 of the layer (an abstract machine system) is shown in Fig. 2.37 (compare with Fig. 2.29). We added the NO dimension for this represen- tation as well to show in which node the element state machines (ESM) reside. Note that when a standard does not apply the actor-agent separation, there are no abstract protocol names to refer. We therefore name abstract protocols by taking the message protocol name (e.g., DSS1 L3) and put it in between slashes, e.g., /DSS1 L3/. 2 Layer Structures SCP resources T-ROs TE resources TE resources all/DSS1 L3/ all/DSS1 L3/ T-ROs all /ISUP/ T-ROs TE resources LE/SSP resources: LE resources TE resources Subscriber data manager Switching machine TR resources - B-channel manager - Etc. LE/SSP SCP Fig. 2.37 The call-handling LSM in an ISDN The designers of the ISDN call-handling layer knew of course exactly how the layer was supposed to operate, but they fragmented their knowledge into (at least) three different message protocol specifications. Since actors are almost always sep- arated from agents in implementations, it became a considerable burden on the part of interpreters of standards (implementors's R&D departments and students of ISDN) to find out how the LSM was supposed to work. A lot of effort has been spent by implementors over the years to create simpler and more intelligible descriptions of call handling than what is represented by the around 2000 pages of the message protocol standards. Implementor's documents stay however as propri- etary documents18 All this effort could have been avoided if the standard, instead of fragmenting the knowledge, had provided a description of the ISDN call han- dling layer in terms of its LPM and LSM models instead. The exception to the rule in ITU-T standards is the TCAP common agent layer. Although not as general as the AMLn common agent layer, the TCAP layer can be used by a number of ISDN actor layers simultaneously, as long as they define their abstract protocols according to the operation classes that ROSE defines, and use the SAG defined