ns3源码阅读(二)net-device和channel

 仿真，主要模拟的就是网络实体的处理速率与链路之间的时延。
 ns3的网卡处理速率反映在NetDevice这个类中，而链路时延则反映在Channel中。上层的数据包发送速率可能比NetDevice处理速率要快，因此类NetDevice要有一个队列，缓存下来不及发送的数据包。
 数据包进入队列处理逻辑

bool
PointToPointNetDevice::Send (
  Ptr packet, 
  const Address &dest, 
  uint16_t protocolNumber)
  {
    if (m_queue->Enqueue (packet))
    {
      //
      // If the channel is ready for transition we send the packet right now
      // 
      if (m_txMachineState == READY)
        {
          packet = m_queue->Dequeue ();
          m_snifferTrace (packet);
          m_promiscSnifferTrace (packet);
          bool ret = TransmitStart (packet);
          return ret;
        }
      return true;
    }
  // Enqueue may fail (overflow)
  m_macTxDropTrace (packet);
  return false;
  }

void PointToPointNetDevice::TransmitStart (Ptr<Packet> p)
{
  m_txMachineState = BUSY;/设置发送状态
  Time txTime = m_bps.CalculateBytesTxTime (p->GetSize ());//模拟链路发送时延
  Time txCompleteTime = txTime + m_tInterframeGap;

  Simulator::Schedule (txCompleteTime, &PointToPointNetDevice::TransmitComplete, this);//数据成功发送后的回调

  bool result = m_channel->TransmitStart (p, this, txTime);//发送到channel中模拟链路传输时延
}
void PointToPointNetDevice::TransmitComplete (void)
{
  m_txMachineState = READY;
  Ptr<Packet> p = m_queue->Dequeue ();//弹出缓冲队列中的数据包
  TransmitStart (p);//数据包发送
}

 channel中的TransmitStart函数：

bool
PointToPointChannel::TransmitStart (
  Ptr<const Packet> p,
  Ptr src,
  Time txTime)
{
  Simulator::ScheduleWithContext (m_link[wire].m_dst->GetNode ()->GetId (),txTime + m_delay, &PointToPointNetDevice::Receive, m_link[wire].m_dst, p->Copy ());//经过txTime + m_delay时延后，通过回调进行数据上传
}

 而且此处ScheduleWithContext对应的模板函数是：

template <typename MEM, typename OBJ,
          typename T1>
void Simulator::ScheduleWithContext (uint32_t context, Time const &delay, MEM mem_ptr, OBJ obj, T1 a1)
{
  return ScheduleWithContext (context, delay, MakeEvent (mem_ptr, obj, a1));
}