CDPD is an enabling technology which provides support for mobility in the WAN environment. To accomplish this, CDPD provides three kinds of services-network services, network application services and network support services. As Figure 3.2 depicts, these services provide support for applications ranging from stationary vending machines to mobile vehicle tracking. The services also support applications ranging from telemetry (extremely low data rates) to interactive PC-based applications such as remote server access.
Network services are data transfer services-the capability of moving data from one location to another. This is the basic service type offered by CDPD. It neither adds value nor content to what the user intends; it is simply data carriage to and from a mobile device.
In terms of data networking, CDPD provides support for routable connectionless peer-to-peer Layer 3 protocols, such as IP and CLNP. Other Layer 3 protocols, such as IPv6 are likely to be supported in the future as well. By definition, Layer 3 is the layer responsible for getting data from one point to another across one or more networks.
Part of basic network service is the provision of access to systems and services-both public and private-external to CDPD. These services can be data resources or networks. In most cases a mobile user sends data to and receives data from a resource external to the CDPD network. CDPD simply provides a means of accessing that external resource and could simply be regarded as an extension of existing data communications networks which are IP- or CLNP-based.
Basic CDPD network services are summarized in this overview chapter and described in more detail in Chapter 4 (mobility management) and Chapter 5 (network access).
Network support services are the services necessary to support the operation of a mobile data network. Network support services include things such as network management, usage accounting and security, which are necessary to operate any network. CDPD network support services also include things such as mobility management and radio resource management, which are necessary for mobile wireless WANs.3.10
In theory, an end-user of the system could use the network services while perfectly oblivious to the existence of the network support services (at least as long as these services are running correctly, save accounting!). In many cases the support services could be considered to add "intelligence" to the system; an example is the fault recovery actions taken by network management when an exceptional condition arises.
CDPD network support services are described in Chapter 6 (security) and Chapter 7 (other support services).
Network application services are services which add value or content to a user's activities above and beyond basic data carriage. The end-user is typically quite aware of these value-added services and often must explicitly subscribe to them. These services in CDPD could leverage off of the mobility of the user, such as subscriber location services or limited size messaging services. It is also possible that these services could be independent of mobility, such as advanced messaging capabilities.
The CDPD specification includes technical descriptions of mobility-enhanced value-added services. Other CDPD network application services could be based on the intrinsic broadcast and multicast capability defined in the CDPD System Specification.
Some of these network application services are described in Chapter 8 (limited size messaging).