NS3中一些难以理解的常数

摘要：在NS3的学习中，PHY MAC中总有一些常数，需要理解才能修改。如帧间间隔等。那么，本文做个简单分析，帮助大家理解。针对802.11标准中MAC协议。

 

void
WifiMac::Configure80211b (void)
{
  SetSifs (MicroSeconds (10));
  SetSlot (MicroSeconds (20));
  SetEifsNoDifs (MicroSeconds (10 + 304));
  SetPifs (MicroSeconds (10 + 20));
  SetCtsTimeout (MicroSeconds (10 + 304 + 20 + GetDefaultMaxPropagationDelay ().GetMicroSeconds () * 2));
  SetAckTimeout (MicroSeconds (10 + 304 + 20 + GetDefaultMaxPropagationDelay ().GetMicroSeconds () * 2));
}

304是怎么来的呢？？

 

1、PHY

采用DSSS,1Mbps模式下。在802.11-2012中，17.2.2.3节中，有PPDU format规定了帧格式。如下图：

其中，大家比较关心的2个参数就是 PLCP Preamble 和 PLCP Header，分别为144bits和48bits。也就是192us，英文为192 MicroSeconds。

计算时间的相关代码，在NS3中 wifi-phy.cc中，代码如下：

uint32_t
WifiPhy::GetPlcpHeaderDurationMicroSeconds (WifiMode payloadMode, WifiPreamble preamble)
{
  switch (payloadMode.GetModulationClass ())
    {
    case WIFI_MOD_CLASS_OFDM:
      {
        switch (payloadMode.GetBandwidth ())
          {
          case 20000000:
          default:
            // (Section 18.3.3 "PLCP preamble (SYNC))" and Figure 18-4 "OFDM training structure"; IEEE Std 802.11-2012)
            // also (Section 18.3.2.4 "Timing related parameters" Table 18-5 "Timing-related parameters"; IEEE Std 802.11-2012)
            // We return the duration of the SIGNAL field only, since the
            // SERVICE field (which strictly speaking belongs to the PLCP
            // header, see Section 18.3.2 and Figure 18-1) is sent using the
            // payload mode.
            return 4;
          case 10000000:
            // (Section 18.3.2.4 "Timing related parameters" Table 18-5 "Timing-related parameters"; IEEE Std 802.11-2012)
            return 8;
          case 5000000:
            // (Section 18.3.2.4 "Timing related parameters" Table 18-5 "Timing-related parameters"; IEEE Std 802.11-2012)
            return 16;
          }
      }
     //Added by Ghada to support 11n
    case WIFI_MOD_CLASS_HT:
      { //IEEE 802.11n Figure 20.1
         switch (preamble)
            {
             case WIFI_PREAMBLE_HT_MF:
               // L-SIG
               return 4;
             case WIFI_PREAMBLE_HT_GF:
               //L-SIG
               return 0;
             default:
               // L-SIG
               return 4;
            }
      }
    case WIFI_MOD_CLASS_ERP_OFDM:
      return 4;

    case WIFI_MOD_CLASS_DSSS:
      if (preamble == WIFI_PREAMBLE_SHORT)
        {
          // (Section 17.2.2.3 "Short PPDU format" and Figure 17-2 "Short PPDU format"; IEEE Std 802.11-2012)
          return 24;
        }
      else // WIFI_PREAMBLE_LONG
        {
          // (Section 17.2.2.2 "Long PPDU format" and Figure 17-1 "Short PPDU format"; IEEE Std 802.11-2012)
          return 48;
        }

    default:
      NS_FATAL_ERROR ("unsupported modulation class");
      return 0;
    }
}

uint32_t
WifiPhy::GetPlcpPreambleDurationMicroSeconds (WifiMode payloadMode, WifiPreamble preamble)
{
  switch (payloadMode.GetModulationClass ())
    {
    case WIFI_MOD_CLASS_OFDM:
      {
        switch (payloadMode.GetBandwidth ())
          {
          case 20000000:
          default:
            // (Section 18.3.3 "PLCP preamble (SYNC))" Figure 18-4 "OFDM training structure"
            // also Section 18.3.2.3 "Modulation-dependent parameters" Table 18-4 "Modulation-dependent parameters"; IEEE Std 802.11-2012)
            return 16;
          case 10000000:
            // (Section 18.3.3 "PLCP preamble (SYNC))" Figure 18-4 "OFDM training structure"
            // also Section 18.3.2.3 "Modulation-dependent parameters" Table 18-4 "Modulation-dependent parameters"; IEEE Std 802.11-2012)
            return 32;
          case 5000000:
            // (Section 18.3.3 "PLCP preamble (SYNC))" Figure 18-4 "OFDM training structure"
            // also Section 18.3.2.3 "Modulation-dependent parameters" Table 18-4 "Modulation-dependent parameters"; IEEE Std 802.11-2012)
            return 64;
          }
      }
    case WIFI_MOD_CLASS_HT:
      { //IEEE 802.11n Figure 20.1 the training symbols before L_SIG or HT_SIG
           return 16;
      }
    case WIFI_MOD_CLASS_ERP_OFDM:
      return 16;

