wifi: Align EDCA transmissions to slot boundaries

CHANGES.html

@@ -88,6 +88,12 @@ and <b> BuildingsHelper </b> classes, a building aware pathloss models, e.g.,
 <b> HybridBuildingsPropagationLossModel </b> is now able to accurately compute 
 the pathloss for a node moving in and out of buildings in a simulation. See <a href=https://gitlab.com/nsnam/ns-3-dev/issues/80>issue 80</a> 
 for discussion.</li>
+<li> The implementation of the wifi channel access functions has been improved
+to make them more adherent to the IEEE 802.11-2016 standard. Concerning the DCF,
+the backoff procedure is no longer invoked when a packet is queued for transmission
+and the medium has not been idle for a DIFS, but it is invoked if the medium is busy
+or does not remain idle for a DIFS after the packet has been queued. Concerning the
+EDCAF, tranmissions are now correctly aligned at slot boundaries.</li>
 </ul>
 
 <hr>

RELEASE_NOTES

@@ -29,6 +29,8 @@ Bugs fixed
 ----------
 - Issue #119 - Waf --lcov-report option was broken
 - Issue #84 - Wi-Fi removing wrong header due to copy-paste error
+- wifi: a zero value for the backoff timer might be discarded and a new value
+  might be generated by an erroneous call to NotifyCollision().
 
 Known issues
 ------------

src/wifi/doc/source/wifi-design.rst

@@ -578,9 +578,28 @@ where the backoff timer duration is lazily calculated whenever needed since it
 is claimed to have much better performance than the simpler recurring timer
 solution.
 
-The backoff procedure of DCF is described in section 9.2.5.2 of [ieee80211]_.
+The DCF basic access is described in section 10.3.4.2 of [ieee80211-2016]_.
 
-*  “The backoff procedure shall be invoked for a STA to transfer a frame 
+*  “A STA may transmit an MPDU when it is operating under the DCF access method
+   [..] when the STA determines that the medium is idle when a frame is queued
+   for transmission, and remains idle for a period of a DIFS, or an EIFS
+   (10.3.2.3.7) from the end of the immediately preceding medium-busy event,
+   whichever is the greater, and the backoff timer is zero. Otherwise the random
+   backoff procedure described in 10.3.4.3 shall be followed."
+
+Thus, a station is allowed not to invoke the backoff procedure if all of the
+following conditions are met:
+
+*  the medium is idle when a frame is queued for transmission
+*  the medium remains idle until the most recent of these two events: a DIFS
+   from the time when the frame is queued for transmission; an EIFS from the
+   end of the immediately preceding medium-busy event (associated with the
+   reception of an erroneous frame)
+*  the backoff timer is zero
+
+The backoff procedure of DCF is described in section 10.3.4.3 of [ieee80211-2016]_.
+
+*  “A STA shall invoke the backoff procedure to transfer a frame
    when finding the medium busy as indicated by either the physical or 
    virtual CS mechanism.”
 *  “A backoff procedure shall be performed immediately after the end of 
@@ -588,19 +607,43 @@ The backoff procedure of DCF is described in section 9.2.5.2 of [ieee80211]_.
    type Data, Management, or Control with subtype PS-Poll, even if no 
    additional transmissions are currently queued.”
 
-Thus, if the queue is empty, a newly arrived packet should be transmitted 
-immediately after channel is sensed idle for DIFS.  If queue is not empty 
-and after a successful MPDU that has no more fragments, a node should 
-also start the backoff timer.
+The EDCA backoff procedure is slightly different than the DCF backoff procedure
+and is described in section 10.22.2.2 of [ieee80211-2016]_. The backoff procedure
+shall be invoked by an EDCAF when any of the following events occur:
 
