Design of AMS, an Atelier for Modeling and Simulating Open
Contents
1. This file contains the simulation results which are recorded according to the user s choices 2 2 Functional description of AMS The end user has four processes to interact with them Figure 2 shows the order to be followed in order to perform the whole work which includes the successive phases editing systems editing scenarios and results generating code and launching simulations During the edition phase the end user constructs an architecture of the system using available simple icons Each simple icon represents a basic model such as a TCP Transfer Control Protocol entity or an Ethernet network A simple icon has one display Besides he has two facilities to edit the architecture generic icon display facility and sub model icon edition facility A generic icon also represents a basic model such as a bridge but has several displays The display that appears in the window edition depends on some attributes and or on which basic model it is connected to For instance the display of a bridge is different if it connects two Ethernets or two token rings The second facility is a powerful mechanism proposed by the atelier A sub model icon is a special icon representing a whole sub model A sub model constitutes a part of the system under study and is composed of several basic models So it is constructed by using either a simple icon or a generic one For instance a stack icon is a special icon inside which the end user can edit a stack of2. the outside Some facets may remain alone in this case we say that these particular facets belong to the special superfacet called the zero superfacet The second notion introduced in this section is the notion of face It is exclusively used by the atelier to validate the connectivity of an architecture A face is composed of several superfacets and or several facets belonging to the zero superfacet The modeler defines a set of faces so that the intersection of any couple of them is the empty set A DBM has to be connected by all its faces to other DBMs namely at least a superfacet or a facet of each face is used to interconnect the current DBM 4 Example In this section we examine how to edit a communication system using the atelier The example chosen is a complex communication system based on satellite backbone figure 9 This system is intended to offer communication services through the efficient use of the satellite network It is composed of geographically scattered Customer Network Facilities CNF interworking through a backbone satellite which will allow direct communication paths between the earth stations located at the CNFs The CNF consists of a LAN connecting several workstations The LANs could be High Speed LAN such as FDDI or Medium Speed LANs such as Ethernet Satellite v 4 x A r v 2 Gateway O A a Gateway GX ra Host j xk
3. Design of AMS an Atelier for Modeling and Simulating Open Communication Systems Atika COHEN and Radouane MRABET it Libre de Bruxelles Service T l matique et Communication Bd du Triomphe CP 230 Brussels Belgium Tel 322 650 57 04 Fax 322 641 38 16 cohen helios iihe rtt be and mrabet helios iihe rtt be C be ADMD rtt PRMD iihe O helios S cohen or S mrabet E Abstract The article describes a work related to the OSISIM project Open System Integrated Simulator The main objective of this project is to set up an atelier for the modelization of communication systems and the analysis of their performances The representation of a system to be simulated is based on models of several standard networks available in the library which is the kernel of the atelier Starting with an outline of the components of the atelier we describe the basic models to be included in it Moreover a concrete example of communication system shows a methodology to be applied when using the atelier 1 Introduction The main objective of the OSISIM project is to set up an atelier for the modelization of communication systems and the analysis of their performances In the literature we find the description of different toolkits dedicated to this field such as TOPNET 1 which is based on PROT net a class of Petri nets NETMOD 2 which is based on simple analytical models BONeS 3 which is based on block oriented modeling paradig
4. M project Besides we gratefully acknowledge the financial support granted by IRSIA
5. a Gateway a Host Host Host Host Figure 9 Communication system based satellite network In the following we illustrate how this communication system will be edited using the atelier The edition will be done in a top down manner Based on figure 8 the system can be broken down into three main components On the one hand there is a satellite backbone component and on the other hand many CNFs components each CNF is connected via a gateway component to the satellite Figure 10 illustrates this first step of the edition phase The number of CNFs is set to three The satellite is represented by a simple icon and the gateway by a generic icon Because a CNF is a complex component each CNF will be represented by a sub model icon At this level of edition we can specify the parameters characterizing the satellite and the gateway components In our model the parameters of a satellite are the propagation time e g 270 ms the bandwidth e g 2 Mbit s and the lost packet probability e g 10 8 and the parameters of the gateway are its storage capacity e g 2 Mbytes and its packet switching time e g 0 1 ms Model Edition Win Figure 10 Decomposition of the communication system The second step in this edition phase is editing the content of each CNF In our example a CNF is composed of an Ethernet network connecting several hosts the number of hosts can be different for each CNF Figure 11 illustrates
