CDPD mobility management is based on principles similar to mobile-lP. The details are shown in Figure 9.4. The MD-IS is the central element in the process. An MD-IS is logically separated into a home MD-IS and a serving MD-IS. A home MD-IS contains a subscription database for its geographic area. Each subscriber is registered in his home MD-IS associated with his home area. The IP address of a subscriber points to his home MD-IS. At the home MD-IS, an MHF maintains information about the current location of MHs associated with (homed at) that home MD~lS. The MHF also encapsulates any packet that is addressed to an M-ES  homed with it directing it to an MSF associated with the serving MD-IS, whose serving area the M-ES is currently visiting. A serving MD-IS manages one serving area. Mobile data BSs that provide coverage in this area are connected to the serving MD-IS, whose MSF contains information about all subscribers currently visiting the area and registered with it. The MSF employs the mobile network location protocol (MNLP) to notify the MHF about the presence of the M-ES in its service area. The channel stream in which a subscriber is active is also indicated. The MSF decapsulates forwarded packets and routes them to the correct channel stream in the cell.