-Some users have observed that the 802.11 MAC with an empty queue on an 
-idle channel will transmit the first frame arriving to the model 
-immediately without waiting for DIFS or backoff, and wonder whether this 
-is compliant.  According to the standard, “The backoff procedure shall 
-be invoked for a STA to transfer a frame when finding the medium busy 
-as indicated by either the physical or virtual CS mechanism.”  So in 
-this case, the medium is not found to be busy in recent past and the 
-station can transmit immediately. 
+*  a frame is "queued for transmission such that one of the transmit queues
+   associated with that AC has now become non-empty and any other transmit queues
+   associated with that AC are empty; the medium is busy on the primary channel"
+*  "The transmission of the MPDU in the final PPDU transmitted by the TXOP holder
+   during the TXOP for that AC has completed and the TXNAV timer has expired, and
+   the AC was a primary AC"
+*  "The transmission of an MPDU in the initial PPDU of a TXOP fails [..] and the
+   AC was a primary AC"
+*  "The transmission attempt collides internally with another EDCAF of an AC that
+   has higher priority"
+*  (optionally) "The transmission by the TXOP holder of an MPDU in a non-initial
+   PPDU of a TXOP fails"
+
+Additionally, section 10.22.2.4 of [ieee80211-2016]_ introduces the notion of
+slot boundary, which basically occurs following SIFS + AIFSN * slotTime of idle
+medium after the last busy medium that was the result of a reception of a frame
+with a correct FCS or following EIFS - DIFS + AIFSN * slotTime + SIFS of idle
+medium after the last indicated busy medium that was the result of a frame reception
+that has resulted in FCS error, or following a slotTime of idle medium occurring
+immediately after any of these conditions.
+
+On these specific slot boundaries, each EDCAF shall make a determination to perform
+one and only one of the following functions:
+
+*  Decrement the backoff timer.
+*  Initiate the transmission of a frame exchange sequence.
+*  Invoke the backoff procedure due to an internal collision.
+*  Do nothing.
+
+Thus, if an EDCAF decrements its backoff timer on a given slot boundary and, as
+a result, the backoff timer has a zero value, the EDCAF cannot immediately
+transmit, but it has to wait for another slotTime of idle medium before transmission
+can start.
 
 The higher-level MAC functions are implemented in a set of other C++ classes and
 deal with:

src/wifi/doc/source/wifi-references.rst

@@ -7,6 +7,8 @@ References
 
 .. [ieee80211] IEEE Std 802.11-2012, *Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications*
 
+.. [ieee80211-2016] IEEE Std 802.11-2016, *Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications*
+
 .. [pei80211b] \G. Pei and Tom Henderson, `Validation of ns-3 802.11b PHY model <http://www.nsnam.org/~pei/80211b.pdf>`__ 
 
 .. [pei80211ofdm] \G. Pei and Tom Henderson, `Validation of OFDM error rate model in ns-3 <http://www.nsnam.org/~pei/80211ofdm.pdf>`__ 

src/wifi/model/channel-access-manager.cc

@@ -262,6 +262,21 @@ ChannelAccessManager::RequestAccess (Ptr<Txop> state, bool isCfPeriod)
       m_accessTimeout = Simulator::Schedule (delay, &ChannelAccessManager::DoGrantPcfAccess, this, state);
       return;
     }
+  /*
+   * EDCAF operations shall be performed at slot boundaries (Sec. 10.22.2.4 of 802.11-2016)
+   */
+  Time accessGrantStart = GetAccessGrantStart () + (state->GetAifsn () * m_slot);
+
+  if (state->IsQosTxop () && state->GetBackoffStart () > accessGrantStart)
+    {
+      // The backoff start time reported by the EDCAF is more recent than the last
+      // time the medium was busy plus an AIFS, hence we need to align it to the
+      // next slot boundary.
+      Time diff = state->GetBackoffStart () - accessGrantStart;
+      uint32_t nIntSlots = (diff / m_slot).GetHigh () + 1;
+      state->UpdateBackoffSlotsNow (0, accessGrantStart + (nIntSlots * m_slot));
+    }
+
   UpdateBackoff ();
   NS_ASSERT (!state->IsAccessRequested ());
   state->NotifyAccessRequested ();

