No. 4 ESS: System Objectives and Organization

System Objectives and Organization By A. E. RITCHIE and L. S. TUOMENOKSA (Manuscript received February 8, 1977) This article is an introduction to a series of articles that describe the No. 4 Electronic Switching System. Objectives and the organization of the system are given and the overall operation is explained. I. INTRODUCTION 1.1 Background A new toll Electronic Switching System, No. 4 ESS, has been developed to meet the expanding needs of the telecommunications network, including the effects of continuing toll message growth. The first four No. 4 ESS offices were placed in service during 1976, starting with the Chicago 7 office on January 17. The purpose of this article is to serve as an introduction to the ten more technically detailed articles that follow. This introduction highlights the objectives of the new system in comparison to existing Bell System toll, or trunk, switching systems and outlines the features and characteristics of No. 4 ESS that differ from previous electronic switching systems, particularly the No. 1 ESS. Expectation of a trunk switching version was implicit in the early planning and development of the first Bell System local electronic switching system, which came to be known as No. 1 ESS. It was clear even then (late 1950s) that if the anticipated improvements in flexibility, reliability, and economy were realized, these advantages would be equally applicable to toll and tandem switching. The opportunity to test this thesis came early. The first local No.