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# <center><i class="fa fa-edit"></i> 5G/NR - MAC</center>
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[TOC]
### High Level View of NR MAC Layer
High level MAC functionality and its interaction with lower and higher layer can be summarized as in the following diagram.

At high level view, NR MAC function/operation is very similar to LTE MAC function/operation. For your reference, I put the NR MAC function and LTE MAC function side-by-side as below


### Channel Mapping at MAC Layer

The illustration shown above may show you a little bit detailed picture of MAC process, but it may not be so clear about the channel mapping unless you follow through each lines very carefully. In terms of channel mapping, the tables in 38.321 would be clearer and simple to understand and my illustration to the right would be even more clear and intuitive.


As you see, most of channels from Logical channel to Transport channel is one-to-one or many-to-one relation, but BCCH case it maps to BCH and DL-SCH.It means some BCCH data maps to BCH and some BCCH data maps to DL-SCH.
If you are familiar with LTE, you would know there are largely two types of BCCH in LTE. One is MIB and the others are SIBs. MIB goes through BCCH-BCH path and SIBs go through BCCH-DL SCH path. NR would use the same pattern of channel mapping.

### NR MAC Subheader Structure

Followings shows the structure of MAC Subheaders in NR and meaning of each field in the sub headers.
R/F/LCID/L MAC subheader with 8-bit L field

R/F/LCID/L MAC subheader with 16-bit L field

R/LCID MAC subheader