src/wifi/model/txop.cc

@@ -341,7 +341,10 @@ Txop::GenerateBackoffUponAccessIfNeeded (void)
       if (!m_channelAccessManager->IsBusy ())
         {
           // medium idle. If this is a DCF, use immediate access (we can transmit
-          // in a DIFS if the medium remains idle).
+          // in a DIFS if the medium remains idle). If this is an EDCAF, update
+          // the backoff start time kept by the EDCAF to the current time in order
+          // to correctly align the backoff start time at the next slot boundary
+          // (performed by the next call to ChannelAccessManager::RequestAccess())
           Time delay = (IsQosTxop () ? Seconds (0) : m_low->GetSifs () + GetAifsn () * m_low->GetSlotTime ());
           UpdateBackoffSlotsNow (0, Simulator::Now () + delay);
         }

src/wifi/test/block-ack-test-suite.cc

@@ -868,14 +868,29 @@ BlockAckAggregationDisabledTest::DoRun (void)
   packetSocket.Install (wifiStaNode);
   packetSocket.Install (wifiApNode);
 
-  Ptr<PacketSocketClient> client = CreateObject<PacketSocketClient> ();
-  client->SetAttribute ("PacketSize", UintegerValue (1400));
-  client->SetAttribute ("MaxPackets", UintegerValue (14));
-  client->SetAttribute ("Interval", TimeValue (MicroSeconds (0)));
-  client->SetRemote (socket);
-  wifiStaNode.Get (0)->AddApplication (client);
-  client->SetStartTime (Seconds (1));
-  client->SetStopTime (Seconds (3.0));
+  // the first client application generates a single packet, which is sent
+  // with the normal ack policy because there are no other packets queued
+  Ptr<PacketSocketClient> client1 = CreateObject<PacketSocketClient> ();
+  client1->SetAttribute ("PacketSize", UintegerValue (1400));
+  client1->SetAttribute ("MaxPackets", UintegerValue (1));
+  client1->SetAttribute ("Interval", TimeValue (MicroSeconds (0)));
+  client1->SetRemote (socket);
+  wifiStaNode.Get (0)->AddApplication (client1);
+  client1->SetStartTime (Seconds (1));
+  client1->SetStopTime (Seconds (3.0));
+
+  // the second client application generates 13 packets. Even if when the first
+  // packet is queued the queue is empty, the first packet is not transmitted
+  // immediately, but the EDCAF waits for the next slot boundary. At that time,
+  // other packets have been queued, hence a BA agreement is established first.
+  Ptr<PacketSocketClient> client2 = CreateObject<PacketSocketClient> ();
+  client2->SetAttribute ("PacketSize", UintegerValue (1400));
+  client2->SetAttribute ("MaxPackets", UintegerValue (13));
+  client2->SetAttribute ("Interval", TimeValue (MicroSeconds (0)));
+  client2->SetRemote (socket);
+  wifiStaNode.Get (0)->AddApplication (client2);
+  client2->SetStartTime (Seconds (1.5));
+  client2->SetStopTime (Seconds (3.0));
 
   Ptr<PacketSocketServer> server = CreateObject<PacketSocketServer> ();
   server->SetLocal (socket);
@@ -892,7 +907,7 @@ BlockAckAggregationDisabledTest::DoRun (void)
 
   Simulator::Destroy ();
 
-  // The client application generates 14 packets, so we expect that the wifi PHY
+  // The client applications generate 14 packets, so we expect that the wifi PHY
   // layer transmits 14 MPDUs, the server application receives 14 packets, and
   // two BARs are transmitted.
   NS_TEST_EXPECT_MSG_EQ (m_txTotal, 14, "Unexpected number of transmitted packets");

src/wifi/test/inter-bss-test-suite.cc

@@ -231,11 +231,11 @@ TestInterBssConstantObssPdAlgo::SetupSimulation ()
 
