When CDPD was first conceived and specified in the early 1990's, the CDMA (IS-95) and TDMA (IS-54, now IS-136) standards efforts for North American cellular voice services were well underway. In fact, TDMA digital voice services were already being deployed in a number of markets. It was a foregone conclusion that both of these competing digital voice standards would eventually support data services as well.
Unfortunately, the schism between the North American cellular service providers which support CDMA and those which support TDMA shows little sign of closing. In early 1996, the only common North American cellular standard is still the analog AMPS (Advanced Mobile Phone System) standard. The common denominator for data services likewise remains CDPD.
In the future, it is likely that both CDMA and TDMA will be deployed throughout North America, thanks in large part to the new PCS spectrum. These standards are being extended to support data services, both in circuit-switched and packet-switched modes. The packet-switched modes are based on a CDPD system design-all that is changed is the necessary radio modulation techniques and protocols, primarily at Physical and MAC Layers. So rather than replacing CDPD, these digital cellular standards will instead adopt CDPD for their base data services-providing architecture.
Another more recent adoption of CDPD architecture is embodied in the new personal Air Communications Technology (pACT) announced by AT& T Wireless Services and others. This two-way messaging technology modifies CDPD to use the narrowband PCS channels auctioned in 1994-another example of CDPD operating over alternative airlinks. pACT is discussed in Chapter 9.
CDPD architecture was conceived with this kind of extensibility in mind. CDPD is more than an airlink specification-it is a system architecture for mobile data services which can support multiple RF technologies. One of the more common misperceptions in the trade press has been the eventual "replacement" of CDPD by emerging standards which were based on the new RF technologies.3.9
Another initiative is that of the Portable Computer and Communications Association (PCCA), which has defined standard APIs for the mobile computing industry since 1993. Their recent STD-201 wireless modem standard is based on the commonly-used Microsoft Network-Device Driver Interface Specification (NDIS), and will allow software developers to support wireless modes of operation to Windows applications. STD-201 complements the earlier STD-101 standard, also developed in the PCCA, which defined hardware- and network-independent extensions to the popular Hayes AT modem command set for wireless operation. CDPD is one of the primary target networks for these interface specifications.