LoRaWAN® supports three different class of devices: Class A, Class B, Class C.
Class A devices are battery powered, when an uplink is sent to the server the device open two short downlink windows for eventual commands, if the server cannot send a downlink communication in these two short windows, it will have to wait till the next uplink message.
Class B devices are battery powered, in addition to the two short windows of Class A, Class B have an extra downlink windows that is opened at scheduled times. This windows is synchronized with the server using a Beacon from the gateway and allows the server to know when the end device is listening.
Class C devices are electrically powered, the receive windows are nearly continuously open, only during transmission are closed.