Notes on the X.25 procedures for virtual call establishment and clearing

The packet level of the CCITT Recommendation X.25 describes how the virtual circuit communication service of public packet-switched data networks can be used. The recommendation contains in particular a specification of how to establish and clear virtual circuits through a given DTE-DCE interface. We comment in this paper on the following three aspects of these procedures for call establishment and clearing: (a) The recommendation contains state diagrams to clarify the operation of the DTE-DCE interface. The reader may assume that the diagrams show, at each instant in time, the unambiguous state of the interface. This is, however, not always true since the DTE and the DCE may be in different states. (b) We have analysed the operation of the interface using a finite state validation method, and have found that in some rare cases a conflicting situation (i.e. when the DTE and the DCE are in different states) may sustain through the repetitions of some cyclic behavior. (c) The procedures of X.25 are very symmetrical. However, the packet level call establishment procedure contains a non-symmetrical element for the handling of call collisions. We consider as alternatives some symmetrical procedures for this purpose. For each of these aspects we try to explain the problem and give some suggestions for its solution.