6. ault value will be used if the end user does not change it Otherwise the new value he provides will be taken into account This value is to be selected from a defined range if any proposed to him Of course the defined range is proposed in order to help him choose a correct value Behavior Engine Interface 1 Interface 2 Interface N Measurement Block figure 3 Internal structure of a DBM 3 1 2 The Interfaces Block A DBM can have several interfaces Indeed a DBM can not be used alone it must be connected with one or several other DBMs in order to make up a complex system The number of interfaces N in figure 3 can either be a fixed value known during the modelization phase or can vary so that the modeler can fix only the minimal and the maximal values The validity of the interconnection between several DBMs is not checked by the DBMs themselves but by the Editor and the Generator processes Thus the DBM assumes that it was correctly connected In section 3 3 three notions are introduced which are used to verify the validity of the interconnection Figure 4 shows how an interface using messages reacts with its BE and with the outside world i e another interface belonging to another DBM I n t Anoth apis e Behavior DBM r Engine f a c e DBM figure 4 A DBM with its BE and one Interface Figure 5 represents a
7. ch exchange messages between themselves and with the outside A message is modeled as a customer which enters a station waits in a queue before receiving a service and finally is destroyed or transmitted to another queue A customer may be of different types depending on the kind of messages it models Inside a same DBM the modeler is free to choose the type In contrast if a message transits between stations not belonging to the same DBM the type of message has to meet a specific format This specific format 9 is used in order to homogenize the interactions between all DBMs Hereunder the format of messages which transit between DBMs A customer modeling such a message has the type named CUST_TT described in figure 6 We note that this kind of message is divided into two parts The first one contains data referenced by the RFMS pointer This data has to be transferred in a transparent manner for the intermediate DBMs Indeed those DBMs do not know the structure of the data object this structure is only known by the DBM which created it and by the recipient DBM The second part of the message contains control data referenced by the RFCT pointer The content of this part is to be used by the DBM which receives the message Indeed this second part contains all the information require for the connected DBMs to coordinate their work DECLARE CUSTOMER OBJECT CUST_TT REF ANY RFMS amp this variable references an object carrying data amp The
8. dition phase the end user can activate the module which in charge of verifying the coherence of the system Besides information at different levels are available to assist him during this primordial phase Once this phase is finished the end user is invited by the atelier to save his work in a specific file so he can modify it later The analysis of the system will be studied by means of other phases not described here Nevertheless as explained earlier the atelier offers the necessary tools to do this 5 Conclusion This paper describes the AMS atelier which provides a set of tools for modeling and performance evaluation of complex open communication systems It provides a powerful human machine interface which use the new graphics technology It also uses the object oriented paradigm namely all the basic models are designed as objects which can be instanced several times and their behaviors can be tuned by means of parameters and their types of connectivity In this paper the design of the basic model has been detailed namely its internal structure the interconnection between DBMs and the format of messages exchanged between them In order to clarify how to use the atelier to construct a complex model a real communication system has been presented and the necessary steps to edit the system have been studied We note that other important aspects of the atelier will be detailed in the future such as The automatic generation of interes
9. end of the simulation 2 1 2 The processes The following processes allow the end user either to access the objects described above or to generate code and launch simulations a Editor Process This process allows the end user to edit his communication system graphically To edit the architecture he can create several instances of basic models connect them by links and if necessary he modifies the values of some basic model parameters in order to configure his system as he wants The Editor generates in a manageable manner the description of the edited system This description called Arch Descrip is stored as a file and can be re used at any time in order to re configure the system and or to modify its architecture During the edition phase the editor verifies the validity of the architecture For example the connections between basic models have to be correct nevertheless the complete verification is fully done before the code is generated b Scenario and Result Process This process helps the end user to define the scenario and the type of simulation results A scenario is defined mainly by varying the values of some parameters called free variables Results can be presented in the form of charts tables lists or a combination of them c Generator Process Through the description of the edited communication system and its parameters this process generates the Qnap2 code Before the code generation a complete verification wil
10. instances of a same DBM can be created Figure 8 shows connections C L between our current DBM and two others DBM1 and DBM2 The connections C and L are also considered as types In this case we say that DBM has two facets which are DBM1 and DBM2 Facet DBM DBM1 DBM2 Figure 8 The facets of DBM Two other notions are going to be introduced in order to have a more flexible interconnection between DBMs In addition they lead to a high degree of abstraction to validate the connections during the generation code phase These two notions are superfacet and face A superfacet is a set of facets which offer he same service to the outside even if they connect different DBMs to the current one For instance a DBM modeling the Internet Protocol DBM IP can be connected to the DBM modeling Transfer Control Protocol DBM TCP and the DBM modeling User Datagram Protocol DBM UDP thus the DBM IP has two facets If we consider that these two facets are the same because the service offered to IP is the same we deduce that they will belong to the same superfacet On the other hand the DBM IP can be connected also to the DBM modeling a Logical Link Control DBM LLC thus the DBM IP has a third facet but it cannot belong to the previous superfacet because the service offered to DBM IP is different So the notion of superfacet will divide the set of a DBM facets into several superfacets each one regroups the facets offering the same service to