   // AP2 sends a packet 0.5s later.
   Simulator::Schedule (Seconds (2.0), &TestInterBssConstantObssPdAlgo::SendOnePacket, this, ap_device2, sta_device2, m_payloadSize2);
-  Simulator::Schedule (Seconds (2.0) + MicroSeconds (1), &TestInterBssConstantObssPdAlgo::CheckPhyState, this, ap_device2, WifiPhyState::TX);
+  Simulator::Schedule (Seconds (2.0) + MicroSeconds (5), &TestInterBssConstantObssPdAlgo::CheckPhyState, this, ap_device2, WifiPhyState::TX);
   // All other PHYs should have stay idle until 4us (preamble detection time).
-  Simulator::Schedule (Seconds (2.0) + MicroSeconds (2), &TestInterBssConstantObssPdAlgo::CheckPhyState, this, sta_device1, WifiPhyState::IDLE);
-  Simulator::Schedule (Seconds (2.0) + MicroSeconds (2), &TestInterBssConstantObssPdAlgo::CheckPhyState, this, sta_device2, WifiPhyState::IDLE);
-  Simulator::Schedule (Seconds (2.0) + MicroSeconds (2), &TestInterBssConstantObssPdAlgo::CheckPhyState, this, ap_device1, WifiPhyState::IDLE);
+  Simulator::Schedule (Seconds (2.0) + MicroSeconds (6), &TestInterBssConstantObssPdAlgo::CheckPhyState, this, sta_device1, WifiPhyState::IDLE);
+  Simulator::Schedule (Seconds (2.0) + MicroSeconds (6), &TestInterBssConstantObssPdAlgo::CheckPhyState, this, sta_device2, WifiPhyState::IDLE);
+  Simulator::Schedule (Seconds (2.0) + MicroSeconds (6), &TestInterBssConstantObssPdAlgo::CheckPhyState, this, ap_device1, WifiPhyState::IDLE);
   // All PHYs should be receiving the PHY header if preamble has been detected (always the case in this test).
   Simulator::Schedule (Seconds (2.0) + MicroSeconds (10), &TestInterBssConstantObssPdAlgo::CheckPhyState, this, sta_device1, WifiPhyState::RX);
   Simulator::Schedule (Seconds (2.0) + MicroSeconds (10), &TestInterBssConstantObssPdAlgo::CheckPhyState, this, sta_device2, WifiPhyState::RX);
@@ -251,18 +251,18 @@ TestInterBssConstantObssPdAlgo::SetupSimulation ()
   // AP2 sends another packet 0.1s later.
   Simulator::Schedule (Seconds (2.1), &TestInterBssConstantObssPdAlgo::SendOnePacket, this, ap_device2, sta_device2, m_payloadSize2);
   // STA1 sends a packet 100us later. Even though AP2 is still transmitting, STA1 can transmit simultaneously if it's PHY was reset by OBSS_PD SR.
-  Simulator::Schedule (Seconds (2.1) + MicroSeconds (100), &TestInterBssConstantObssPdAlgo::SendOnePacket, this, sta_device1, ap_device1, m_payloadSize1);
+  Simulator::Schedule (Seconds (2.1) + MicroSeconds (90), &TestInterBssConstantObssPdAlgo::SendOnePacket, this, sta_device1, ap_device1, m_payloadSize1);
   if (expectPhyReset)
     {
       // In this case, we check the TX power is restricted (and set the expected value slightly before transmission should occur)
       double expectedTxPower = std::min (m_txPowerDbm, 21 - (m_obssPdLevelDbm + 82));
-      Simulator::Schedule (Seconds (2.1) + MicroSeconds (99), &TestInterBssConstantObssPdAlgo::SetExpectedTxPower, this, expectedTxPower);
+      Simulator::Schedule (Seconds (2.1) + MicroSeconds (89), &TestInterBssConstantObssPdAlgo::SetExpectedTxPower, this, expectedTxPower);
     }
   // Check simultaneous transmissions
-  Simulator::Schedule (Seconds (2.1) + MicroSeconds (105), &TestInterBssConstantObssPdAlgo::CheckPhyState, this, sta_device1, expectPhyReset ? WifiPhyState::TX : WifiPhyState::RX);
-  Simulator::Schedule (Seconds (2.1) + MicroSeconds (105), &TestInterBssConstantObssPdAlgo::CheckPhyState, this, ap_device1, WifiPhyState::RX);
-  Simulator::Schedule (Seconds (2.1) + MicroSeconds (105), &TestInterBssConstantObssPdAlgo::CheckPhyState, this, sta_device2, WifiPhyState::RX);
-  Simulator::Schedule (Seconds (2.1) + MicroSeconds (105), &TestInterBssConstantObssPdAlgo::CheckPhyState, this, ap_device2, WifiPhyState::TX);
+  Simulator::Schedule (Seconds (2.1) + MicroSeconds (100), &TestInterBssConstantObssPdAlgo::CheckPhyState, this, sta_device1, expectPhyReset ? WifiPhyState::TX : WifiPhyState::RX);
+  Simulator::Schedule (Seconds (2.1) + MicroSeconds (100), &TestInterBssConstantObssPdAlgo::CheckPhyState, this, ap_device1, WifiPhyState::RX);
+  Simulator::Schedule (Seconds (2.1) + MicroSeconds (100), &TestInterBssConstantObssPdAlgo::CheckPhyState, this, sta_device2, WifiPhyState::RX);
+  Simulator::Schedule (Seconds (2.1) + MicroSeconds (100), &TestInterBssConstantObssPdAlgo::CheckPhyState, this, ap_device2, WifiPhyState::TX);
 
