size recCnIRepCnf(...,SDU Sequence not guaranteed Unreliable CNL-CSP No flow control Fig. 5.32 Possible functions of a transport service adaption handler (TAH) The EIA553 Case Study 5.5.1 Introduction The primary purpose of the discrimination model (Fig. 5.13) and the message-han- dling model (Fig. 5.22) is to support forward engineering of protocol and logical networks, according to the following design procedure: Protocol networks are first specified on the abstract protocol level (as service networks or abstract networks), where all operations that are handled by the net- work are related to service objects or abstract machines. Each operation is spec- ified in a symbolic operation specification. Abstract networks are then allocated in logical networks. As a result, the opera- tions and ASPs in each protocol network interface are identified. (Note that 5.5 The EIA553 Case Study some abstract protocols may exist only inside logical nodes, in which case they can be implemented by mapping directly on node platform communication primitives.) Common message-handling functions (OPH, FOH, ENC) according to the mes- sage handling model are then added. This implies, e.g. producing message for- mats and selecting encoding methods. Transport service adaption (TAH) functions may also be added. Finally, association and discrimination functions are added. A logical node integrator may contain all or just some message-handling func- tions and discrimination levels. For example, the integrator may perform discrimi- nation only. As an alternative, it could support adaption and encoding functions for all service elements, while service elements must define their own FOH and OPH (often hidden in the code of an implementation), or it could perform all functions in the same way for all service elements (like an object-oriented platform does). If the discrimination and message-handling models are used when designing logical networks, all protocols would be specified in a uniform way, e.g. defined by specialized logical node models,