14.6. 14.6 Simulation modelling systems

14.6.1. 14.6.1 Data collection tools and network analysers

The section summaries the main features of the widely used discrete event simulation tools, OPNET and COMNET, and the supporting network analysers, Network Associates' Sniffer and OPNET's Application Characterisation Environment.

OPtimized Network Engineering Tools (OPNET) is a comprehensive simulation system capable of modelling communication networks and distributed systems with detailed protocol modelling and performance analysis. OPNET consists of a number of tools that fall into three categories corresponding to the three main phases of modelling and simulation projects: model specification, data collection and simulation, and analysis.

14.6.2. 14.6.2 Model specification

During model specification the network modeller develops a representation of the network system under study. OPNET implements the concept of model reuse, i.e., models are based on embedded models developed earlier and stored in model libraries. The model is specified at various levels of details using specification editors. These editors categorise the required modelling information corresponding to the hierarchical structure of an actual network system. The highest level editor, the Project Editor develops network models consisting of network topology, subnets, links, and node models specified in the Node Editor. The Node Editor describes nodes' internal architecture, functional elements and data flow between them. Node models in turn, consist of modules with process models specified by the Process Editor. The lowest level of the network hierarchy, the process models, describes the module's behaviour in terms of protocols, algorithms, and applications using finite state machines and a high-level language.

There are several other editors to define various data models referenced by process- or node-level models, e.g., packet formats and control information between processes. Additional editors create, edit, and view probability density functions (PDFs) to control certain events, such as the interarrival time of sending or receiving packets, etc. The model-specification editors provide a graphical interface for the user to manipulate objects representing the models and the corresponding processes. Each editor can specify objects and operations corresponding to the model's abstraction level. Therefore, the Project Editor specifies nodes and link objects of a network, the Node Editor specifies processors, queues, transmitters, and receivers in the network nodes, and the Process Editor specifies the states and transitions in the processes. Figure 14.12 depicts the abstraction level of each editor:

Figure 14.12.  The three modelling abstraction levels specified by the Project, Node, and Process editors.

The three modelling abstraction levels specified by the Project, Node, and Process editors.

14.6.3. 14.6.3 Data collection and simulation

OPNET can produce many types of output during simulation depending on how the modeller defined the types of output. In most cases, modellers use the built in types of data: output vectors, output scalars, and animation:

  • Output vectors represent time-series simulation data consisting of list of entries, each of which is a time-value pair. The first value in the entries can be considered as the independent variable and the second as the dependent variable.

  • Scalar statistics are individual values derived from statistics collected during simulation, e.g., average transmission rate, peak number of dropped cells, mean response time, or other statistics.

  • OPNET can also generate animations that are viewed during simulation or replay after simulation. The modeller can define several forms of animations, for instance, packet flows, state transitions, and statistics.

14.6.4. 14.6.4 Analysis

Typically, much of the data collected during simulations is stored in output scalar and output vector files. In order to analyse these data OPNET provides the Analysis Tool which is a collection of graphing and numerical processing functions. The Analysis Tool presents data in the form of graphs or traces. Each trace consists of a list of abscissa and ordinate pairs. Traces are held and displayed in analysis panels. The Analysis Tool supports a variety of methods for processing simulation output data and computing new traces. Calculations, such as histograms, PDF, CDF, and confidence intervals are included. Analysis Tool also supports the use of mathematical filters to process vector or trace data. Mathematical filters are defined as hierarchical block diagrams based on a predefined set of calculus, statistical, and arithmetic operators. The example diagrams below (Figures 14.13 and 14.14) shows graphs generated by the Analysis Tool:

Figure 14.13.  Example for graphical representation of scalar data (upper graph) and vector data (lower graph).

Example for graphical representation of scalar data (upper graph) and vector data (lower graph).

Figure 14.14.  Figure 14.14 shows four graphs represented by the Analysis Tool.

Figure 14.14 shows four graphs represented by the Analysis Tool.

Figure 14.14 Analysis Tool Showing Four Graphs.

COMNET is another popular discrete-event simulation system. We will discuss it briefly and demonstrate its features in Section 14.9.

14.6.5. 14.6.5 Network Analysers

There is an increasing interest in predicting, measuring, modelling, and diagnosing application performance across the application lifecycle from development through deployment to production. Characterising the application's performance is extremely important in critical application areas, like in eCommerce. In the increasingly competitive eCommerce, the application's performance is critical, especially where the competition is just “one click” away. Application performance affects revenue. When an application performs poorly it is always the network that is blamed rather than the application. These performance problems may result from several areas including application design or slow database servers. Using tools, like ACE and Network Associates' Sniffer, network modellers can develop methodologies to identify the source of application slowdowns and resolve their causes. After analysing the applications, modellers can make recommendations for performance optimisation. The result is faster applications and better response times. The Application Characterisation Environment (ACE) is a tool for visualising, analysing, and troubleshooting network applications. Network managers and application developers can use ACE to

  • Locate network and application bottlenecks.

  • Diagnose network and application problems.

  • Analyse the affect of anticipated network changes on the response time of existing applications.

  • Predict application performance under varying configurations and network conditions

The performance of an application is determined by network attributes that are affected by the various components of a communication network. The following list contains some example for these attributes and the related network elements:

  • Network media

    • Bandwidth (Congestion, Burstiness)

    • Latency (TCP window size, High latency devices, Chatty applications)

  • Nodes

  • Clients

    • User time

    • Processing time

    • Starved for data

  • Servers

    • Processing time

    • Multi-tier waiting data

    • Starved for data

  • Application

    • Application turns (Too many turns – Chatty applications)

    • Threading (Single vs. multi-threaded)

    • Data profile (Bursty, Too much data processing)

Analysis of an application requires two phases:

  • Capture packet traces while an application is running to build a baseline for modelling an application. We can use the ACE's capturing tool or any other network analysers to capture packet traces. The packet traces can be captured by strategically deployed capture agents.