   // Verify transmit power restrictions are not applied if access to the channel is requested after ignored OBSS transmissions.
 
@@ -280,14 +280,14 @@ TestInterBssConstantObssPdAlgo::SetupSimulation ()
   // This test checks whether this sequence preserves transmit power restrictions if CCA resets occurred, since STA 1 has been deferring during ignored OBSS transmissions.
 
   Simulator::Schedule (Seconds (2.4), &TestInterBssConstantObssPdAlgo::SendOnePacket, this, sta_device2, ap_device2, m_payloadSize2 / 10);
-  Simulator::Schedule (Seconds (2.4) + MicroSeconds (5), &TestInterBssConstantObssPdAlgo::SendOnePacket, this, ap_device2, sta_device2, m_payloadSize2 / 10);
-  Simulator::Schedule (Seconds (2.4) + MicroSeconds (65), &TestInterBssConstantObssPdAlgo::SendOnePacket, this, ap_device1, sta_device1, m_payloadSize1 / 10);
-  Simulator::Schedule (Seconds (2.4) + MicroSeconds (105), &TestInterBssConstantObssPdAlgo::SendOnePacket, this, ap_device3, sta_device3, m_payloadSize3 / 10);
+  Simulator::Schedule (Seconds (2.4) + MicroSeconds (15), &TestInterBssConstantObssPdAlgo::SendOnePacket, this, ap_device2, sta_device2, m_payloadSize2 / 10);
+  Simulator::Schedule (Seconds (2.4) + MicroSeconds (270), &TestInterBssConstantObssPdAlgo::SendOnePacket, this, ap_device1, sta_device1, m_payloadSize1 / 10);
+  Simulator::Schedule (Seconds (2.4) + MicroSeconds (300), &TestInterBssConstantObssPdAlgo::SendOnePacket, this, ap_device3, sta_device3, m_payloadSize3 / 10);
   if (expectPhyReset)
     {
       // In this case, we check the TX power is restricted (and set the expected value slightly before transmission should occur)
       double expectedTxPower = std::min (m_txPowerDbm, 21 - (m_obssPdLevelDbm + 82));
-      Simulator::Schedule (Seconds (2.4) + MicroSeconds (450), &TestInterBssConstantObssPdAlgo::SetExpectedTxPower, this, expectedTxPower);
+      Simulator::Schedule (Seconds (2.4) + MicroSeconds (400), &TestInterBssConstantObssPdAlgo::SetExpectedTxPower, this, expectedTxPower);
     }
 
