CDPD architecture defines three key external interfaces, as depicted in Figure 3.4. Because these interfaces-"A", "E" and "I"-form the logical boundaries for a CDPD service provider's network, they are essential to the proper operation of CDPD.
Other lesser interfaces are defined within the CDPD service provider "cloud." However, since these internal interfaces are under the control of a single CDPD service provider, their specifications could be considered to be recommendations rather than requirements.
Although this may sound contrary to our previous discussion, flexibility is required in internal interfaces because of the different network implementation approaches favored by the various CDPD service providers. It is important to differentiate between technical specification and implementation requirements.
Each of the interfaces defined in the CDPD specification includes a profile representing the protocols supported or required at each layer in the OSI Reference Model. An example of these profiles is displayed in Figure 3.5. Well-defined primitives at each layer request services of the layer below; the services provided to each layer consists of the collective set of services provided by all of the underlying layers.
The A-Interface or airlink is the interface between the CDPD mobile device and the CDPD network and contains much of the "invention" of CDPD. This is the point at which CDPD network services are accessed by a subscriber and is described in detail in Chapter 5 (network access). It is defined by Parts 400 through 409 in the CDPD System Specification. Although the airlink receives much attention, this is only one part of the overall CDPD system architecture.
The External or E-Interface of CDPD is the means by which CDPD interoperates with the rest of the world and is key to the provision of CDPD network services. Conventional data networking protocols are used for data carriage between CDPD and external data services such as the Internet, VANs, wide area transport providers or private networks.
The Layer 3 protocols supported at the E-Interface include the same connectionless protocols as within CDPD-IP and CLNP. IPv6 is likely to be supported at a later time as it matures. Other protocols, such as APPN (via MPTN) or IPX, could be supported either via encapsulation (say within IP packets) or via protocol translation gateways. Either of these techniques is outside the scope of the CDPD specifications.
Border gateway protocols such as BGP-4 and IDRP are also recommended at the E-Interface. This is necessary because the E-Interface specifies a boundary between two autonomous systems-that of a CDPD service provider and that of an external party. Initially, static routing is likely to be employed at the E-Interface for CLNP traffic.
The E-Interface is intended to be no different than the interface to any other autonomous system. All of the issues of security, routing, name-to-address translation, etc., apply. The fact that the CDPD service provider supports mobility is transparent to the E-Interface, by design.
As we discussed in Chapter 2, the North American cellular service environment is partitioned into markets which are served by a multiplicity of service providers. Seamless nationwide coverage (a goal of CDPD) requires these service providers to be capable of supporting each others' customers. Seamless coverage also requires the capability for a transparent and smooth transfer from one system to its neighbors.
The Inter-service provider or I-Interface defines the means by which the CDPD service providers can collectively provide a seamless nationwide service. From a purely technical viewpoint, there is nothing preventing CDPD service from being offered worldwide.
The I-Interface supports the same protocols as the E-interface plus the Mobile Network Location Protocol or MNLP. This protocol is the means by which mobile users from one system are supported by another system and is a key piece of the CDPD mobility management scheme. Additional protocols for network management (CMIP) and accounting (X.400-based) are also defined for the I-interface. All of the protocols across the I-interface are based on CLNP.3.14