Introduction This Part specifies the Cellular Digital Packet Data (CDPD) Mobile Data Intermediate System (MD-IS) . The CDPD Network provides data communications services to subscribers over a wide geographic area. The distinctive nature of the CDPD Network lies primarily in the ability to provide this data connectivity to mobile users. The Mobile End System (M-ES) may therefore change its subnetwork point of attachment at will. The CDPD Network thus provides the service of managing the mobile data connectivity of its subscribers. The MD-IS has the primary responsibility for providing this mobile data connectivity management. Within the MD-IS@ the Mobility Management function is defined as follows: a. Mobile Home Function (MHF) b. Mobile Serving Function (MSF). The MD-IS participates in the provision of the CDPD Network support services. The support services involving the MD-IS include: a. Network Management Services b. Accounting Services c. Multicast Service d. Broadcast Service e. Subscriber Authentication and Authorization Service f. Subscriber Location Service g. Airlink Encryption Service h. Compression Service. The following sections describe the MD-IS role within each of the functions and services. Section 11 subsequently details the functional and communications protocol profiles for the MD-IS.