<specification, protocol> (FDT) A formal method for developing telecomunications services and protocols. FDTs range from abstract to implementation-oriented descriptions. All FDTs offer the means for producing unambiguous descriptions of OSI services and protocols in a more precise and comprehensive way than natural language descriptions. They provide a foundation for analysis and verification of a description. The target of analysis and verification may vary from abstract properties to concrete properties. Natural language descriptions remain an essential adjunct to formal description, enabling an unfarmiliar reader to gain rapid insight into the structure and function of services and protocols.