the subscribers and to support operations, administration, and maintenance (OA&M) for the wireless service provider. In ANSI-41, the notion of a wireless system is the MSC and its associated base sta- tions. Elsewhere we have referred to a collection of MSCs and base stations as a single system because it is owned by one vendor and it serves one subscriber community. For the rest of this chapter, however, a system is an MSC, and intersystem activity is any- thing going from one MSC to another, whether the two MSCs are operated by the same provider or not. The HLR is the home location register, a system that keeps track of the status of each user terminal at its home base. Even the simple mobile telephone call described above may involve some communication between MSC and the HLR, so ANSI-41 may yet be involved in a typical call. The interface between MSC and HLR is the C interface de- fined in ANSI-41. The VLR is the visitor location register. Its B interface to the MSC and its D interface directly to the HLR are both defined in ANSI-41. VLRs are allowed to communicate with each other using a G interface not defined in ANSI-41. HLRs can communicate with authentication centers using an H interface not defined in ANSI-41. MSCs communicate directly with other MSCs over ANSI-41-defined E interfaces. The ANSI-41 network reference model has grown to look like Fig. 15.2. 2 The basic picture in Fig. 15.1 has changed a little, but a lot has been added. The CSS has been renamed the mobile station (MS), and the H interface between the HLR and the au- thentication center (AC) is now part of ANSI-41. Thus we have a fully specified signal- ing network within ANSI-41 connecting MSCs, HLRs, VLRs, and ACs to authenticate mobile subscribers and to let them roam and hand off where they please. The SMS message center (MC) is connected to the MSC by the Q interface and to the HLR by the N interface, both ANSI-41-specified. The M 1 interface from the MC to the short message entity (SME) is specified. Also specified in ANSI-41 are the 