er between the MAC and the PHY allows for capturing the start of the next MAC PDU in case of erroneous FEC blocks. A typical MAC PDU transmission is shown in Figure 13.11, the MAC PDU format is illustrated in Figure 13.12, and the generic MAC header is shown in Figure 13.13. Fragmentation involves partitioning a MAC SDU into fragments to be transported in multiple MAC PDUs. Each connection can only be a single fragmentation state at any time. The fragmentation subheader contains a 2-bit fragmentation control (FC) This tells us that it is unfragmented, the last fragment, first fragment, or continuing fragment. There is also a 3-bit fragmentation sequence number (FSN), which is a continuous counter across SDUs. It is required to detect missing continuing fragments. Figure 13.13 illustrates the generic MAC header. It has a fixed format, and one or more MAC subheaders may be part of the payload. The presence of subheaders is indicated by a type field in the generic MAC header. "Packing" is the process of combining multiple MAC SDUs (or fragments thereof) into a single MAC PDU. On connections with variable-length MAC SDUs, the packed PDU contains a subheader for each packed SDU (or fragment thereof). On connections with fixed-length MAC SDUs, no packing subheader is necessary. Packing and fragmentation can be combined. This process can, under certain situations, save up to 10% of the system bit rate capacity. Figure 13.14 shows packing fixed-length SDUs, and Figure 13.15 shows METROPOLITAN AREA NETWORKS Type (6) (1) RSV G-111 msb (3) LEN Isb (8) CID msb (8) CID Isb (8) HCS (8) Figure 13.13 Generic MAC header. k MAC SDUs Heada fixed length fixed length fixed length fixed length MAC SDU MAC SDU MAC SDU MAC SDU length = n length = n length = n length = n Figure 13.14 Packing fixed-length SDUs. k MAC SDUs Addressor variable length variable length variable lenght MAC SDU MAC SDU MAC SDU Lending length = a length = b Length length = C Figure 13.15 Packing variable-length SDUs. packing variable-length SDUs. In Figure 13.15 i