wifi: (fixes #1073) Document that beacons use PIFS access with zero backoff

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

@@ -948,6 +948,14 @@ Station Manager. As a result, PPDUs are transmitted on the largest idle primary
 For example, if a STA is operating on a 40 MHz channel and the secondary20 channel
 is indicated to be busy, transmissions will occur on the primary20 channel.
 
+In |ns3|, by default, beacons for both QoS and non-QoS APs access the channel after a PIFS interval (SIFS
+plus one slot time), and with zero backoff (CWmin and CWmax both set to zero), giving
+them higher priority than other access categories.  Beacons are given a
+separate Txop from data frames.  The DCF parameters described above are non-standard,
+but appear to be what some vendors have implemented in their products.  To change the
+DCF parameters for |ns3| access points, the ``ApWifiMac::DoCompleteConfig()`` method
+must be manually edited.
+
 The higher-level MAC functions are implemented in a set of other C++ classes and
 deal with:
 

src/wifi/model/ap-wifi-mac.cc

@@ -261,6 +261,7 @@ ApWifiMac::DoCompleteConfig()
 {
     NS_LOG_FUNCTION(this);
     m_beaconTxop->SetWifiMac(this);
+    // DCF behavior may be edited here; the default is PIFS access with zero backoff
     m_beaconTxop->SetAifsns(std::vector<uint8_t>(GetNLinks(), 1));
     m_beaconTxop->SetMinCws(std::vector<uint32_t>(GetNLinks(), 0));
     m_beaconTxop->SetMaxCws(std::vector<uint32_t>(GetNLinks(), 0));

src/wifi/model/ap-wifi-mac.h

@@ -63,7 +63,11 @@ using AssocReqRefVariant = std::variant<std::reference_wrapper<MgtAssocRequestHe
  * \ingroup wifi
  *
  * Handle association, dis-association and authentication,
- * of STAs within an infrastructure BSS.
+ * of STAs within an infrastructure BSS.  By default, beacons are
+ * sent with PIFS access, zero backoff, and are generated roughly
+ * every 102.4 ms by default (configurable by an attribute) and
+ * with some jitter to de-synchronize beacon transmissions in
+ * multi-BSS scenarios.
  */
 class ApWifiMac : public WifiMac
 {