1、what is the margin time protect for？
// classBScheduleMargin contains a Class-B scheduling margin to make sure
// there is enough time between scheduling and the actual Class-B ping-slot.
const classBScheduleMargin = 5 * time.Second
Note that the enqueuing and the scheduler (reading from the DB and sending to the gateway) are two processes. So if you lower this margin, it could mean that the downlink is enqueued for a ping-slot before the scheduler is able to send it to the gateway on that slot.
3、I want to shorten the margin time，can you please give me some advise？
Feel free to create a pull-request to make this config variable adjustable. I think it makes sense to add this margin under the
[network_server.network_settings.class_b] config section.