[IOTCELL-1181] Using higher data rates to connect

The stack was trying to connect with default data rates which happened to be
the lowest data rates in a specific region. In the beginning device and NS do
not have agreed upon tx rx parameters and there can be synchronization issues.
When we use lower datarates, we may end up having a minute and a half long
transmissions that hnot only blocks the channel for a long time but also reduce
the chance of proper synch between device and NS. That's why we have decided to
begin with higher data rates and gradually decrease datarate if we do not hear
from the network server.

features/lorawan/lorastack/mac/LoRaMac.cpp

@@ -1755,9 +1755,7 @@ lorawan_status_t LoRaMac::initialize(EventQueue *queue)
     _params.timers.mac_init_time = _lora_time.get_current_time();
 
     _params.sys_params.adr_on = MBED_CONF_LORA_ADR_ON;
-
-    _params.is_nwk_public = MBED_CONF_LORA_PUBLIC_NETWORK;
-    _lora_phy->setup_public_network_mode(MBED_CONF_LORA_PUBLIC_NETWORK);
+    _params.sys_params.channel_data_rate = _lora_phy->get_default_max_tx_datarate();
 
     return LORAWAN_STATUS_OK;
 }

features/lorawan/lorastack/phy/LoRaPHY.cpp

@@ -514,13 +514,16 @@ void LoRaPHY::reset_to_default_values(loramac_protocol_params *params, bool init
 
     params->sys_params.channel_tx_power = get_default_tx_power();
 
-    params->sys_params.channel_data_rate = get_default_tx_datarate();
+    // We shall always start with highest achievable data rate.
+    // Subsequent decrease in data rate will mean increase in range henceforth.
+    params->sys_params.channel_data_rate = get_default_max_tx_datarate();
 
     params->sys_params.rx1_dr_offset = phy_params.default_rx1_dr_offset;
 
     params->sys_params.rx2_channel.frequency = get_default_rx2_frequency();
 
-    params->sys_params.rx2_channel.datarate = get_default_rx2_datarate();
+    // RX2 data rate should also start from the maximum
+    params->sys_params.rx2_channel.datarate = get_default_max_tx_datarate();
 
     params->sys_params.uplink_dwell_time = phy_params.ul_dwell_time_setting;
 
@@ -560,6 +563,11 @@ uint8_t LoRaPHY::get_default_tx_datarate()
     return phy_params.default_datarate;
 }
 
+uint8_t  LoRaPHY::get_default_max_tx_datarate()
+{
+    return phy_params.default_max_datarate;
+}
+
 uint8_t LoRaPHY::get_default_tx_power()
 {
     return phy_params.default_tx_power;

features/lorawan/lorastack/phy/LoRaPHY.h

@@ -423,6 +423,14 @@ public:
      */
     uint8_t get_default_tx_datarate();
 
+    /**
+     * @brief get_default_max_tx_datarate Gets the maximum achievable data rate for
+     *        LoRa modulation. This will always be the highest data rate achievable with
+     *        LoRa as defined in the regional specifications.
+     * @return Maximum achievable data rate with LoRa modulation.
+     */
+    uint8_t get_default_max_tx_datarate();
+
     /**
      * @brief get_default_tx_power Gets the default TX power
      * @return Default TX power