Recently, there has been much discussion about interoperability among and between the various public safety entities. The Congress of the United States has expressed a large measure of concern and willingness to resolve the interoperability issues that were exhibited during such national disasters as the attack on the World Trade Center and Hurricane Katrina along the Gulf Coast. Resolution of these issues has even generated substantial federal government funding, as exemplified by the $1 billion grant program that Congress allotted for the Department of Homeland Security from the Department of Commerce. Consequently, this article looks at the fundamental precepts of interoperability, that is, at what it means and how it is achieved.

How It Is Defined

Definitions of interoperability range from purely generic interpretations to highly technical interpretations that apply to specific types of hardware, software, or systems.

  • Webster's Dictionary
    The Webster's Dictionary definition, based on a purely generic interpretation, defines the term as "the ability of a system (as a weapons system) to work with or use the parts or equipment of another system."
  • ATIS Telecom Glossary 2000
    Approved by the American National Standards Institute, the ATIS glossary provides five definitions from the Telecom Glossary 2000:
    1. The ability of systems, units or forces to provide services to and accept services from other systems, units, or forces and to use the services so exchanged to enable them to operate effectively together. [JP 1-02]
    2. The condition achieved among communications-electronics systems or items of communications-electronics equipment when information or services can be exchanged directly and satisfactorily between them and/or their users. The degree of interoperability should be defined when referring to specific cases. [JP 1-02]
    3. Allows applications executing on separate hardware platforms, or in multi-processing environments on the same platform, to share data and cooperate in processing it through communications mechanisms such as remote procedure calls, transparent file access, etc. [JP 1-02]
    4. The ability of a set of modeling and simulation to provide services to and accept services from other modeling and simulation, and to use the services for exchange enabling them to operate effectively together. [JP 1-02]
    5. The capability to provide useful and cost-effective interchange of electronic data among, e.g., different signal formats, transmission media, applications, industries, or performance levels.

International Standards Organization
This worldwide federation suggests in its draft technical report (ISO/IEC 2382-01, Information Technology Vocabulary, Fundamental Terms) that interoperability be defined as "the capability to communicate, execute programs, or transfer data among various functional units in a manner that requires the user to have little or no knowledge of the unique characteristics of those units."

IEEE Standard Computer Dictionary
The Institute of Electrical and Electronic Engineers (IEEE) provides the generally accepted definition of interoperability, at least from a technical perspective. It defines the term as "the ability of two or more systems or components to exchange information and to use the information that has been exchanged." See IEEE Standard Computer Dictionary: A Compilation of IEEE Standard Computer Glossaries (New York, NY: 1990).

The Federal Communications Commission has adopted the following definition of interoperability. Interoperability is defined in Section 90.7 of the Commission's rules as "[a]n essential communications link within public safety and public service wireless communications systems which permits units from two or more different entities to interact with one another and to exchange information according to a prescribed method in order to achieve predictable results."

How It Is Achieved

To an engineer, the multiple definitions above are interesting in that they describe interoperability for hardware, software, and systems but frankly are of little import because the engineer's job is to make things work. Thus, the traditional radio engineer attempts to ensure interoperability, at least among various radio entities, by achieving three objectives:

  1. Using compatible communications equipment operating on the same frequencies, with the same signaling characteristics, and the same operating procedures;
  2. Ensuring adequate signal coverage (i.e. transmit power, which translates to received signal power) over the area of interest (Recall – "If it doesn't work the first time, increase power!"); and
  3. Scaling the size of the radio network with additional transmitter/receivers by sharing the mutually agreed upon procedures.

Same operating parameters
The first objective is relatively easy to achieve by pre-ordaining the appropriate transmit/receive frequencies, modulation format, and other operating parameters with those entities wishing to share an interoperable radio network.  Hence, the Commission established the national mutual aid channels among other standards with published rules (Part 90 of 47 CFR) for their use by multiple entities.  The point here is that the first principle of interoperability is for all entities to use the same operating parameters. This fundamental precept is valid whether the network is one that uses radio transmissions or the network is a local, wide area, or international computer network.

Adequate signal coverage
The second objective is of limited flexibility since the Commission limits the amount of power that a particular transmitter may use depending on the frequency range of operation.  By nature, this constraint limits the potential range over which a signal can be transmitted.  Hence the robustness of radio interoperability is always dependent on the radio coverage afforded all of the users.  This also includes alternative connectivity provided by auxillary transmissions such as those used by some satellite services that are augmented by terrestrial transmitters.

The third objective is inherent in the operation of the network.  The operation of the network should not be degraded by the addition of more transmitter/receivers - a chore actually easy to suggest, but more difficult to achieve in practice due to one very important and thus far unmentioned factor.

Cautionary notes
Although the three objectives apply principally to purely radio networks, new alternative approaches are achieving interoperability by using "bridging" or networking approaches to solve the interoperability issue. These approaches include the use of radio network bridges or "gateways" that provide direct interface between disparate radio networks.  Alternatively, interoperability is accomplished by using gateway devices that convert the radio network traffic to/from Internet Protocol based messages for transmission via computer networks including the Internet at large.

Also note that the interoperability of a radio network completely depends on the ability of users to reach agreement over how that network is to operate.  Technically, the conditions and standards of the network must and will be established a priori but, when the information exchange procedures are not agreed on in advance and adhered to in operation, interoperability will not be attained.  Thus, the human factor may be the most important aspect of interoperability.

The SAFECOM Program, administered by the U.S. Department of Homeland Security, is attempting to solve these interoperability issues, as are many other government agencies including the Federal Communications Commission.