t should be noted that there is a 2-byte packing sub-header before each SDU. The length of an SDU (service data unit) is 11 bits; fragmentation control (FC) is 2 bits long; and the fragmentation sequence number (FS) is 3 bits long. 13.6 IEEE 802.1 16 STANDARD r MAC SDUs unfrag- last fragment unfragmented first fragment mented of MAC SDU MAC SDU of MAC SDU MAC SDU length = a length = b length = d length = C r+ 1 MAC PDUs continuing continuing continuing fragment of fragment of fragment of MAC SDU MAC SDU MAC SDU length = e length = f length = g t MAC PDUs unfrag- last fragment unfragmented mented unfragmented of MAC SDU MAC SDU MAC SDU MAC SDU length = h length = j length = k length = j Figure 13.16 Packing with fragmentation. Figure 13.16 illustrates packing with fragmentation. Downlink transmissions consist of two kinds of bursts: TDM and TDMA. All bursts are identified by a DIUC (downlink interval usage code). The TDMA bursts have a resync preamble allowing more flexible scheduling. Each terminal listens to all bursts at its operational IUC (interval usage code), or at a more robust one, except when told to transmit. Each burst may contain data for several terminals. A subscriber station (SS) must recognize the PDUs with known CIDs (connection identifiers). A DL-MAP message signals downlink usage. The "downlink channel descriptor" is used for advertising downlink burst profiles. The burst profile of a DL broadcast channel is well known; all others are acquired. Burst profiles can be changed on the fly without interrupting service. It is not intended a super-adaptive modulation. The downlink channel descriptor establishes association between DIUC and actual PHY parameters. Figure 13.17 illustrates burst profiles. The following should be noted in Figure 13.17: Each burst profile has mandatory exit threshold and minimum entry threshold. A subscriber station may request a less robust DIUC once above the minimum entry level. A subscriber station must request fall back to more robust DIUC once at mandatory exit thres