SSyyed Riifffat AAlli DDiiggiittaall SSwwiittcchhiinngg SSyysstteemmss ((Syystemm Reliaabbiilliittyy aandd AAnnalysis) Bell Communications Research, Inc. Piscataway, New Jersey McGraw-Hill, Inc. New York • San Francisco • Washington, DC. Auckland • BogotA • Cara- cas • Lisbon • London Madrid • Mexico City • Milan • Montreal • New Delhi San Juan • Singapore • Sydney • Tokyo • Toronto 2 PREFACE The motive of this book is to expose practicing telephone engineers and other graduate engineers to the art of digital switching system (DSS) analysis. The concept of applying system analysis techniques to the digital switching sys- tems as discussed in this book evolved during the divestiture period of the Bell Operating Companies (BOCs) from AT&T. Bell Communications Research, Inc. (Bellcore), formed in 1984 as a research and engineering company support- ing the BOCs, now known as the seven Regional Bell Operating Companies (RBOCs), conducted analysis of digital switching system products to ascertain compatibility with the network. Since then Bellcore has evolved into a global provider of communications software, engineering, and consulting services. The author has primarily depended on his field experience in writing this book and has extensively used engineering and various symposium publications and advice from many subject matter experts at Bellcore. This book is divided into six basic categories. Chapters 1, 2, 3, and 4 cover digital switching system hardware, and Chaps. 5 and 6 cover software ar- chitectures and their impact on switching system reliability. Chapter 7 primarily covers field aspects of digital switching systems, i.e., system testing and accep- tance and system maintenance and support. Chapter 8 covers networked aspects of the digital switching system, including STf SCP, and AIN. Chapter 9 devel- ops a hypothetical digital switching system and explores system analysis needs. The object of chapter 10 is to introduce the hardware and software of various major digital switching systems in use today in the North American network. Summary This book develops hardware and software architectures related to modern digital switching systems. It uses with an inside-out approach, i.e., starting from basic building blocks and then gradually adding more advanced functionalities. It then applies advanced techniques to hardware and software reliability analysis for class 5 (end-office) applications. This book covers Markov chain analysis for hardware reliability and an enhanced root-cause and metrics model methodology for improving software reliability. The first four chapters introduce various aspects of digital switching sys- tem architecture, the internal communications and control necessary for digital switching system subsystems, and the basics of switching fabric, i.e., switching elements within a switch. Chapters 3 and 4 develop basic theory related to reli- ability modeling based on Markov chain analysis. Chapters 5 and 6 introduce the basics of digital switching system software architecture and software analy- sis methodologies. Chapter 7 discusses some practical aspects of digital switch 3 maintenance. Chapter 8 extends Markov chain analysis to networked switching elements such as the signaling transfer point (STP). Chapter 9 develops a ge- neric digital switching system and discusses switching system analysis needs. Chapter 10 describes some basic hardware and software architectures of major digital switching systems currently deployed in the North American Network. Major subjects covered under each chapter are as follows: - Chapter 1: Switching System Fundamentals: Central office linkages, switching system hierarchy building blocks of digital switching sys- tem, and basic call processing. - Chapter 2: Communications and Control: Levels of control, basic functionalities of digital switch subsystems, control architectures, mul- tiplexed highways, switching fabric, programmable junctors, network redundancy. - Chapter 3: Reliability Modeling: Downtimes in digital switching sys- tems, reliability assessment techniques, Markov models, failure mod- els, and sensitivity analysis. - Chapter 4: Switching System Reliability Analysis: Analysis of central processor community, clock subsystem, network controller subsystem, switching network, line and trunk downtimes, call cutoffs, ineffective machine attempts, and partial downtimes. - Chapter 5: Digital Switching Software: Software architecture, operat- ing systems, database management, digital switching system software classification, call models, software linkages during a call, and feature interaction. - Chapter 6: Quality Analysis of Switching System Software: Life cycle of switching system software, software development, a methodology for assessing switching software quality, software testing, CMM, and other models. - Chapter 7: Maintenance of Digital Switching Systems: Interfaces of central office, system outage causes, software patches, generic pro- gram upgrade, problem reporting, firmware deployment, maintainabil- ity metrics, a strategy for improving software quality, and defect analysis. - Chapter 8: Analysis of Networked Switching Systems: Switching in a networked environment, network reliability requirements, Markov model of a hypothetical STP, dependence of new technologies on digi- tal switch, ISDN, AIN, and future trends in digital switching systems. - Chapter 9: A Generic Digital Switching System Model: Hardware ar- chitectures of a central processor, network control processors, interface controllers, interface modules, software architecture, recovery strategy, calls through the switching system, and analysis report. 4 - Chapter 10: Major Digital Switching Systems in the North American Network: High-level hardware and software architectures. Acknowledgments The author extends his appreciation and gratitude to Gobin Ganguly, Fred Hawley, Chi-Ming Chen, Adrian Dolinsky, and Bob Thien for their help and support. 5 CHAPTER 1 SWITCHING SYSTEM FUNDAMENTALS 1.1. Introduction This book assumes that the reader is familiar with the fundamentals of te- lephony. The author will use concepts and language commonly known and un- derstood in the telecommunications industry. However/ for the reader who needs to review some basic concepts, a reference list is provided at the end of this chapter [1]. A telephony system can be divided into three categories: 1. Circuit switching 2. Station equipment 3. Transmission This book will deal primarily with practical, as opposed to theoretical, as- pects of the first category above, circuit switching. References will be made to the other two categories, station equipment and transmission systems, only when necessary for the reader to understand the functions of circuit switching; other- wise, these subjects will not be covered in this book. 1.2. Digital Switching System Analysis System analysis and design is defined as the process of developing user requirements and designing systems to achieve them effectively [2]. The main object of this book will be to expose the reader to various aspects of digital switching system requirements, design analysis, and mathematical techniques which could help him or her to analyze a digital switching system. An exchange, or central office1, is a large, complex system comprising many subsystems, each with unique characteristics and functionality. A basic understanding of these subsystems and their interaction with the rest of the sys- tem is needed for a digital switching system to be effectively analyzed. 1.2.1. Purpose of Analysis The reliability of digital switching systems is becoming increasingly im- portant for users of telephone services. Currently most Internet access takes place through digital switching systems. Almost all electronic money transfers depend on the reliability of digital switching systems. The federal government requires that all network outages exceeding 30 minutes be reported to the Fed- eral Communications Commission (FCC). The Bell Operating Companies re- quire that the outage of digital switches not exceed 3 minutes per system per year. All this shows that the reliability of digital switching systems is a very se- 1 Throughout this book, the term central office is used instead of exchange. The term exchange is predominantly used in Europe and Asia. 6 rious matter; it can impact a nation's commerce, security and efficiency. But how does the engineering profession ensure that the telecommunications net- work is reliable and will operate within prescribed limits? This, in effect, is what this book tries to answer. There are many excellent books on telecommunica- tions, networks, and reliability. However, this book attempts to bring these dis- ciplines under an umbrella of reliability analysis. Headlines like "String of Phone Failures Perplexes Companies and US. Investigators" [3] or "Regional Phone Systems on Both Coasts Are Disrupted by Glitches in Software" [4] appear frequently in the news media. The nature of such system outages needs to be understood and methodologies put in place that will alleviate such problems. The current telephone network is becoming very complex; it has multiple owners and is equipped by many different suppliers. This book will address the root cause of many outages, which usually involves a breakdown of a network element such as a digital switching system. Digital switching systems represent very complex systems. They are mul- tifaceted and require an analyst to explore many avenues during the analysis process. This book provides guidance and answers