The general packet structure is shown in the figure on this slide, which also highlights the different fields. The length of each field is shown in the diagram. The preamble pattern starts the data transfer and is always a series of alternating ones and zeroes, starting with a zero. All the fields identified have programmable lengths to accommodate different applications. For example, the header field can consist of up to four bytes controlled by header enable registers. The most common CRC polynomials are also available for selection. Automatically adding these fields to the data payload greatly reduces the amount of communication between the CPU and the radio, and reduces the required computational power of the microcontroller.