The class A end-devices allow for bi-directional communications whereby each end-device's up-link transmission is followed by two short down-link receive windows. The transmission slot scheduled by the end-device is based on its own communication needs with a small variation based on a random time basis (ALOHA-type of the protocol).
This Class A operation is the lowest power end-device system for applications that only require down-link communication from the server shortly after the end-device has sent an up-link transmission. Down-link communications from the server at any other time will have to wait until the next scheduled up-link.