    case WIFI_MOD_CLASS_DSSS:
      if (preamble == WIFI_PREAMBLE_SHORT)
        {
          // (Section 17.2.2.3 "Short PPDU format)" Figure 17-2 "Short PPDU format"; IEEE Std 802.11-2012)
          return 72;
        }
      else // WIFI_PREAMBLE_LONG
        {
          // (Section 17.2.2.2 "Long PPDU format)" Figure 17-1 "Long PPDU format"; IEEE Std 802.11-2012)
          return 144;
        }
    default:
      NS_FATAL_ERROR ("unsupported modulation class");
      return 0;
    }
}

double
WifiPhy::GetPayloadDurationMicroSeconds (uint32_t size, WifiTxVector txvector)
{
  WifiMode payloadMode=txvector.GetMode();

  NS_LOG_FUNCTION (size << payloadMode);
 
  switch (payloadMode.GetModulationClass ())
    {
    case WIFI_MOD_CLASS_OFDM:
    case WIFI_MOD_CLASS_ERP_OFDM:
      {
        // (Section 18.3.2.4 "Timing related parameters" Table 18-5 "Timing-related parameters"; IEEE Std 802.11-2012
        // corresponds to T_{SYM} in the table)
        uint32_t symbolDurationUs;

        switch (payloadMode.GetBandwidth ())
          {
          case 20000000:
          default:
            symbolDurationUs = 4;
            break;
          case 10000000:
            symbolDurationUs = 8;
            break;
          case 5000000:
            symbolDurationUs = 16;
            break;
          }

        // (Section 18.3.2.3 "Modulation-dependent parameters" Table 18-4 "Modulation-dependent parameters"; IEEE Std 802.11-2012)
        // corresponds to N_{DBPS} in the table
        double numDataBitsPerSymbol = payloadMode.GetDataRate () * symbolDurationUs / 1e6;

        // (Section 18.3.5.4 "Pad bits (PAD)" Equation 18-11; IEEE Std 802.11-2012)
        uint32_t numSymbols = lrint (ceil ((16 + size * 8.0 + 6.0) / numDataBitsPerSymbol));

        // Add signal extension for ERP PHY
        if (payloadMode.GetModulationClass () == WIFI_MOD_CLASS_ERP_OFDM)
          {
            return numSymbols * symbolDurationUs + 6;
          }
        else
          {
            return numSymbols * symbolDurationUs;
          }
      }
    case WIFI_MOD_CLASS_HT:
      {
         double symbolDurationUs;
         double m_Stbc;
        //if short GI data rate is used then symbol duration is 3.6us else symbol duration is 4us
        //In the future has to create a stationmanager that only uses these data rates if sender and reciever support GI
         if (payloadMode.GetUniqueName() == "OfdmRate135MbpsBW40MHzShGi" || payloadMode.GetUniqueName() == "OfdmRate65MbpsBW20MHzShGi" )
           {
             symbolDurationUs=3.6;
           }
         else
           {
             switch (payloadMode.GetDataRate ()/ (txvector.GetNss()))
               { //shortGi
                  case 7200000:
                  case 14400000:
                  case 21700000:
                  case 28900000:
                  case 43300000:
                  case 57800000:
                  case 72200000:
                  case 15000000:
                  case 30000000:
                  case 45000000:
                  case 60000000:
                  case 90000000:
                  case 120000000:
                  case 150000000:
                    symbolDurationUs=3.6;
                    break;               
                 default:
                    symbolDurationUs=4;
              }
           }
         if  (txvector.IsStbc())
            m_Stbc=2;
         else
           m_Stbc=1;
         double numDataBitsPerSymbol = payloadMode.GetDataRate () *txvector.GetNss()  * symbolDurationUs / 1e6;
         //check tables 20-35 and 20-36 in the standard to get cases when nes =2
         double Nes=1;
        // IEEE Std 802.11n, section 20.3.11, equation (20-32)
        uint32_t numSymbols = lrint (m_Stbc*ceil ((16 + size * 8.0 + 6.0*Nes) / (m_Stbc* numDataBitsPerSymbol)));
       
        return numSymbols * symbolDurationUs;
         
      }
    case WIFI_MOD_CLASS_DSSS:
      // (Section 17.2.3.6 "Long PLCP LENGTH field"; IEEE Std 802.11-2012)
      NS_LOG_LOGIC (" size=" << size
                             << " mode=" << payloadMode
                             << " rate=" << payloadMode.GetDataRate () );
      return lrint (ceil ((size * 8.0) / (payloadMode.GetDataRate () / 1.0e6)));

    default:
      NS_FATAL_ERROR ("unsupported modulation class");
      return 0;
    }
}

