SI=3 for SCCP. The remaining twelve SI values are intended for other hosted networks, which (with the exception of SCCP) all perform some type of call-handling functions in PSTN/ISDN networks. Still, almost 20 years after the standard was published, only seven of these values are assigned. MTP L2 nodes are interconnected by link routes that are called signaling data links (SDL). These rely primarily on 64-kbps circuit-switched connections through the switching network (often referred to as MTP L1 routes). SDLs are semi-perma- nent links, established by MTP L2 management. An MTP L3 node must handle a routing table (T-ROs), since every node in an MTP L3 network can both route and terminate data. To allow for some flexibility regarding failures on the MTP L1 level, as well as for higher transfer rates than 64 kbps, several SDLs may connect the same two MTP L3 nodes. A specific layer that runs on SI=0 defines and manages such groups of SDLs. 6 It calls such a group sig- naling link set (SLset), and the individual channels signaling link (SL). T-ROs and T-RE tables in MTP L3 therefore rely on SLset, SL, and SDL, as shown in Fig. 3.26. Note that an MTP L3 message also includes a parameter called signaling link selection (SLS). This parameter has several applications, depending on the hosted network that initiated the message. One is to identify the signaling link SO that data can be transferred in sequence in the MTP L3 network, similar to the purpose with the virtual circuit identifier in CON networks (as e.g., in ATM). This feature actu- ally makes MTP L3 behave like a CON network without having to establish routes on-demand. Note that this layer is not described as a separate layer in the standard, but just as a function, well intertwined with other functions in MTP L3. This could not happen if AMLn had been used for modeling. A similar set of management functions are used in IP (see Sect. 3.4.3.4), but they are described as separate layers (as they should be). 3.4 Route Type Examples Requirements on reliability of 