   Simulator::Stop (Seconds (2.5));

src/wifi/test/wifi-test.cc

@@ -1954,14 +1954,14 @@ private:
    */
   void RunSubtest (PointerValue apErrorModel, PointerValue staErrorModel);
 
-  uint8_t m_receivedNormalMpduCount; ///< Count received normal MPDU packets on STA
-  uint8_t m_receivedAmpduCount;      ///< Count received A-MPDU packets on STA
-  uint8_t m_droppedActionCount;      ///< Count dropped ADDBA request/response
-  uint8_t m_addbaEstablishedCount;   ///< Count number of times ADDBA state machine is in established state
-  uint8_t m_addbaPendingCount;       ///< Count number of times ADDBA state machine is in pending state
-  uint8_t m_addbaRejectedCount;      ///< Count number of times ADDBA state machine is in rejected state
-  uint8_t m_addbaNoReplyCount;       ///< Count number of times ADDBA state machine is in no_reply state
-  uint8_t m_addbaResetCount;         ///< Count number of times ADDBA state machine is in reset state
+  uint16_t m_receivedNormalMpduCount; ///< Count received normal MPDU packets on STA
+  uint16_t m_receivedAmpduCount;      ///< Count received A-MPDU packets on STA
+  uint16_t m_droppedActionCount;      ///< Count dropped ADDBA request/response
+  uint16_t m_addbaEstablishedCount;   ///< Count number of times ADDBA state machine is in established state
+  uint16_t m_addbaPendingCount;       ///< Count number of times ADDBA state machine is in pending state
+  uint16_t m_addbaRejectedCount;      ///< Count number of times ADDBA state machine is in rejected state
+  uint16_t m_addbaNoReplyCount;       ///< Count number of times ADDBA state machine is in no_reply state
+  uint16_t m_addbaResetCount;         ///< Count number of times ADDBA state machine is in reset state
 };
 
 Bug2470TestCase::Bug2470TestCase ()
@@ -2096,8 +2096,10 @@ Bug2470TestCase::RunSubtest (PointerValue apErrorModel, PointerValue staErrorMod
   Config::Connect ("/NodeList/*/DeviceList/*/$ns3::WifiNetDevice/Phy/$ns3::WifiPhy/PhyRxDrop", MakeCallback (&Bug2470TestCase::RxDropCallback, this));
   Config::Connect ("/NodeList/*/DeviceList/*/$ns3::WifiNetDevice/Mac/$ns3::RegularWifiMac/BE_Txop/BlockAckManager/AgreementState", MakeCallback (&Bug2470TestCase::AddbaStateChangedCallback, this));
 
-  Simulator::Schedule (Seconds (0.5), &Bug2470TestCase::SendPacketBurst, this, 5, apDevice.Get (0), staDevice.Get (0)->GetAddress ());
-  Simulator::Schedule (Seconds (0.8), &Bug2470TestCase::SendPacketBurst, this, 5, apDevice.Get (0), staDevice.Get (0)->GetAddress ());
+  Simulator::Schedule (Seconds (0.5), &Bug2470TestCase::SendPacketBurst, this, 1, apDevice.Get (0), staDevice.Get (0)->GetAddress ());
+  Simulator::Schedule (Seconds (0.5) + MicroSeconds (5), &Bug2470TestCase::SendPacketBurst, this, 4, apDevice.Get (0), staDevice.Get (0)->GetAddress ());
+  Simulator::Schedule (Seconds (0.8), &Bug2470TestCase::SendPacketBurst, this, 1, apDevice.Get (0), staDevice.Get (0)->GetAddress ());
+  Simulator::Schedule (Seconds (0.8) + MicroSeconds (5), &Bug2470TestCase::SendPacketBurst, this, 4, apDevice.Get (0), staDevice.Get (0)->GetAddress ());
 
   Simulator::Stop (Seconds (1.0));
   Simulator::Run ();