Modernization of the Suburban ESS: Adding Data Links to an Existing ESS

Adding Data Links to an Existing ESS By C. E. ISHMAN,* R. B. S A N D E R S O N , * L. M. TAFF,* D. P. TRUAX,* and C. T. TULLOSS* (Manuscript received June 21, 1982) As a major modernizing improvement data links for three important functions have been added to the 2B Electronic Switching System (ESS). The data-link system was designed with a layered architecture using an enhanced subset of the X.25 protocol. Levels 1 and 2 of the protocol are implemented in the hardware and software of a new device, the Serial Peripheral Unit Controller/Data Link (SPUC/DL). Within the SPUC/DL itself, check circuitry and software are used for error detection. A second SPUC/DL for each critical application provides redundancy on either a dedicated or dial-up basis. An elaborate diagnostic program within the 2B ESS can be invoked automatically or manually. This paper discusses some of the issues in retrofitting the BX.25 protocol to pre-X.25-speaking machines. I. INTRODUCTION This paper describes the addition of real-time data-link capability to the No. 2B Electronic Switching System (ESS) 1 . The paper combines a technical description of the project with explanations of certain * Bell Laboratories. Acronyms and abbreviations used in this paper are defined at the back of the Journal. + ^Copyright 1983, American Telephone & Telegraph Company. Copying in printed form for private use is permitted without payment of royalty provided that each reproduction is done without alteration and that the Journal reference and copyright notice are included on the first page.