irst fragment of first byte of a MAC SDU. The last byte of data a MAC SDU or a MAC PDU with one or more in the MAC PDU is not the last byte of a MAC non-fragmented MAC SDUs followed by the first fragment of the subsequent MAC SDU. The first byte of data in the MAC PDU is not the A MAC PDU with only the last fragment of first byte of a MAC SDU. The last byte of data a MAC SDU or MAC PDU with the last fragment in the MAC PDU is the last byte of a MAC SDU. of a MAC SDU followed by one or more non- fragmented subsequent MAC SDUs. The first byte of data in the MAC PDU is not the A MAC PDU with only the middle fragment of first byte of a MAC SDU. The last byte of data a MAC SDU or a MAC PDU with the last in the MAC PDU is not the last byte of a MAC fragment of a MAC SDU followed by zero or more non-fragmented MAC SDUs followed by the first fragment of the subsequent MAC SDU. The Fragmentation Control (FC) bits are defined in Table 6-2. An example usage of FEH/PEH/ RFPEH/MCEH and MEH extended headers in a MAC PDU is illustrated in Figure 6-7. 6.3 MAC SIGNALING HEADERS 6.3.1 Legacy MAC Signaling Headers MAC signaling headers are different types of headers that are used in MAC PDUs with no payload. They are sent as standalone or concatenated with other MAC PDUs. The IEEE 802. 16-2009 standard specifies two types of signaling headers. This MAC header format is applicable to the uplink direction. The MAC header is not followed by any MAC PDU payload and CRC. The structure of Type I legacy MAC signaling headers are shown in Figure 6-8, with the description of their specific fields given in Table 6-3. The type II MAC signaling header is also UL-specific. There is no payload following the MAC signaling header. The structure of MAC signaling header type II is illustrated in Figure 6-9. The feedback header is sent by a legacy MS either as a response to a Feedback Polling IE or as an unsolicited feedback. When sent as a response to a Feedback Polling IE, the legacy MS sends a feedback header using the assigned resource indicated 