NS3中一些難以理解的常數 - khldragon - 博客園

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NS3中一些難以理解的常數

摘要：在NS3的學習中，PHY MAC中總有一些常數，需要理解才能修改。如幀間間隔等。那麼，本文做個簡單分析，幫助大家理解。針對802.11標準中MAC協議。

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是怎麼來的呢？？

1、PHY

採用DSSS,1Mbps模式下。在802.11-2012中，17.2.2.3節中，有PPDU format規定了幀格式。如下圖：

其中，大家比較關心的2個參數就是 PLCP Preamble 和 PLCP Header，分別爲144bits和48bits。也就是192us，英文爲192 MicroSeconds。

計算時間的相關代碼，在NS3中 wifi-phy.cc中，代碼如下：

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);
}

在函數CalculateTxDuration中，duration的計算方法。

那麼，假如你開啓4次握手機制，那麼rts的duration如何計算呢？

也就是當你生成pacp文件，用wiresharp打開時，看到rts幀中，那個duration是怎麼得到的呢？

如下圖中17342 是怎麼得到的呢？

你需要知道應用層的包是如何封裝的，這涉及到計算機網絡的知識。這裏以上面的包大小舉例說明,packet =2000bytes.

上圖中可以看到：data—>udp（8）—>ip（20）—>llc（8）—>mac （28）包封裝過程

ip和udp封裝包頭大小，一般計算機網絡書中有介紹。llc 這個沒搞懂爲啥是8個。mac數據幀可以看下圖：

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

好了，我們開始計算，但是還需要看一個代碼在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);
}

公式就是上面這個代碼中提取出來的。sifs查這個802.11-2012中上圖

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

結果符合wiresharp中那個duration。

    </div>
    <div class = "postDesc">posted @ <span id="post-date">2014-11-28 22:31</span> <a href='http://www.cnblogs.com/khldragon/'>khldragon</a> 閱讀(<span id="post_view_count">...</span>) 評論(<span id="post_comment_count">...</span>)  <a href ="https://i.cnblogs.com/EditPosts.aspx?postid=4129518" rel="nofollow">編輯</a> <a href="#" onclick="AddToWz(4129518);return false;">收藏</a></div>
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