H) may be inserted in the MAC PDUs immediately following the generic MAC header. If both the FSH and GMSH are present, the GMSH takes precedence. In the DL, the FFSH always appears as the last per-PDU sub-header. The ESF bit in the generic MAC header indicates that one or more extended sub-headers are present in the PDU. The extended sub-headers always appear immediately after the generic MAC header and before all other sub-headers. The extended sub-headers are not encrypted. The only per-SDU sub-header is the Packing Sub-Header (PSH). It may be inserted before each MAC SDU, if indicated by the Type field. The PSH and FSH are mutually exclusive and cannot present within the same MAC PDU. When present, per-PDU sub-headers always precede the first per-SDU sub-header. The GMSH is 2 bytes in length and is used by the MS to convey bandwidth management requests to the serving BS. This sub-header is encoded differently based on the type of UL scheduling service for the connection as given by the CID. The support of GMSH at both BS and MS is optional. When packing is used, the MAC may pack multiple SDUs into a single MAC PDU. When packing variable- length MAC SDUs, the MAC prefixes each one with a PSH. If the ARQ feedback payload bit in the MAC Type field is set, the ARQ feedback payload is transported. If packing is used, it is transported as the first packed payload. Note that this bit does not address the ARQ feedback payload contained inside an ARQ feedback message. 6.2.2 IEEE 802. 16m MAC Headers To mitigate the excessive overhead of the legacy MAC header for small payload applications, IEEE 802.16m has specified new MAC headers. The structure of an IEEE 802.16m MAC PDU is shown in 6.2 MAC PDU headers Extended MAC MAC Header Payload (Variable Size) Header (optional) FIGURE 6-3 Structure of a MAC PDU [2] Figure 6-3. Each MAC PDU consists of a MAC header and is optionally followed by one or more extended headers. The MAC PDU contains a variable-sized payload. Multiple MAC SDUs and/or SDU fragments from different unica