Re-compute correct timeslot on applyModemConfig (#4469)

* Re-compute correct timeslot on applyModemConfig

* Cap contention window max at 7

src/mesh/RadioInterface.cpp

@@ -520,6 +520,7 @@ void RadioInterface::applyModemConfig()
     saveChannelNum(channel_num);
     saveFreq(freq + loraConfig.frequency_offset);
 
+    slotTimeMsec = computeSlotTimeMsec(bw, sf);
     preambleTimeMsec = getPacketTime((uint32_t)0);
     maxPacketTimeMsec = getPacketTime(meshtastic_Constants_DATA_PAYLOAD_LEN + sizeof(PacketHeader));
 

src/mesh/RadioInterface.h

@@ -71,18 +71,20 @@ class RadioInterface
       - roundtrip air propagation time (assuming max. 30km between nodes);
       - Tx/Rx turnaround time (maximum of SX126x and SX127x);
       - MAC processing time (measured on T-beam) */
-    uint32_t slotTimeMsec = 8.5 * pow(2, sf) / bw + 0.2 + 0.4 + 7;
+    uint32_t slotTimeMsec = computeSlotTimeMsec(bw, sf);
     uint16_t preambleLength = 16;      // 8 is default, but we use longer to increase the amount of sleep time when receiving
     uint32_t preambleTimeMsec = 165;   // calculated on startup, this is the default for LongFast
     uint32_t maxPacketTimeMsec = 3246; // calculated on startup, this is the default for LongFast
     const uint32_t PROCESSING_TIME_MSEC =
         4500;                // time to construct, process and construct a packet again (empirically determined)
     const uint8_t CWmin = 2; // minimum CWsize
-    const uint8_t CWmax = 8; // maximum CWsize
+    const uint8_t CWmax = 7; // maximum CWsize
 
     meshtastic_MeshPacket *sendingPacket = NULL; // The packet we are currently sending
     uint32_t lastTxStart = 0L;
 
+    uint32_t computeSlotTimeMsec(float bw, float sf) { return 8.5 * pow(2, sf) / bw + 0.2 + 0.4 + 7; }
+
     /**
      * A temporary buffer used for sending/receiving packets, sized to hold the biggest buffer we might need
      * */