st connections corresponding to the same MS can be multiplexed into a single MAC PDU. The multiplexed unicast connections are associated with the same security association. The IEEE 802.16m specifies three types of MAC headers: (1) Advanced Generic MAC Header (AGMH) that is used for MAC PDUs containing either MAC management messages or user payload; (2) Short-Packet MAC Header (SPMH) that is utilized in conjunction with persistent or group allo- cations; and (3) Signaling MAC header. The MAC header formats are mutually exclusive and are not used simultaneously for the same connection. It must be noted that the Advanced Generic MAC header is different from that specified in the IEEE 802.16-2009 standard [1]. The Generic MAC Header (GMH) specified by the latter consists of 6 bytes that was used for all MAC PDUs containing MAC management messages or user data. While the overhead resulting from the MAC header was negligible for large user payloads, it could become a considerable overhead for the small payload applications such as VoIP and interactive gaming. It was further observed that the GMH specified by the IEEE 802.16-2009 standard contains many information fields that are not typically used. To mitigate this problem, the generic MAC header was restructured and redundant fields were removed to improve efficiency. The AGMH MAC header format is shown in Figure 6-4. In Figure 6-4, different header fields are defined as follows: Flow Identifier (Flow ID or FID): this field indicates the service flow that is addressed. Extended Header (EH) Presence Indicator: when set to 1, this field indicates that an Extended Header is present following this AGMH. Length (11 bits): length of the MAC PDU in number of bytes including the AGMH and extended header (if present). The short-packet MAC header is used for connections with persistent and group allocation (small payload applications). The structure of the short-packet MAC header is illustrated in Figure 6-5. The EH field denotes that the Extended Header when set to 1 indicate