11. l be done in order to confirm the description provided by the end user The verification includes i completeness of the architecture namely if the communication system architecture contains what is required for the system to function properly ii connectivity namely if all the elements icons created by the end user are linked in such a manner as to avoid having two or several portions not connected to each other d Simulator Process This process compiles and executes the generated code The execution depends on the scenarios defined by the end user At the end of the execution the desired results are produced visualized and stored as decided earlier by the end user Library of Basic Models Local Measures 4 Information lobal Parameters nani 4 Re HesgH rSStHtS4 Figure 1 Architecture of the AMS 2 1 3 The files The following files are the important ones handled by the atelier a Arch Descrip This file contains the description of the communication system architecture edited by the end user The description is written using the Graphical Description Language GDL 6 b Generated Code It is the code of the modelization of the communication system based on the user s description This code is mainly written in the Qnap2 language c Scenario and Result Description This file contains the description of the proposed scenario and the set of desired results types d Results
12. ll defined sets of functions while having clearly specified interfaces In the section 2 this paper describes the architectural elements of the AMS 7 and how the latter can be used The section 3 focuses on the design of a basic model 8 its components the structure of messages exchanged and the interconnection of two basic models The section 4 is devoted to the presentation of how an end user can build a communication system using the atelier Since the example selected is a complete one it is built in a top down way While working out the example we show how to decompose the system under study how to define the environment parameters These packages are trademarks of SIMULOG a French company specialized in the field of simulation i e those related to the modeled system and characterizing each part of the system and how to choose the desired measurements A conclusion is drawn in the last section 2 Outline of the atelier 2 1 Architectural elements of AMS The main components of the AMS are shown in figure 1 On the one hand we can find components to be handled by the end user such as objects to describe a communication system to be studied i e the basic models processes to edit the architecture and parameters of the system under study and to generate the code modeling system On the other hand there is a set of files that contain the internal representation of the system described by the end user Those files are
13. m written in C Unlike these approaches ours is based on queueing networks Our atelier named AMS Atelier for Modelization and Simulation integrates the facilities and the tools in way as to be easy to use by the end user It gives the end user some powerful tools to easily edit a new communi system i in a graphical environment The AMS is based on the latest techniques in software engineering such as graphics i ing pop up menus and the object oriented programming paradigm QNAP2 4 5 Queueing Network Analysis Package 2 and GSS4 6 Graphical Support System 4 are the main packages that will be used to operate the AMS One component of this atelier is a library of basic models including most of the standard networks such as LANs Ethernet Token Ring FDDI etc WANs X25 TCP IP satellite and radio networks It is obvious that the usefulness of the atelier heavily depends on the number of available models necessary to make up transmission systems and networks The end user of the AMS is not expected to be a specialist in modeling or performance analysis however he should be a communication system designer He will use the AMS to build and validate an architectural choice or to compare several possible ways of solving a problem Models have to be constructed in a very modular fashion That is why we have to build basic models which will make up other models of more complex systems Each basic model implements uniform and we
14. protocols These special icons allow the end user to edit a large communication system in a hierarchical manner In addition a special icon can enclose one or several other sub model icons For instance when several local area networks are to be interconnected by a satellite backbone each local area network can be represented by a sub model icon A complete example is studied in section 4 figure 2 Processes call order 3 Design of Basic Models 3 1 Internal Structure of Basic Models Each basic model is to be detailed so as to reflect its exact behavior Hence we have to specify the functions performed by the basic model as exactly as possible Hereafter basic models will be called Detailed Basic Models DBM The primary advantages of this approach is to validate easily the model and to have accurate measurements when the DBM is simulated as a component of a complex system Figure 3 shows the structure of a DBM there are three blocks Behavior Engine BE Interfaces Int and Measurement Block MB 3 1 1 The Behavior Engine Block In the BE block we find the modelization of the behavior of the DBM The behavior is an open network of stations each station includes a queue with limited or unlimited capacity and one or several servers Besides the DBM has a large number of parameters which make it flexible from the end user s point of view Each parameter has an initial value set by the modeler during the modelization phase This def
