EI value to the receiving hosted node. Let's also take a look at how hosted nodes on the network side make use of LAPD. Figure 3.23 shows the hosted node on SAPI=0, which in ISDN is part of the call-handling logical network. This node depends on a very large number of LAPD hosting networks (one for each subscriber line), of which we only indicate two, here denoted nw1={n11,n12} and nw2={n21,n22}. The call-handling network is called nw3={n31,n32,...). Local exchange LAPD node in terminal SAPI=0 Call-handling layer element {na1,...} message on te 127 T-ROs T-ROs te5,te9,te127 AddrPar te127 (na1) {sap0,te127 REdef (na2) Fig. 3.23 LAPD layer elements in a local exchange A scenario is depicted: The call-handling layer element in the local exchange receives an (ISUP-) mes- sage over SS7 that requests a call to a subscriber terminal, identified by the ISDN network address na1 (actually called "subscriber number" in ISDN). The layer element analyzes this message and then creates a call-handling (DSS1 L3-) message to be sent to the actual subscriber terminal. It must then find the LAPD route to that terminal. Since there are many such routes, this requires a routing analysis and therefore a routing table, T-ROs. The routing table gives the REi=n32, which is the call handling node in n12. The call-handling layer element in the local exchange then uses a T-RE table to find the LAPD hosting network to use. As was shown previously, LAPD does not require any address parameters, which is why only the NWi part of an REdef is needed in this table. The data of the DSS1L3 message is then sent over the broadcast connection te127. The reason is that (in ISDN) a terminal site may actually comprise many internal physical terminals, each having an n32 node. All of them listen to te127, but the node that actually takes the call must, when answering, use a TEI that is assigned to it, which in this scenario is either te5 or te9. The chosen TEI will then be used in the dialogue that follows between this terminal node and n31. 3.4 Route Type