Software Considerations in the Control of Digital Communications Switching Systems

Abstract

Today's complex implementations of integrated packet and circuit switched digital communications networks demand that the software used for controlling these systems be robust, fault tolerant, and capable of runtime recovery from all but the most severe of operational errors. The typical modern switched communications system includes the use of multiple circuit switches, each with potentially thousands of end-user interfaces. Further, these switches are often inter-connected to each other via high-capacity trunks. A single connection between two end-user interfaces often traverses a number of intermediate circuit switches in order to effect the end-to-end communications desired. In this complex, distributed environment, the establishment and dissolution of end-to-end user connections involve far more than simple binary connection states indicating the existence, or non-existence, of a link. More commonly, a single end-to-end connection requires multiple node links across multiple, heterogeneous interfaces. The command and control software used to establish, monitor, and dissolve these connections must be capable of dealing with errors which arise at any node along the way in a consistent and reliable manner. Most critically, the system software must be capable of maintaining an accurate, multi-level mapping of distributed resources' availability, allocation, and status. Further, the software must have the capability of "healing itself" during operational run-time when it can, and of accurately reporting the nature of inconsistencies caused by anomalous events that cannot be fixed on the fly. The Edwards Digital Switch (EDS), developed by CSTI, provides a case study of possible solutions, and potential pitfalls, that can arise in the design, development, and implementation of the controlling software in today's dynamic, distributed communications' system architectures.