 By the time an ARQ system discovers an error and re-transmits it, the re-sent data will arrive too late to be usable.Applications where the transmitter immediately forgets the information as soon as it is sent (such as most television cameras) cannot use ARQ; they must use FEC because when an error occurs, the original data is no longer available.Applications that use ARQ must have a return channel; applications having no return channel cannot use ARQ.