IEEE 802.16 STANDARD
A typical MAC PDU transmission.
Generic MAC Header
MAC PDU format.
support. We can expect possible future support in this sublayer for PPP (point-to-point
protocol; see RFC 1661), MPLS (multi-protocol label switching), and others.
MAC addressing specifies a 48-bit address capability for the subscriber station (SS)
and 48-bit base station (BS) ID that is not a MAC address. There is a 24-bit operator
indicator. There is also a 16-bit connection ID (CID) that is used in MAC PDUs.
MAC PDUs are transmitted in PHY bursts. A single PHY burst can contain multiple
concatenated MAC PDUs. The PHY burst can also contain multiple FEC (forward error
correction) blocks. The MAC PDUs can span FEC block boundaries. The transmission
convergence layer 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