The difficulty of interconnecting networks or network components when they obey different architectures is an important inhibitor to computer communication growth and flexibility. The author reviews the state of the protocol conversion art from the architectural point of view. The aim is to state the problem succinctly, propose consistent definitions, review current ad hoc solutions, and speculate about more effective and general methodologies. To do this, the author first analyzes what happens on an access path between two end users at the discontinuity point where the path passes from a portion of the network obeying one architecture into a portion obeying another. This discontinuity can be designed to occur at a choice of nodes along the access path and at a choice of protocol layer level at each such node.