wifi: Fix EDCA immediate channel access

...when channel access is requested at a slot boundary.

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

@@ -17,6 +17,7 @@
 #include "ns3/log.h"
 #include "ns3/simulator.h"
 
+#include <cmath>
 #include <sstream>
 
 #undef NS_LOG_APPEND_CONTEXT
@@ -591,13 +592,13 @@ ChannelAccessManager::RequestAccess(Ptr<Txop> txop)
      */
     Time accessGrantStart = GetAccessGrantStart() + (txop->GetAifsn(m_linkId) * GetSlot());
 
-    if (txop->IsQosTxop() && txop->GetBackoffStart(m_linkId) > accessGrantStart)
+    if (const auto diff = txop->GetBackoffStart(m_linkId) - accessGrantStart;
+        txop->IsQosTxop() && diff.IsStrictlyPositive())
     {
-        // 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 = txop->GetBackoffStart(m_linkId) - accessGrantStart;
-        uint32_t nIntSlots = (diff / GetSlot()).GetHigh() + 1;
+        // 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.
+        const auto div = diff / GetSlot();
+        const uint32_t nIntSlots = div.GetHigh() + (div.GetLow() > 0 ? 1 : 0);
         txop->UpdateBackoffSlotsNow(0, accessGrantStart + (nIntSlots * GetSlot()), m_linkId);
     }
 

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

@@ -1746,15 +1746,15 @@ LargestIdlePrimaryChannelTest::DoRun()
  * that the frame is not transmitted. A backoff value is kept being generated as long as the
  * frame is kept in the queue.
  *
- *                                                       Backoff                             Last
- * Backoff                 Backoff        Backoff        value #3,                          backoff
- * value #0                value #1       value #2       unblock queue                       value
- *  |              ┌─────┐    |              |              |              ┌─────┐   ┌────┐    |
- *  |       ┌───┐  │Assoc│    |    |Decrement|    |Decrement|    |Decrement│ADDBA│   │QoS │    |
- *  |       │ACK│  │Resp │    |AIFS| backoff |slot| backoff |slot| backoff │ Req │. .│data│    |
- * ──┬─────┬┴───┴──┴─────┴┬───┬────────────────────────────────────────────┴─────┴───┴────┴┬───┬──
- *   │Assoc│              │ACK│                                                            │ACK│
- *   │ Req │              └───┘                                                            └───┘
+ *                                                      Backoff                          Last
+ * Backoff                 Backoff          Backoff     value #3,                       backoff
+ * value #0                value #1         value #2    unblock queue                    value
+ *  |              ┌─────┐    |                |           |           ┌─────┐   ┌────┐    |
+ *  |       ┌───┐  │Assoc│    |    | Decrement | Decrement | Decrement │ADDBA│   │QoS │    |
+ *  |       │ACK│  │Resp │    |AIFS|  backoff  |  backoff  |  backoff  │ Req │. .│data│    |
+ * ──┬─────┬┴───┴──┴─────┴┬───┬────────────────────────────────────────┴─────┴───┴────┴┬───┬──
+ *   │Assoc│              │ACK│                                                        │ACK│
+ *   │ Req │              └───┘                                                        └───┘
  *   └─────┘
  *
  * The ProactiveBackoff test checks the generation of backoff values when the attribute is set
@@ -2164,10 +2164,6 @@ BackoffGenerationTest::BackoffGenerated(AcIndex ac, uint32_t backoff, uint8_t li
                                   offset,
                                   "Backoff value generated too early");
             m_nextBackoffGen.Cancel();
-
-            // we get here when the backoff expired but no transmission occurred, thus we have
-            // generated a new backoff value and we will start decrementing the counter in a slot
-            delay = m_apMac->GetWifiPhy(linkId)->GetSlot();
         }
 
         if (m_nGenBackoff < nValues)

src/wifi/test/wifi-dynamic-bw-op-test.cc

@@ -155,8 +155,8 @@ WifiUseAvailBwTest::Transmit(uint8_t bss,
         client->SetAttribute("Interval", TimeValue(MicroSeconds(0)));
         client->SetRemote(m_sockets[0]);
         m_apDevices.Get(0)->GetNode()->AddApplication(client);
-        client->SetStartTime(Seconds(0)); // start now
-        client->SetStopTime(Seconds(1));  // stop in a second
+        client->SetStartTime(TimeStep(1)); // start shortly after the start of PPDU in BSS 1
+        client->SetStopTime(Seconds(1));   // stop in a second
         client->Initialize();
 
         // after 1us (to allow for propagation delay), the largest idle primary
@@ -470,20 +470,18 @@ WifiDynamicBwOpTestSuite::WifiDynamicBwOpTestSuite()
     AddTestCase(
         new WifiUseAvailBwTest({"{50, 160, BAND_5GHZ, 5}", "{42, 80, BAND_5GHZ, 2}"}, MHz_u{80}),
         TestCase::Duration::QUICK);
-    // clang-format off
     /**
-     *                                                                                 ───────────────────────────── primary 160 ─────────────────────────────
-     *                                                                                                    primary20
-     *          ┌────────┬────────┬────────┬────────┬───────┬────────┬────────┬────────┌────────┬────────┬────────┬────────┬───────┬────────┬────────┬────────┐
-     *  BSS 0   │   1    │   5    │   9    │   13   │  17   │   21   │   25   │   29   │   33   │   37   │   41   │   45   │  49   │   53   │   57   │   61   │
-     *          └────────┴────────┴────────┴────────┴───────┴────────┴────────┴────────└────────┴────────┴────────┴────────┴───────┴────────┴────────┴────────┘
+     *                                                  ────────────── primary 160 ──────────────
+     *                                                           primary20
+     *          ┌────┬────┬────┬────┬────┬────┬────┬────┬────┬────┬────┬────┬────┬────┬────┬────┐
+     *  BSS 0   │  1 │  5 │  9 │ 13 │ 17 │ 21 │ 25 │ 29 │ 33 │ 37 │ 41 │ 45 │ 49 │ 53 │ 57 │ 61 │
+     *          └────┴────┴────┴────┴────┴────┴────┴────┴────┴────┴────┴────┴────┴────┴────┴────┘
      *
-     *          ┌────────┬────────┬────────┬────────┬───────┬────────┬────────┬────────┐
-     *  BSS 1   │   1    │   5    │   9    │   13   │  17   │   21   │   25   │   29   │
-     *          └────────┴────────┴────────┴────────┴───────┴────────┴────────┴────────┘
-     *                                                                         primary20
+     *          ┌────┬────┬────┬────┬────┬────┬────┬────┐
+     *  BSS 1   │  1 │  5 │  9 │ 13 │ 17 │ 21 │ 25 │ 29 │
+     *          └────┴────┴────┴────┴────┴────┴────┴────┘
+     *                                            primary20
      */
-    // clang-format on
     AddTestCase(
         new WifiUseAvailBwTest({"{31, 320, BAND_6GHZ, 10}", "{15, 160, BAND_6GHZ, 7}"}, 160),
         TestCase::Duration::QUICK);