BOX 2.4 Layers in the Open Systems Interconnection Reference Model
In the area of communications, the Open Systems Interconnection reference model1 provides a useful framework for thinking about different levels or layers at which interoperability must be considered for C4I systems.
connectors and signaling. Physical interoperability depends on things like electronic/photonic interconnectivity (the commonality of voltages and waveforms that allow communications over physical interconnections) and the mechanical compatibility of connectors. Interoperability problems that arise when two C4I system components have not been designed to share a common communications grid are also in this layer.
Data link, network, and transport--the transmission of data (e.g., a message, binary representation of a picture, and so on) between C4I applications. For example, C4I applications that use different field radios with different framing and addressing conventions would not interoperate at the data link layer. To take another example, consider an application that produces and transmits data via a shared file and another application that expects to receive that data via a TCP stream. These two applications would not be interoperable, even if the applications agree completely on the semantics of the data and the battlespace models used to interpret the data.
Session, presentation, and application--the interpretation of successfully communicated
data. For example, two C4I applications may communicate position as three
spatial coordinates but not agree on whether spherical or Cartesian
coordinate systems are to be used, or which Earth model is to be used to
interpret spatial coordinates, and thus they will not interoperate. More
subtly, they will not interoperate if they do not agree on a common model of
the battle space or have a means of translating between two different models
(e.g., the scope and detail of information presented to a division command
in a common operating picture vis-à-vis that presented to a tank commander,
or the scope and detail of a common air picture for a naval air engagement
vis-à-vis that required for a land engagement).
1 International Organization for Standardization/International Electrotechnical Commission. 1994. Open Systems Interconnection--Basic Reference Model: The Basic Model, ISO/IEC 7498-1, International Organization for Standardization, Geneva.