 MAP, but for GSM systems. BSSAP and its subprotocols are sometimes referred to as GSM-MAP. BSSAP does not use TCAP but rather uses the services of SCCP directly. 14.4 Logical and Physical Models When we delve into the world of communications systems and message protocols, we run into the distinction between logical models and physical models. 2 Both the logical physical models of wireless system connectivity fall into the category of network reference models that show the interfaces among the basic functional components of a network. A logical model describes the components for how a system functions and the inter- faces between relevant pairs of components, whereas a physical model shows how the actual parts are put together. A simple example is the user terminal described in Sec. 3.1.1, where we consider the radio transmitter and receiver as logically separate items, even though they are physically designed as one transceiver unit. Consider three MSCs, each connected to the other two. Each MSC has an associated home location register (HLR) and visitor location register (VLR). We will explain what HLRs and VLRs are in more detail in Chap. 15, but for now, let us look at the logical network. Each MSC is connected to all three HLRs and to its own VLR. The logical model of this network is shown in Fig. 14.4. The physical layout may look nothing like Fig. 14.4, however. In real life, the MSC, HLR, and VLR may all live in the same cabinet, as shown in Fig. 14.5. The logical links connecting each MSC to its own HLR and VLR are software subroutines because they share the same computer memory and disk drives. Between each pair of boxes is a DS-1 pipe with 2 DS-0s used for each MSC-to-HLR connection and the other 20 DS-0s used for MSC-to-MSC data. If we were to walk in and ask to see the 9 MSC-to-HLR links shown in Fig. 14.4, then we would be told that 6 of them are time slots inside the three DS-1 links. Three of them do not physically exist as links at all. In fact, the left two MSCs in Fig. 14.5 may be colocated with