LTE structures its radio communication through three layers of channels that progressively transform abstract data services into concrete radio transmissions. Logical channels define the type of information being carried, such as user traffic versus control messages versus system broadcast data. Transport channels describe how that information is processed and delivered, whether shared among users or sent as a broadcast. Physical channels map the processed data onto specific resource elements in the time-frequency grid. Data flows downward through these three layers during transmission and upward through them during reception.