  • Import the capture file to create a representation of the application's transactions called an application task for further analysis of the messages and protocol data units generated by the application.

After creating the application task, we can perform the following operations over the captured traffic traces:

  • View and edit the captured packet traces on different levels of the network protocol stack in different windows. We can also use these windows to remove or delete sections of an application task. In this way, we focus on transactions of our interest.

  • Perform application level analysis by identifying and diagnosing bottlenecks. We can measure the components of the total response time in terms of application level time, processing time, and network time and view detailed statistics on the network and application. We can also decode and analyse the network and application protocol data units from the contents of the packet traces.

  • Predict application performance in “what-if” scenarios and for testing projected changes.

Without going into specific details we illustrate some of the features above through a simple three-tier application. We want to determine the reason or reasons of the slow response time from a Client that remotely accesses an Application Server (App Server) to retrieve information from a Database Server (DB Server). The connection is over an ADSL line between the client and the Internet, and a 100Mbps Ethernet connection between the App Server and the DB Server. We want to identify the cause of the slow response time and recommend solutions. We deployed capture agents at the network segments between the client and the App Server and between the servers. The agents captured traffic traces simultaneously during a transaction between the client and the App Server and the App Server and the DB Server respectively. Then, the traces were merged and synchronised to obtain the best possible analysis of delays at each tier and in the network.

After importing the trace into ACE, we can analyse the transaction in the Data Exchange Chart, which depicts the flow of application messages among tiers over time.

Figure 14.15.  Data Exchange Chart.

Data Exchange Chart.


The Data Exchange Chart shows packets of various sizes being transmitted between the Client and the servers. The overall transaction response time is approximately 6 seconds. When the “Show Dependencies” checkbox is checked, the white dependency lines indicate large processing delays on the Application Server and Client tiers. For further analysis, we generate the “Summary of Delays” window showing how the total response time of the application is divided into four general categories: Application delay, Propagation delay, Transmission delay and Protocol/Congestion delay. Based on this chart we can see the relation between application and network related delays during the transaction between the client and the servers. The chart clearly shows that the application delay far outweighs the Propagation, Transmission, and Protocol/Congestion delays slowing down the transaction.

Figure 14.16.  Summary of Delays.

Summary of Delays.


The “Diagnosis” function (Figure 14.17) provides a more granular analysis of possible bottlenecks by analysing factors that often cause performance problems in networked applications. Values over a specified threshold are marked as bottlenecks or potential bottlenecks.

Figure 14.17.  Diagnosis window.

Diagnosis window.


The diagnosis of the transaction confirms that the primary bottleneck is due to Processing Delay on the Application Server. The processing delay is due to the file I/O, CPU processing, or memory access. It also reveals another bottleneck: the chattiness of the application that leads us to the next step. We investigate the application behaviour in terms of application turns that can be obtained from the transaction statistics. An application turn is a change in direction of the application-message flow.

The statistics of the transaction (Figure 14.18) disclose that the number of application turns is high, i.e., the data sent by the transaction at a time is small. This may cause significant application and network delays. Additionally, a significant portion of application processing time can be spent processing the many requests and responses. The Diagnosis window indicates a “Chattiness” bottleneck without a “Network Cost of Chattiness” bottleneck, which means the following:

Figure 14.18.  Statistics window.

Statistics window.

  • The application does not create significant network delays due to chattiness.

  • The application creates significant processing delays due to overhead associated with handling many small application level requests and responses.

  • The application's “Network Cost of Chattiness” could dramatically increase in a high-latency network.

The recommendation is that the application should send fewer, larger application messages. This will utilise network and tier resources more efficiently. For example, a database application should avoid sending a set of records one record at a time.

Would the response time decrease significantly if we added more bandwidth to the link between the client and the APP Server (Figure 14.19)? Answering this question is important because adding more bandwidth is expensive. Using the prediction feature we can answer the question. In the following chart we selected the bandwidth from 128K to 10Mbps. The chart shows that beyond approximately 827 Kbps there is no significant improvement in response time, i.e., for this application the recommended highest bandwidth is no more than 827Kbps, which can be provided by a higher speed DSL line.

Figure 14.19.  Impact of adding more bandwidth on the response time.

Impact of adding more bandwidth on the response time.


After the analysis of the application's performance, we can immediately create the starting baseline model from the captured traffic traces for further simulation studies as illustrated in Figure 14.20.

Figure 14.20.  Baseline model for further simulation studies.

Baseline model for further simulation studies.

14.6.6. 14.6.6 Sniffer

Another popular network analyser is Network Associates' Sniffer. (Network Associates has recently renamed it to Netasyst.) It is a powerful network visualisation tool consisting of a set of functions to:

  • Capture network traffic for detailed analysis.

  • Diagnose problems using the Expert Analyzer.

  • Monitor network activity in real time.

  • Collect detailed utilisation and error statistics for individual stations, conversations, or any portion of your network.

  • Save historical utilisation and error information for baseline analysis.

  • Generate visible and audible real-time alarms and notify network administrators when troubles are detected.

  • Probe the network with active tools to simulate traffic, measure response times, count hops, and troubleshoot problems.

For further details we refer the reader to the vendors' documentations on http://www.nai.com.