Time
WifiPhy::CalculateTxDuration (uint32_t size, WifiTxVector txvector, WifiPreamble preamble)
{
  WifiMode payloadMode=txvector.GetMode();
  double duration = GetPlcpPreambleDurationMicroSeconds (payloadMode, preamble)
    + GetPlcpHeaderDurationMicroSeconds (payloadMode, preamble)
    + GetPlcpHtSigHeaderDurationMicroSeconds (payloadMode, preamble)
    + GetPlcpHtTrainingSymbolDurationMicroSeconds (payloadMode, preamble,txvector)
    + GetPayloadDurationMicroSeconds (size, txvector);
  return MicroSeconds (duration);
}

在函数CalculateTxDuration中，duration的计算方法。

那么，假如你开启4次握手机制，那么rts的duration如何计算呢？

也就是当你生成pacp文件，用wiresharp打开时，看到rts帧中，那个duration是怎么得到的呢？

如下图中17342 是怎么得到的呢？

你需要知道应用层的包是如何封装的，这涉及到计算机网络的知识。这里以上面的包大小举例说明,packet =2000bytes.

上图中可以看到：data—>udp（8）—>ip（20）—>llc（8）—>mac （28）包封装过程

ip和udp封装包头大小，一般计算机网络书中有介绍。llc 这个没搞懂为啥是8个。mac数据帧可以看下图：

一共40字节，但是地址4，qos，ht不用。ns3中使用的是non qos mac。

好了，我们开始计算，但是还需要看一个代码在mac-low.cc：

void
MacLow::SendRtsForPacket (void)
{
  NS_LOG_FUNCTION (this);
  /* send an RTS for this packet. */
  WifiMacHeader rts;
  rts.SetType (WIFI_MAC_CTL_RTS);
  rts.SetDsNotFrom ();
  rts.SetDsNotTo ();
  rts.SetNoRetry ();
  rts.SetNoMoreFragments ();
  rts.SetAddr1 (m_currentHdr.GetAddr1 ());
  rts.SetAddr2 (m_self);
  WifiTxVector rtsTxVector = GetRtsTxVector (m_currentPacket, &m_currentHdr);
  Time duration = Seconds (0);

  WifiPreamble preamble;
  //standard says RTS packets can have GF format sec 9.6.0e.1 page 110 bullet b 2
  if ( m_phy->GetGreenfield()&& m_stationManager->GetGreenfieldSupported (m_currentHdr.GetAddr1 ()))
    preamble= WIFI_PREAMBLE_HT_GF;
  else if (rtsTxVector.GetMode().GetModulationClass () == WIFI_MOD_CLASS_HT)
    preamble= WIFI_PREAMBLE_HT_MF;
  else
    preamble=WIFI_PREAMBLE_LONG;

  if (m_txParams.HasDurationId ())
    {
      duration += m_txParams.GetDurationId ();
    }
  else
    {
      WifiTxVector dataTxVector = GetDataTxVector (m_currentPacket, &m_currentHdr);
      duration += GetSifs ();
      duration += GetCtsDuration (m_currentHdr.GetAddr1 (), rtsTxVector);
      duration += GetSifs ();
      duration += m_phy->CalculateTxDuration (GetSize (m_currentPacket, &m_currentHdr),
                                              dataTxVector, preamble);
      duration += GetSifs ();
      duration += GetAckDuration (m_currentHdr.GetAddr1 (), dataTxVector);
    }
  rts.SetDuration (duration);

  Time txDuration = m_phy->CalculateTxDuration (GetRtsSize (), rtsTxVector, preamble);
  Time timerDelay = txDuration + GetCtsTimeout ();

  NS_ASSERT (m_ctsTimeoutEvent.IsExpired ());
  NotifyCtsTimeoutStartNow (timerDelay);
  m_ctsTimeoutEvent = Simulator::Schedule (timerDelay, &MacLow::CtsTimeout, this);

  Ptr<Packet> packet = Create<Packet> ();
  packet->AddHeader (rts);
  WifiMacTrailer fcs;
  packet->AddTrailer (fcs);

  ForwardDown (packet, &rts, rtsTxVector,preamble);
}

 

公式就是上面这个代码中提取出来的。sifs查这个802.11-2012中上图

duration += GetSifs ();  10

duration += GetCtsDuration (m_currentHdr.GetAddr1 (), rtsTxVector);  cts:14*8+192=304

duration += GetSifs (); 10

duration += m_phy->CalculateTxDuration (GetSize (m_currentPacket, &m_currentHdr), dataTxVector, preamble); 2064*8+192=16704

duration += GetSifs ();10

duration += GetAckDuration (m_currentHdr.GetAddr1 (), dataTxVector); ack:14*8+192=304

duration = 10+304+10+16704+10+304=17342

结果符合wiresharp中那个duration。