Applying layering principles to legacy systems: Link 16 as a case study

Layering communications systems (for better horizontal integration and to foster interoperability) is central to the Global Grid architecture. Accordingly, it is important to show how legacy systems can be layered in order to direct future development efforts toward achieving greater compatibility with the Global Grid. This paper assesses whether the Link 16 legacy system can be layered within a Global Grid model that has been developed with military requirements in mind. This model is slightly different from the standard OSI 7-layer one and has the following 7 layers: mission, application, service, transport, network, link and physical. Two approaches to layering are considered. First, in a general way,the feasibility is assessed of layering the entire communications process at every level of the protocol stack. Each layer is examined to see if the different functions can be naturally separated into distinct sublayers. This approach emphasizes the desirability of being able to upgrade system performance by replacing one layer at a time without altering other layers. The second approach is to consider the layering potential of systems in terms of their compatibility with the emerging Global Grid. The main thrust of these concepts is that all systems that are part of the Global Grid should be compatible with its transport and network layers (i.e., TCP/IP and/or some variants). Link 16 is a prime example of a "stovepipe" system, designed to maximize performance in a particular tactical environment with high levels of jamming. Since it was designed before layering principles were in wide use, it presents certain difficulties to the layering process.