The A-Interface or airlink is the most visible of the required CDPD interfaces and is key to the provision of CDPD network services to mobile hosts. This is the interface between the mobile end-user and the CDPD system. Because of the numbers of mobile units (projected to be in the millions before the end of the decade) and the fact that they are owned and configured by end-users, getting the A-interface right is essential to the success of the CDPD technology. This is where the bulk of the "invention" in CDPD is located.
The A-interface is the profile of protocols between the M-ES and MDBS stacks in Figure 5.1. One of the more interesting aspects of this interface is the separation of network-side endpoints for the MAC and LLC sublayers. The shaded area in the figure highlights the protocols and services described in this chapter and defined in the 400's series of Parts5.1 of the CDPD System Specification.
In the following sections, we examine each layer of the A-Interface protocol stack. In our discussion, we will build up from the physical layer and progress towards the network layer.