Service Elements

This chapter refines the possible structuring of a service by discussing a number of general purpose and frequently used functional service elements. A service element is characterised by the temporal pattern in the exchange of service primitives. These elements can be fit in a general purpose service and then lead to parameters of service primitives. An important representative of these parameters is the Service Data Unit (SDU) concept. A SDU is a data string that is transferred transparently, i.e., unchanged and uninspected by the service, between peer-to-peer protocol entities. The SDU concept is a necessary prerequisite for the design approach called separation of design concerns, which leads to a layered protocol structure. To this end, the SDU is coded on bit level. We explain three general purpose service types that aim at the transfer of a single SDU in one direction only, namely the unconfirmed, provider confirmed and user confirmed data service. Together they are called connectionless services. These elements can be used to define more complex service types of which we discuss one in particular, the general purpose connection-oriented service.