The approach taken by the CDPD specification team assumed that all objects could be defined by their interfaces and functions. Initially this philosophy was applied to the entire system.
Upon examination of the basic design goals and considerations of the CDPD network, the specification team found that they could be satisfied with a network providing an "over-the-air" interface to the mobile devices, an external interface to land-line hosts, and an inter-service provider interface to link multiple cooperating CDPD service providers.
This philosophy led to the development of the "CDPD cloud"3.13 depicted in Figure 3.3. With this model, it is necessary and sufficient to fully define the CDPD network by specifying the detailed interface to the mobile devices, external networks and other CDPD networks. Within the CDPD network "cloud", the necessary support functions of mobility management and data delivery can be defined separately.
Although this "cloud" approach addresses the stated requirements of the CDPD network, it does not address the practical side of developing a new network service. Any new network service deployment requires the development of new network infrastructure equipment. The technical specification of the new service must also define these components.
While the "cloud" approach to network system specification defines all the necessary information for network equipment development, it may result in vastly different internal network architectures. Different equipment vendors may conceive of different sets of equipment to provide the same functionality and interfaces. This is true regardless of the level of detail attained in the system specification, assuming it doesn't go so far as to specify an actual implementation.
Some of the service providers expressed concerns about the "cloud" approach of system specification. They recognized that this approach could result in networks that interoperate (over the I - Interface) but cannot share internal components. They were concerned that the RF equipment of one vendor would only operate with network routing equipment from the same vendor. This type of limitation would severely restrict a service provider's flexibility in equipment vendor selection. Indeed, most of these service providers have already lived under these types of captive marketing approaches in the cellular telephony world. They did not want this vendor-dependence to continue.
It was the service provider's initial discomfort with this aspect of the CDPD "cloud" that drove the specification team to then define individual components within the CDPD network. Some of these components are depicted in Figure 3.4.
