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•Each MAC PDU corresponds to a single Transport Block (TB)
•There is one sub-header for each MAC Control Element in the PDU and each MAC SDU in the PDU

3. Each MAC subheader consists of a Logical Channel ID (LCID) and a Length (L) field.
4. The LCID indicates whether the corresponding part of the MAC payload is a MAC Control Element, and if not, to which logical channel the related MAC SDU belongs.
5. The L field indicates the size of the related MAC SDU or MAC Control Element.

MAC header consists of multiple sub-headers

•One sub-header for each Control Element, MAC PDU or Padding

•Each sub-header is 1 or 2, 3 bytes in length

–[R/R/E/LCID]: Used for fixed length MAC SDUs and MAC Control Elements

–[R/R/E/LDID/F/Length]: Used for variable length MAC SDUs

MAC Control Elements are used for MAC-level peer-to-peer signalling, including delivery of BSR information and reports of the UE’s available power headroom in the uplink, and in the downlink DRX commands and timing advance commands. For each type of MAC Control Element, one special LCID is allocated. When a MAC PDU is used to transport data from the PCCH or BCCH logical channels, the MAC PDU includes data from only one logical channel. In this case, because multiplexing is not applied, there is no need to include the LCID field in the header. In addition, if there is a one-to-one correspondence between a MAC SDU and a MAC PDU, the size of the MAC SDU can be known implicitly from the transport block size. Thus, for these cases a headerless MAC PDU format is used as a transparent MAC PDU.