$BOOL $BCDINT $IA5String Data types (encoding information) pdu (PDU instance) 400805 "H2X3R4" Value SDU instance 00100011 01000000 00001000 00000101 00000001 01001000 00110010 01011000 00110011 01010010 00110100 CNL-SAP DATA request (SAPI, SDU) ICI instance {DATA} Service provider {Request} Fig. 3.10 The OSI RM data flow model for communication over SAPs 3.2 The Communication Service Provider It is a pity that these concepts are SO poorly understood, since they are very effective for modelling. SM&AML relies on these concepts. In the remaining part of this book it is expected that you have a good understanding of these concepts and how they relate to the ASP concept. A short description of Fig. 3.10 follows: The protocol data unit (PDU) represents the information that is exchanged between peer users. This information is described both from a semantic point of view (the identifiers) and from a data point of view. Using the latter, we describe data types, value range and encoding methods (associated with the data type name). The interface data unit (IDU) represents the information that is exchanged between the user and the service provider. The IDU is the data representation of the ASP. It consists of two parameters: the service data unit (SDU) and the interface control information (ICI). An SDU instance is the bit-representation of the instance of a PDU. SDUs are the data that are transferred between SAPs. The service provider handles SDUs of a certain size, structure and rate, but otherwise has no knowledge of what the bits of the SDU mean. An instance of an ICI comprises all information of an ASP that is not the SDU. In the exemplified case, the ICI consists of the values DATA (an operation identi- fier), Request (a primitive type) and a SAPI value. The data representation of these values is not provided. It is regarded as a local matter, since ICI data are not necessarily transferred between SAPs (although some parts of the ICI, in this case, the SAPI value, will be transferred between nodes inside the service 