CHAPTER ONE

INTRODUCTION

1.           BACKGROUNDOF THE STUDY

The importance of protocol arisesfrom the fact that protocols form the nervous system of teleprocessing networksand as such are responsible for ensuring that the pieces of the system work asa harmonious whole. The complexity and size of today’s systems and the factthat they are put together from subsystem components manufactured in differentlocations and even by different companies, virtually demand formalspecification and there are many side benefits. And so, as open systems and standard networkarchitectures encompass an ever growing segment of the computing industry, theneed for clear and precise protocol specification becomes more important.Traditional methods of informal narrative specifications and ad hoc validationhave demonstrated their shortcomings as protocol bugs and incompatibleimplementations crop up. Problems of ambiguous and incomplete specificationsare particularly severe for the ever growing number of protocol standards thatmust be implemented by a wide community of users with diverse equipment (Rudin,2003).            

There has been recent work on formalprotocol specification and verification. In addition to individual researchers,several national and international standards organizations have become activein this area.  These include theInternational Standard Organization (ISO) TC16/1 working group on FormalDefinition Technique (FDT), the Consultative Committee for InternationalTelegraphy and Telephony (CCITT) SG VII special report on question 39-SystemDescription Techniques (SDT), and major protocol development projects by theNational Bureau of Standards (NBS) and Defense Communication Engineering Center(DCEC) in the US. It ispertinent to note that good protocol specification methods or languages provideprecise notations to facilitate implementation of standard and enhancetechnical quality. As their syntax and semantics are precisely specified, everyword and symbol has a well-defined meaning and its use must follow exact rules.This makes standardized specifications unambiguous while improvingintuitiveness, increasing consistency and making it possible to detect errorsduring standardization rather than implementation (ETSI, 2013). Thus to make protocol specificationless complex, the use of Petri net as a modeling tool, graphical notation aswell as a compact way to specify behaviour (protocol) has been employed. Mostmodeling languages have graphical notations, and these have good reasons.Models are used as a means to specify concept and ideas, and to communicatethem between humans. Nearly everybody would use some kind of graphics toexpress his or her understanding of a system, even without using any explicitmodeling languages. It does not need psychological research to state thatgraphics employing two dimensions allow for a better understanding of complexstructures than one dimensional text. Since specification of systems andcommunication of models are the main application of Petri net in practice,understandability for human is among the most crucial quality criteria formodeling languages. Petri nets have nice graphical representation using onlyvery few different types of elements, which is good basis for an easyunderstandability of a model and for the learnability of the language. Thesetwo criteria for modeling languages belong to the most important onesrecognized in the “Guidelines of Modeling” (Desel and Juhas, 2001).          

Manufacturers who use precise andgraphical languages report good results and considerable productivity gains.Their use within standardization increased along with the general increasedacceptance of such techniques (ETSI, 2013). The most core issue of Petri netsis that they model behavioral aspects of distributed systems, i.e., systemswith components that are locally separated and communicate which each other(Desel and Juhas, 2001). Since its invention by Carl Petri (Murata, 1989),Petri nets have been found useful in modeling systems with distributed statessuch as concurrency, synchronization, conflict(choice or decision) etc.