elivers all SDUs in the same order as they were sent, it accepts SDUs of any length and it is always ready for sending and receiving new SDUs. In case the CSP cannot meet these requirements, the CSP must at least be able to tell the user about transport problems (these are, more or less, the properties of the OSI presentation service). When that is not the case, the integrator should also implement transport service adaption functions, that com- 5.8 Summary pensate for a less-than-ideal CSP. Such functions are segmentation, sequencing, error handling and flow control. There are two ways in which protocol elements in a protocol network can access each other's functions: either by direct associations, or over relaying chains. Two forms of relaying are identified: message relaying and event relaying. Message relaying implies that messages are transported transparently, or by modifying mes- sage format and encoding through a protocol element, but without changing param- eters that represent the information elements of the ASPs that are carried by the messages. We say that the abstract protocol is the same on both sides of the relay- ing element. Event relaying implies that the protocol element translates between two different abstract protocols (as, e.g. between ANSI 41 and MAP). Relaying chains represent a flexible method for introducing functional modifi- cations in a protocol network. They can, however, also be hazardous to the robust- ness of a system, unless there is one element in a relaying chain that performs the overall state control in relation to other protocol elements that are external to the chain. Such an element is an agent. Transaction specifications define how an agent maps external events on internal events that are handled by other elements in the relaying chain. 6 Process Control System Modelling Introduction Our basic structural definition of any processing system is that it consists of two major subsystems: a resource system (RS) and a process control system (PCS), the latter being the "brain" 