Wimax standards and Security The Wimax

MAC Protocol Data Unit

Each protocol data unit (PDU), as seen in Figure 11.2, is comprised of a generic MAC header (GMH), a payload, and an optional cyclic redundancy check (CRC). The GMH defines the contents of the payload and starts at the most significant bit (MSB). The payload consists of zero or more subheaders and MAC service data units (SDUs). The length of the payload may vary. The CRC is optional in the SC PHY layer but mandatory for SCa, OFDM, and OFDMA PHY layers [5].
There are two formats defined for the MAC header. The GMH is used for MAC PDUs that contain MAC management messages or convergence sub- layer data. The bandwidth request header is used when requesting additional bandwidth. The two headers are distinguished by the single-bit header type (HT) field, which is zero for the generic header and one for the bandwidth request header.
The GMH, shown in Figure 11.3, is encoded from the HT field on. It is 6 bytes in length and consists of 12 fields. Two of these fields, which are 1 bit in length each, are reserved for future use. The remaining fields are defined in Table 11.1.

MSB

LSB
The type field of the GMH is used to indicate what type of subheader or special payload is included in the message. The possible type values and corresponding meanings are defined in Table 11.2.

Generic MAC header

Payload (optional)

CRC (optional)

FIGURE 11.2
MAC PDU.

MSB

HT = 0(1)	EC (1)	Type (6)	Rsv (1)	CI (1)	EKS (2)	Rsv (1)	LEN MSB (3)
LEN LSB (8)			CID MSB (8)
CID LSB (8)			HCS (8)

LSB
FIGURE 11.3
Generic MAC header.