5.1 Introduction

The ITU-T CS-1 IN standard, along with the subsequent ETSI core INAP modifications, was a resounding success. It led to IN installations in most major operators networks because the equipment vendors generally supported the engineering content of the recommendations, or at least a significant sub-set of them. However, in the period leading up to the original CS-1 publication there had been lively debates about the basic network model that should be chosen for the standard. The contenders were the sequential call logic approach, which later became known as the basic call state model , and the connection view state (CVS) model, based on snapshot views of call-states including the connection configurations. The CVS model included transition rules that prescribed the possible connection states between the legs that represented the end-points. In the end the CS-1 standard makers settled on the basic call state model, which cycles through the logical sequence of events on the originating and terminating side, and this approach has generally been the subject of the earlier chapters of this book.

Essentially the debate was about how to represent the mechanics of the underlying switch network to the IN service logic in the SCF. The SCF does not need to be aware of too much of the detail of the telephone network; on the other hand it needs a logically consistent view, comprehensible by the SSF as well...