nodes handle a T-RE where the REdef for a route comprises a single destination point code (DPC). Each MTP L3 node knows its own SPC. When an MTP L3 message is sent by a hosted user, the MTP L3 layer elements adds its SPC as the originating point code (OPC). When a message is just relayed through the node, nothing is changed in the message. MTP L3 applies the layer-APi method. It must therefore consult a T-AP table (NWi<>APi translation) both when sending and receiving. The APi in MTP L3 is called service indicator (SI), and has a range of 16 values. MTP L3 is used both in national and international networks. How to assign SI values to hosted nodes is defined only for international use as follows: SI values 0,1 and 8 are reserved for layer-management networks and 