It's every cellular user's worst nightmare: "I'm sorry, but your phone does not work in this area."

Users expect to be able to take their code division multiple access (CDMA) phones with them when they travel-after all, their analog cellular phones have always worked in other cities. They don't want to find out after they've committed to CDMA that their options are limited based on what city they are in or who the service provider is. They also don't want to find out that although their phones work on the road, the dazzling array of enhanced features don't. For the CDMA industry, these are but a few of the worst-case customer scenarios that a collaborative industry interoperability test program is designed to avoid.

Cooperate...Interoperate What is interoperability, anyway? It is the ability of one manufacturer's equipment to function properly with equipment from other manufacturers without modification. The long-established U.S. analog cellular system is the perfect example. Although the network infrastructure in a given city could be provided by Ericsson, Lucent Technologies, Motorola, Northern Telecom or another manufacturer, users could be certain that they could take their analog cellular phones practically anywhere in the U.S. and make cellular calls.

In fact, the system worked so well that most users probably never even considered that their phones might not work in some locations. Of course, the analog cellular industry is quite mature, and years of experience have made it what it is today.

For CDMA equipment manufacturers and service providers, interoperability means that personal communication services (PCS), cellular or dual-band CDMA handsets from any given handset manufacturer will work wherever there is a CDMA-capable network operating in the corresponding frequency band (cellular and PCS operate at different frequencies), regardless of the network infrastructure manufacturer.

Conversely, interoperability also means that handsets from various manufacturers will all work properly on any given manufacturer's network infrastructure. In a nutshell, a CDMA handset bought and used in Los Angeles will also operate in the CDMA network in New Jersey (or wherever a CDMA-compliant network in the handset's frequency band exists). Although this goal may appear obvious from a customer viewpoint, achieving interoperability is not as easy as it may seem-even with the existence of industry standards.

The concept of interoperability based on industry standards is not new-all telecommunications technologies are governed by industry-accepted standards that define basic operation and functionality. Although strict adherence to industry specifications is critical for system interoperability, it typically will not guarantee compatibility between different manufacturers' equipment. In fact, compliance to industry specifications is only the beginning of the overall interoperability process.

Generally, industry specifications do not dictate the exact implementation of a given feature or how to process a certain message but only what must be included and what should be communicated between the various pieces of equipment. The actual implementation of the functions outlined in the specifications is left to the interpretation of the respective equipment manufacturers.

Therein lies the rub. Each manufacturer's interpretation and subsequent implementation will vary, and a given manufacturer's implementation may be less than optimal or based on an "incorrect" interpretation of a standard. Because of this potential problem, a well-designed interoperability testing and acceptance program is critical to the overall success of any new technology or new application of an existing technology such as CDMA.

Equipment manufacturers and service providers that are committed to successful CDMA deployment realized early on that industry-level cooperation was critical. Although each party has a different set of motivating factors for participating in the interoperability test process and assuring industry interoperability, they all share a common goal: successful CDMA deployment.

For service providers, infrastructure and subscriber equipment interoperability is a prerequisite for doing business with equipment manufacturers. Without interoperability, a service provider would be severely limited in the types of handsets and feature options it could offer to its customers. Currently, many of the larger PCS and cellular service providers are deploying equipment from multiple infrastructure manufacturers across their markets. In addition, for service providers pursuing a "national" presence, establishing a seamless, nationwide network would be impossible without interoperability.

For infrastructure manufacturers, interoperability is important in servicing their primary customers, the service providers. With a limited but growing customer base (thanks to the PCS auctions), infrastructure manufacturers must be able to attract new customers and satisfy existing customers and their corresponding contractual commitments.

For handset manufacturers, interoperability is especially critical. It is expected that handsets from a given manufacturer will be used on multiple types of infrastructure equipment. Without interoperability, a handset manufacturer might have to customize its handset for each type of infrastructure equipment-a logistical and distribution nightmare.

Moreover, should an interoperability problem be discovered after deployment, it is more difficult and painful for everyone-especially the subscriber equipment manufacturers, service providers and affected customers-to recall thousands of handsets than to revise infrastructure software. To ensure interoperability, handset manufacturers must test their handsets with each of the infrastructure manufacturers to help highlight and resolve issues. This requirement is understood by service providers and is frequently reflected in formal contractual commitments.

With interoperability clearly a priority for all parties, the next issue is how to develop a consistent interoperability test program.

The Role of the CDG To facilitate industry interoperability and to address potential standards-related problems and issues in the rollout of the new CDMA technology, the CDMA Development Group has taken an aggressive role in developing interoperability testing procedures for infrastructure and handset manufacturers. These procedures set a minimum standard for CDMA interoperability testing and provide manufacturers with a baseline document to use in creating their own documents, if they so desire. The test categories are shown in Table 1.

As an industry forum, the CDG commissioned a cross-industry team consisting of service providers and subscriber, test and infrastructure equipment manufacturers to develop comprehensive testing procedures and qualification plans that are used as the basis for ensuring interoperability across the industry.

The CDG Interoperability System Test Group has developed testing standards for both CDMA cellular and PCS using a three-stage model, with each subsequent stage encompassing a progressively higher level of interoperability testing (see sidebar).

Ideally, completion of these three stages would be straightforward and ensure full functionality for CDMA equipment, but the application of CDMA technology is still relatively new and thus requires additional fine-tuning and test optimization.

To address and resolve these and other issues that may arise from interoperability testing, the CDG has also formed the Interoperability Analysis Team. This team works in conjunction with the Interoperability System Test Group to ensure resolution of interoperability conflicts and other technical issues, including specification changes and new compatibility problems.

Interoperability testing is a parallel and iterative activity between handset and infrastructure manufacturers (along with service providers and test equipment manufacturers) that are working together to implement an interoperable CDMA system (Figure 1).

One key issue facing the industry is the varying development and market deployment schedules for the wide range of features that CDMA technology enables. For a specific feature to be tested, both sides of the system-infrastructure and subscriber equipment-must support the feature in their software loads, and efforts to accelerate interoperability testing help coordinate and accelerate feature development. This process has fostered an unprecedented level of CDMA industry cooperation in interoperability testing to ensure successful CDMA deployment.

Infrastructure manufacturers have set up dedicated interoperability test lab facilities to enable handset manufacturers to perform Stage 2 testing and resolve specific interoperability problems.

Handset manufacturers are sharing handset design and operational information with both infrastructure manufacturers and service providers, allowing quick resolution of potential interoperability issues.

Service providers are working closely with infrastructure, handset and test equipment manufacturers to optimize overall performance in both the testing labs and the field, where they are allowing their newly deployed CDMA networks to be tested by various manufacturers. For the CDMA industry, interoperability testing and development have been, and will continue to be, excellent vehicles for bringing the two sides of the system together to help speed development of quality services for the market.

Interoperability testing is recognized by the industry as an essential process for ensuring rapid, coordinated and successful CDMA deployment. CDMA interoperability testing has allowed manufacturers to work together to ensure successful CDMA deployment from the beginning. Through continuous optimization facilitated by the CDG interoperability test program, potential customer and system-affecting issues are identified and resolved early in the development process, making solutions less costly for all parties.

Robert Chinn is a Manager and Mark Levy is an Associate at Pittiglio, Rabin, Todd and McGrath (PRTM), Mountain View, Calif.