• 随机变量：在网络仿真科学研究中不可避免地需要计算机提供随机数来模拟现实环境中的一些不确定性行为。正因如此随机数生成器就应运而生，随机数发生器通过一定的算法对事先选定的随机种子做复杂的运算，用产生的结果来近似模拟完全随机数，这种随机数被称作伪随机数。ns-3提供了一个伪随机数生成器（PRNG）。
• 回调：
• 对象模型：ns-3 中提供了 3个对象基类供用户继承。这 3个基类分别是：Object，ObjectBase、SimpleRefCount。ns-3并不强迫每一个类都必须继承自这3个类，但是从这3个基类继承的类包含了ns-3提供的特有特性：属性系统；对象聚合系统；智能指针和引用计数系统。
• 属性系统：
• Tracing系统：模拟现实世界来得出仿真结果供研究人员进一步学习和探索是仿真程序的主要目的，ns-3提供的Tracing系统就是实现这一目的。

/*
 * SPDX-License-Identifier: GPL-2.0-only
 */

#include "ns3/applications-module.h"
#include "ns3/core-module.h"
#include "ns3/csma-module.h"
#include "ns3/internet-module.h"
#include "ns3/mobility-module.h"
#include "ns3/network-module.h"
#include "ns3/point-to-point-module.h"
#include "ns3/ssid.h"
#include "ns3/yans-wifi-helper.h"

// Default Network Topology
//
//   Wifi 10.1.3.0
//                 AP
//  *    *    *    *
//  |    |    |    |    10.1.1.0
// n5   n6   n7   n0 -------------- n1   n2   n3   n4
//                   point-to-point  |    |    |    |
//                                   ================
//                                     LAN 10.1.2.0

using namespace ns3;

NS_LOG_COMPONENT_DEFINE("ThirdScriptExample");

int
main(int argc, char* argv[])
{
    bool verbose = true;
    uint32_t nCsma = 3;
    uint32_t nWifi = 3;
    bool tracing = false;

    CommandLine cmd(__FILE__);
    cmd.AddValue("nCsma", "Number of \"extra\" CSMA nodes/devices", nCsma);
    cmd.AddValue("nWifi", "Number of wifi STA devices", nWifi);
    cmd.AddValue("verbose", "Tell echo applications to log if true", verbose);
    cmd.AddValue("tracing", "Enable pcap tracing", tracing);

    cmd.Parse(argc, argv);

    // The underlying restriction of 18 is due to the grid position
    // allocator's configuration; the grid layout will exceed the
    // bounding box if more than 18 nodes are provided.
    if (nWifi > 18)
    {
        std::cout << "nWifi should be 18 or less; otherwise grid layout exceeds the bounding box"
                  << std::endl;
        return 1;
    }

    if (verbose)
    {
        LogComponentEnable("UdpEchoClientApplication", LOG_LEVEL_INFO);
        LogComponentEnable("UdpEchoServerApplication", LOG_LEVEL_INFO);
    }

    NodeContainer p2pNodes;
    p2pNodes.Create(2);

    PointToPointHelper pointToPoint;
    pointToPoint.SetDeviceAttribute("DataRate", StringValue("5Mbps"));
    pointToPoint.SetChannelAttribute("Delay", StringValue("2ms"));

    NetDeviceContainer p2pDevices;
    p2pDevices = pointToPoint.Install(p2pNodes);

    NodeContainer csmaNodes;
    csmaNodes.Add(p2pNodes.Get(1));//这里用于将PPP网络和CSMA网络链接在一起
    csmaNodes.Create(nCsma);

    CsmaHelper csma;
    csma.SetChannelAttribute("DataRate", StringValue("100Mbps"));
    csma.SetChannelAttribute("Delay", TimeValue(NanoSeconds(6560)));

    NetDeviceContainer csmaDevices;
    csmaDevices = csma.Install(csmaNodes);

    NodeContainer wifiStaNodes;
    wifiStaNodes.Create(nWifi);
    NodeContainer wifiApNode = p2pNodes.Get(0);

    YansWifiChannelHelper channel = YansWifiChannelHelper::Default();
    YansWifiPhyHelper phy;
    phy.SetChannel(channel.Create());

    WifiMacHelper mac;
    Ssid ssid = Ssid("ns-3-ssid");

    WifiHelper wifi;

    NetDeviceContainer staDevices;
    mac.SetType("ns3::StaWifiMac", "Ssid", SsidValue(ssid), "ActiveProbing", BooleanValue(false));
    staDevices = wifi.Install(phy, mac, wifiStaNodes);

    NetDeviceContainer apDevices;
    mac.SetType("ns3::ApWifiMac", "Ssid", SsidValue(ssid));
    apDevices = wifi.Install(phy, mac, wifiApNode);

    MobilityHelper mobility;

    mobility.SetPositionAllocator("ns3::GridPositionAllocator",
                                  "MinX",
                                  DoubleValue(0.0),
                                  "MinY",
                                  DoubleValue(0.0),
                                  "DeltaX",
                                  DoubleValue(5.0),
                                  "DeltaY",
                                  DoubleValue(10.0),
                                  "GridWidth",
                                  UintegerValue(3),
                                  "LayoutType",
                                  StringValue("RowFirst"));

    mobility.SetMobilityModel("ns3::RandomWalk2dMobilityModel",
                              "Bounds",
                              RectangleValue(Rectangle(-50, 50, -50, 50)));
    mobility.Install(wifiStaNodes);

    mobility.SetMobilityModel("ns3::ConstantPositionMobilityModel");
    mobility.Install(wifiApNode);

    InternetStackHelper stack;
    stack.Install(csmaNodes);
    stack.Install(wifiApNode);
    stack.Install(wifiStaNodes);

    Ipv4AddressHelper address;

    address.SetBase("10.1.1.0", "255.255.255.0");
    Ipv4InterfaceContainer p2pInterfaces;
    p2pInterfaces = address.Assign(p2pDevices);

    address.SetBase("10.1.2.0", "255.255.255.0");
    Ipv4InterfaceContainer csmaInterfaces;
    csmaInterfaces = address.Assign(csmaDevices);

    address.SetBase("10.1.3.0", "255.255.255.0");
    address.Assign(staDevices);
    address.Assign(apDevices);

    UdpEchoServerHelper echoServer(9);

    ApplicationContainer serverApps = echoServer.Install(csmaNodes.Get(nCsma));
    serverApps.Start(Seconds(1));
    serverApps.Stop(Seconds(10));

    UdpEchoClientHelper echoClient(csmaInterfaces.GetAddress(nCsma), 9);
    echoClient.SetAttribute("MaxPackets", UintegerValue(1));
    echoClient.SetAttribute("Interval", TimeValue(Seconds(1)));
    echoClient.SetAttribute("PacketSize", UintegerValue(1024));

    ApplicationContainer clientApps = echoClient.Install(wifiStaNodes.Get(nWifi - 1));
    clientApps.Start(Seconds(2));
    clientApps.Stop(Seconds(10));

    Ipv4GlobalRoutingHelper::PopulateRoutingTables();

    Simulator::Stop(Seconds(10));

    if (tracing)
    {
        phy.SetPcapDataLinkType(WifiPhyHelper::DLT_IEEE802_11_RADIO);
        pointToPoint.EnablePcapAll("third");
        phy.EnablePcap("third", apDevices.Get(0));
        csma.EnablePcap("third", csmaDevices.Get(0), true);
    }

    Simulator::Run();
    Simulator::Destroy();
    return 0;
}