NS3中一些难以理解的常数
2024-09-01 01:18:38
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。
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