15. se data are not modified but only transferred REF ANY RFCT amp this variable references an object carrying amp control data These data will be used by the amp DBM which receives this customer END 2ANY is a type of top level it is the root of all the types A pointer of this type can reference any object gt Data 1 RFMS RFCT Control Data 1 figure 6 Format of message exchanged between DBMs 3 3 How to connect two DBMs The exchange of messages between DBMs is done via their interfaces For a given DBM the interface is represented by two stations One station receives messages from outside i e another DBM and sends them to the BE block of this DBM At the opposite the second station receives messages from the BE block of this DBM and sends them to the outside This is illustrated by figure 7 DBM1 DBM2 Intl_in Int2_out lt BEI Intl_out Int2_in a gt figure 7 Two DBMs connected by two interfaces Depending on the system under study a DBM can be connected to a variety of DBMs many instances of the same DBM may be involved For each connection an interface is needed Let us say that this DBM has several facet If we make an analogy with the algorithmic languages a DBM is considered as a type Thus several
16. standard interface with two queues Qio receives messages from the outside which are meant to be sent later to the BE while Qii receives messages from the BE to be sent later to another DBM which is connected to it Seen from the outside all the interfaces are the same but they are different in the way they process the ingoing and outgoing messages On the other hand the main function of the interfaces block is to convey messages from a BE to the outside and vice versa Qi BE Another Qio DBM figure 5 A standard interface 3 1 3 The Measurement Block With regard to measurements the end user can be satisfied with the default list of measurements presented by the atelier Otherwise he can make his choices from the exhaustive list of measurements related to that particular DBM The measurement block contains two types of measures the first type reflects the behavior of the DBM and thus it is associated to the BE the second type is associated to the interfaces and it shows mainly the data flow entering and exiting the DBM All the measures are meaningful for an end user not specialized in the field of network queueing theory All the aspects related to this field are transparent The measures are associated to metrics used in the field of communication networks 3 2 How to construct messages for a DBM As we said in the previous section a DBM is an open network of stations whi
17. ted scenarios in order to minimize the number of simulations The management of the measurements since the atelier provides a lot of measurements which will make their post processing very tedious The integration of analytical models in the library 6 References 1 M A Marsan G Balbo G Bruno and F Neri TOPNET A Tool for the Visual Simulation of Communication Networks IEEE Journal on Selected Areas in Comm Vol 8 no 9 1735 1747 Dec 90 2 D W Bachmann M E Segal M M Srinivasan and T J Teorey NetMod A Design Tool for Large Scale Heterogeneous Campus Networks IEEE Journal on Selected Areas in Comm Vol 9 no 1 1735 1747 15 24 January 91 3 K S Shanmugan V S Frost W LaRue A block Oriented Network Simulator Simulation 83 94 February 92 4 QNAP2 User s Manual Simulog S A 1992 5 QNAP2 Reference Manual Simulog S A 1992 6 GSS4 User s Guide Simulog S A 1992 7 A Cohen and R Mrabet AMS An Integrated Simulator for Open Systems GLOBECOM 93 Conference 656 660 Houston Texas November 29 December 2 1993 8 A Cohen and R Mrabet AMS Internal structure for Basic Models Internal Report IIHE HELIOS B 1 15 92 October 1992 9 A Cohen and R Mrabet How to write a DBM using the QNAP2 package Internal Report STC 93 15 juin 1993 7 Acknowledgment We wish to express our gratitude to SAIT Systems with whom we have been working on the OSISI
18. the edition of one CNF where two hosts are present The Ethernet network is represented by a simple icon while each host is represented by a new sub model icon since a host is a complex component At this level of edition several parameters of the Ethernet network can be specified such as the length of its bus e g 500 m and its bandwidth e g 10 Mbit s Figure 11 Decomposition of a CNF The third step in this phase is editing the content of each host A host is composed of a stack of protocols To edit a stack the end user chooses protocols from the library of basic models where many protocols are available for each layer Thus several stacks can be considered In our example the stack is composed of a Medium Access Control MAC to the Ethernet Protocol a Logical Link Layer type 1 LLC an Internet Protocol IP a Transfer Control Protocol TCP and a File Transfer protocol FTP Figure 12 illustrates this step where each protocol is represented by a simple icon At this level the parameters characterizing each protocol can be specified Figure 12 An example of a HOST At each step a large number of measurements can be chosen such as the throughput of each connection a mean delay for a file transfer the utilization of the satellite bandwidth and so on These measurements can be provided for several scenarios and pictured as charts or lists On the other hand they can be provided periodically in time During the e
19. to be handled by the atelier Hereafter we give more explanations about those components 2 1 1 The objects The main objects offered to the end user in order to describe a communication system are the Library of Basic Models the Graphical Database and the Scenario and Result elements a Library of Basic Models The basic core of the AMS is constituted by a library of basic models Each basic model represented by an icon in figure 1 corresponds to a communication entity such as a medium access control protocol application etc Using these basic models the end user prepares the architecture of the system to be studied In order to facilitate carrying out these models their interconnection and their maintenance see section 3 1 a same internal structure is proposed for all of them Besides each basic model is characterized by a definite number of parameters The parameter values can be edited and modified by the end user within limits of variation associated with each parameter b Graphical Database This database contains all the information about icons pop menu and edition rules which allow the end user to edit his communication system graphically This database is necessary in the edition phase and is not used directly by the end user c Scenario and Result elements These elements help the end user to define the scenarios which will be followed during the simulation and the type of results the end user wants to obtain at the
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