During 1991 IBM and McCaw Cellular Communications, Inc.3.2 , began a collaborative effort to determine the feasibility of overlaying a digital packet-switched data network on the North American AMPS analog cellular system. This joint venture resulted in a proof of concept implementation at McCaw's headquarters in Kirkland, Washington, at the end of the year. The technology was named "Celluplan II" by IBM, the provider of the initial conceptual framework for the technology.3.3
The initial prototype system used a private partition of cellular channels in a single cell to demonstrate the concept of frequency-hopping, also called channel-hopping3.4 . In this demonstration, the RF coverage exceeded that provided by AMPS because of the digital GMSK modulation and the Reed-Solomon (63,39) forward error correction coding employed.
Following the demonstration's success, plans were made for extending the scope of the project to include a field trial of a larger system. The technology was renamed "Data Over Cellular" and again renamed Cellular Digital Packet Data or CDPD. From this point forward the prototype CDPD effort was dominated by schedules which could be charitably characterized as highly aggressive and accompanied by hyperbole to match.
In order to standardize the technology and increase the geographic coverage of the eventual service, McCaw enlisted the support of other large cellular service providers. The public announcement of CDPD and its backing by eight of the largest North American wireless services providers was made in April, 1992. In May, 1992, these wireless services providers (Ameritech, Bell Atlantic, GTE, McCaw, Nynex, PacTel, Southwest Bell, US West) staged a CDPD Technical Conference in Santa Clara.
The Santa Clara conference drew more than 600 attendees, reflecting the widespread interest in mobile data communications. Copies of a preliminary technical specification (Release 0.1) were distributed and plans for an upcoming field trial in the Bay Area were disclosed.
The early CDPD system architecture was telecommunications-oriented. A modified version of SCCP from SS7 provided the connection-oriented transport service, then considered necessary for support of mobile devices. This architecture required gateway services3.5 to interconnect with the rest of the data networking world, not unlike the competing RAM and Ardis systems. The RF channel used the same GMSK modulation with Reed-Solomon (63,39) forward error correction as in the earlier demonstration system. The RF MAC sublayer was specified with both polled and contention-based modes of shared channel operation.
The so-called "field trial" took place in the Bay Area during the latter half of 1992 and validated the radio resource management operation of the CDPD overlay on cellular systems. Channel hopping, cell transfer and interference avoidance were all exhaustively tested. Suspicious police often followed rented antenna-clad Cadillacs occupied by test personnel and equipment slowly cruising Camino Real in the dead of night, when potential cellular voice customer impact would be minimized.
The results of the trial were sufficient to convince seven of the cellular service providers supporting the effort (Ameritech, Bell Atlantic, GTE, McCaw, Nynex, PacTel, Southwest Bell) to continue the development of the technical specifications.
The early focus of the technical specifications effort was on the RF aspects of the system. The end result was a Reed-Solomon (63,47) forward error correction designed to increase the effective user bandwidth of the GMSK-modulated bitstream and a contention resolution scheme, similar to Ethernet, which provides both collision avoidance and collision detection. This MAC protocol is called slotted non-persistent Digital Sense Multiple Access (DSMA) with collision detection.
Although the 1992 CDPD specification team focus was on robust airlink protocols, work continued on the overall system architecture. In the fall, several members of the team concluded that the telephony-based system architecture was inappropriate for the services to be provided by CDPD3.6 . During the first week of December, a five-person subteam designed an alternative architecture, informally dubbed "CDPD Lite."
The "CDPD Lite" architecture was based on existing data networking standards and on the open connectionless Layer 3 protocols (IP and CLNP) that were available. This open architecture eliminated the need for gateways and leveraged conventional network recovery mechanisms to help support transient mobility. Its mobility management scheme was based on the early work of the IETF Mobile IP task force. This architecture, elegant in its simplicity, was later adopted as the "official" CDPD architecture by the group of seven cellular service providers who continued to support the effort.
The first half of 1993 saw the completion of the Bay Area "field trial." Preliminary specification releases of the new CDPD architecture were published in March (Release 0.8) and May (Release 0.9). The first official release of the specification (CDPD System Specification Release 1.0 [CDPD93]) followed in July. All of these published releases embodied the "CDPD Lite" architecture.
The second half of 1993 saw the initial development of legitimate CDPD infrastructure and mobile devices. Separate efforts by McCaw and a collaboration of other cellular service providers resulted in two demonstrations of CDPD operation at the large Comdex trade show in Las Vegas in November. This rapid four-month specification to demonstration timeframe reflects the benefits of the open CDPD architecture.
In 1994, the cellular service providers behind the CDPD specification development created the CDPD Forum to enlarge the base of support for CDPD. This trade association of service providers, infrastructure and mobile unit vendors, and software and applications developers continues to have as its objective the support and promotion of CDPD as a basis for mobile data applications.
The CDPD Forum supported the development of CDPD System Specification and Implementor Guidelines Release 1.1 [CDPD95] during 1994. This release was published in January, 1995, and includes enhancements to the radio resource management procedures, the Mobile Data Link Protocol (MDLP), accounting and multicast capabilities as well as protocol test specifications and implementation guidelines. The primary purpose of Release 1.1 was to finish the definition of incomplete or unclear capabilities of CDPD Release 1.0.
Release 1.1 also restructured the CDPD specification into a System Specification and Implementor Guidelines. This restructuring provided a better distinction between stable protocols (which were placed into the System Specification) and other Parts3.7 which might be less mature or more implementation-specific or otherwise unsuitable for a system standard (and which were placed into the Implementor Guidelines3.8 ).
In 1995, the CDPD Forum developed certification test plans for mobiles and initiated a selection process for an agency to conduct these tests. Further extensions to CDPD were contributed by Forum members and ratified by the Technical Steering Committee in the areas of circuit-switched access to CDPD services and limited size messaging. By mid-1995, the CDPD Forum had almost 100 member companies, reflecting the diverse and dynamic interests in CDPD technology. It continues to actively support the contributions of members to CDPD standards and implementor guidelines development. A standards track process has been developed which is loosely modeled after that of the IETF.
Separately, a Service Provider Corporation was created in 1995 to provide a forum for resolving inter-service provider concerns. This SPCo manages the activities of the CDPD Network Information Center (NIC), which include administration of CDPD network address blocks, DNS names and unique CDPD identifiers amongst the service providers.
Over the course of the year, CDPD service providers deployed services and expanded the customer base for those services. At year-end 1995, CDPD services were offered in over thirty markets, as depicted in Figure 3.1. Inter-service provider testing and interconnection was also well underway, with agreements and interoperation amongst the major service providers in place by early 1996.
Time will tell just how successful CDPD will be in terms of commercial adoption. In any case, the technology is likely to influence future mobile data systems. Both the CDMA and TDMA Forums are developing technical specifications for packet data services whose mobility management mechanisms are identical to and will interoperate with CDPD; this will allow service providers to offer CDPD services via multiple airlinks.