Downlink Modes

Devices do not form connections the same way you might expect other network devices to maintain connections. The Symphony Link Module for example will only establish a connection and will not communicate with the Gateway until it has a message to send or receive. The rules around receiving the DownlinkMessage are what define the Downlink Modes.

Mailbox Mode

When a Symphony Link Module or LTE CAT-M1 Module is in Mailbox Mode, the module does not constantly communicate with its associated gateway outside of initial connection. The module will just “Check its mailbox” at certain intervals. When a Mailbox Check occurs, the module can receive a “Pending” DownlinkMessage that the Gateway has in its Mailbox.

From a cloud perspective, sending a DownlinkMessage to a module in Mailbox Mode should include a time_to_live_s parameter in the Module.send_message() call

my_mod = account.get_module("$301$0-0-0-03000XXXX")
dl_msg = mod.send_message(b'hello world', time_to_live_s=300.0)

This time_to_live_s value represents the amount of time (in seconds) that the Mailbox Message can sit on the gateway’s Mailbox before that message “Expires”. If the module performs a Mailbox Check within the time_to_live_s of the message, since it was received by their gateway, then the module will receive that DownlinkMessage. Otherwise, if the module checks its Mailbox after the message’s time_to_live_s, the gateway will report no messages to downlink to the module.

Downlink Always On

This mode allows constant check-ins from the Symphony Link Module to the Gateway, allowing the fastest possible downlink to occur. This mode does not require any of the optional parameters of the Module.send_message(), however, they can be used when necessary.

Warning

The LTE CAT-M1 Module does not support Downlink Always On Mode.