Tech Topic 1: Interoperability
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 first technical
topic 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
- 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]
- 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]
- 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.
- 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]
- 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:
- Using compatible communications equipment operating on the
same frequencies, with the same signaling characteristics,
and the same operating procedures;
- 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
- 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
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.
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