a large portion of the interface coordination communication a constant flow of effective information the characterization of specific criteria initiation of critical subsystem development the fully integrated test program the product configuration baseline any associated supporting element the incorporation of additional operational constraints the independent function principle a consideration of system and/or subsystem technologies the sophisticated hardware the anticipated third generation equipment the subsystem compatability testing the structural design the preliminary qualification limit the philosophy of commonality and standardization the greater effectiveness and reliability any discrete configuration made the total system rationale the evolution of specifications over a given time the programmability of a microprocessor controlled operating system the specification of least cost routing the future developement of a digital EPABX