Generic MAC Header

The primary task of the WiMAX MAC layer is to provide an interface between the higher transport layers and the physical layer. The MAC layer takes packets from the upper layer - these packets are called MAC service data units (MSDUs) and organizes them into MAC protocol data units (MPDUs) for transmission over the air. For received transmissions, the MAC layer does the reverse.

MAC PDU construction topic shows an example of MAC PDU frame.

Each MAC PDU contains 3 components: GMH, which contains frame control information, variable length frame body and 32-bit CRCMAC PDUs are transmitted in PHY burst. A singal PHY burst contains multiple concatenated MAC PDUs.

Generic MAC Header fields (GMH):
If Header Type (HT) field is '0' then it is GMH header.
CI - CRC indication bit
CID - 16 bit connection identifier
EC - Encryption control bit
EKS - Encryption key sequence
ESF - Extended subheader indication bit
HCS - 8 bit Header Check Sum
HT - Header Type, shall be '0'
Len - 11 bit length of PDU
Type - 6 bit special Payload Type (PSH, FSH, ARQ feedback etc...)

Bandwidth Request fields:
This PDU is applicable to UL only.
It does not contain any payload and should not be encrypted.

BR - 19 bit Bandwidth request
HT - Header Type, shall be '0'
EC - always set to zero
CID - 16 bit connection identifier
HCS - 8 bit Header Check Sum
Type - 3 bit type of BR header