public static void main(String[] args)
    {
        System.out.println("Starting network example ...");

        try
        {
            //////////////////////////////////////////
            // First step: Initialize the GridSim package. It should be called
            // before creating any entities. We can't run this example without
            // initializing GridSim first. We will get run-time exception
            // error.
            int num_user = 2;   // number of grid users
            Calendar calendar = Calendar.getInstance();

            // a flag that denotes whether to trace GridSim events or not.
            boolean trace_flag = true;

            // Initialize the GridSim package
            System.out.println("Initializing GridSim package");
            GridSim.initNetworkType(GridSimTags.NET_FLOW_LEVEL);
            GridSim.init(num_user, calendar, trace_flag);


            //////////////////////////////////////////
            // Second step: Creates one or more GridResource entities

            double baud_rate = 1000; // bits/sec
            double propDelay = 10;   // propagation delay in millisecond
            int mtu = 1500;          // max. transmission unit in byte
            int i = 0;

            // more resources can be created by
            // setting totalResource to an appropriate value
            int totalResource = 10;
            ArrayList resList = new ArrayList(totalResource);
            for (i = 0; i < totalResource; i++)
            {
                GridResource res = createGridResource("Res_"+i, baud_rate,
                                                      propDelay, mtu);

                // add a resource into a list
                resList.add(res);
            }

            //////////////////////////////////////////
            // Third step: Creates one or more grid user entities

            // number of Gridlets that will be sent to the resource
            int totalGridlet = 40;

            // create users
            ArrayList userList = new ArrayList(num_user);
            ArrayList userNameList = new ArrayList(); // for background traffic
            for (i = 0; i < num_user; i++)
            {
                String name = "User_" + i;

                // if trace_flag is set to "true", then this experiment will
                // create User_i.csv where i = 0 ... (num_user-1)
                FlowNetUser user = new FlowNetUser(name, totalGridlet, baud_rate,
                                           propDelay, mtu, trace_flag);

                // add a user into a list
                userList.add(user);
                userNameList.add(name);
            }

            //////////////////////////////////////////
            // Fourth step: Builds the network topology among entities.

            // In this example, the topology is:
            // user(s) --1Mb/s-- r1 --10Mb/s-- r2 --1Mb/s-- GridResource(s)

            // create the routers.
            // If trace_flag is set to "true", then this experiment will create
            // the following files (apart from sim_trace and sim_report):
            // - router1_report.csv
            // - router2_report.csv
            Router r1 = new FlowRouter("router1", trace_flag);   // router 1
            Router r2 = new FlowRouter("router2", trace_flag);   // router 2

            // generates some background traffic using SimJava2 distribution
            // package. NOTE: if you set the values to be too high, then
            // the simulation might finish longer
			TrafficGenerator tg = new TrafficGenerator(
			            new Sim_uniform_obj("freq",1,3),  // num of packets 
			            new Sim_uniform_obj("inter_arrival_time",10,20) );
			                            
            // connect all user entities with r1 with 1Mb/s connection
            // For each host, specify which PacketScheduler entity to use.
            FlowNetUser obj = null;
            for (i = 0; i < userList.size(); i++)
            {
                // A First In First Out Scheduler is being used here.
                // SCFQScheduler can be used for more fairness
                FIFOScheduler userSched = new FIFOScheduler("NetUserSched_"+i);
                obj = (FlowNetUser) userList.get(i);
                r1.attachHost(obj, userSched);
                
                // for even user number
                if (i % 2 == 0)
                {
                    // for each time, sends junk packet(s) to all entities
                    tg.setPattern(TrafficGenerator.SEND_ALL);
                    
                    // sends junk packet(s) to resource and user entities
                    obj.setBackgroundTraffic(tg, userNameList);
                }
                else   // for odd user number
                {
                    // for each time, sends junk packet(s) to only one entity
                    tg.setPattern(TrafficGenerator.SEND_ONE_ONLY);
                    
                    // sends junk packet(s) to resource entities only
                    obj.setBackgroundTraffic(tg);
                }                   
            }

            // connect all resource entities with r2 with 1Mb/s connection
            // For each host, specify which PacketScheduler entity to use.
            GridResource resObj = null;
            for (i = 0; i < resList.size(); i++)
            {
                FIFOScheduler resSched = new FIFOScheduler("GridResSched_"+i);
                resObj = (GridResource) resList.get(i);
                r2.attachHost(resObj, resSched);
            }

            // then connect r1 to r2 with 10Mb/s connection
            // For each host, specify which PacketScheduler entity to use.
            baud_rate = 10000;
            Link link = new FlowLink("r1_r2_link", baud_rate, propDelay, Integer.MAX_VALUE);

            FIFOScheduler r1Sched = new FIFOScheduler("r1_Sched");
            FIFOScheduler r2Sched = new FIFOScheduler("r2_Sched");

            // attach r2 to r1
            r1.attachRouter(r2, link, r1Sched, r2Sched);

            //////////////////////////////////////////
            // Fifth step: Starts the simulation
            GridSim.startGridSimulation();

            //////////////////////////////////////////
            // Final step: Prints the Gridlets when simulation is over

            // also prints the routing table
            r1.printRoutingTable();
            r2.printRoutingTable();

            GridletList glList = null;
            for (i = 0; i < userList.size(); i++)
            {
                obj = (FlowNetUser) userList.get(i);
                glList = obj.getGridletList();
                printGridletList(glList, obj.get_name(), false);
            }

            System.out.println("\nFinish network example ...");
        }
        catch (Exception e)
        {
            e.printStackTrace();
            System.out.println("Unwanted errors happen");
        }
    }