The IEEE® 802.15.4 frame is composed of a header, addressing fields, security header, payload and a Frame Check Sequence (FCS). The header contains the frame control field which indicates the type of frame, and stores information about the frame and what it contains. There are four types of frames: beacon, data, acknowledgment and command frame. Any data frame may contain both source and destination information with the size of the address field between 4 and 20 bytes. The security header indicates the security details contained in the frame. The payload field is variable in length, considering the full data frame must not exceed the 127 bytes for the Media access control Protocol Data Unit (MPDU). The FCS is added at the end of the frame to determine if the packet was received correctly. The PHY handles the preamble sequence, Start Of Frame (SOF) and Frame length bytes. These bytes are not part of the MPDU.