到2018.03.21，MAC相关的协议38.321基本定型了 ，这篇文章简单的介绍一下MAC，进一步的理解，我们要自己去深入321了。
先附上一张MAC层所处的位置，他的上级和下级分别是什么？请看下图：
1.MAC的上级(Higher layer)RLC
2.MAC的下级（Lower layer） PHY
下面附一张NR和LTE对比的图表格：
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5G/NR(38.321)
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LTE(36.321)
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Services
provided to upper layers
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- data transfer
- radio resource allocation
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- data transfer
- radio resource allocation
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MAC Functions
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- mapping between logical channels and transport channels
- multiplexing of MAC SDUs
- demultiplexing of MAC SDUs
- scheduling information reporting;
- error correction through HARQ;
- logical channel prioritisation.
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- mapping between logical channels and transport channels;
- multiplexing of MAC SDUs
- demultiplexing of MAC SDUs
- scheduling information reporting
- error correction through HARQ
- priority handling between UEs
- priority handling between logical channels of one MAC entity
- Logical Channel prioritisation
- transport format selection
- radio resource selection for SL
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Services
expected from physical layer
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- data transfer services;
- signalling of HARQ feedback;
- signalling of Scheduling Request;
- measurements (e.g. Channel Quality Indication (CQI)).
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- data transfer services;
- signalling of HARQ feedback;
- signalling of Scheduling Request;
- measurements (e.g. Channel Quality Indication (CQI)).
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MAC层的信道映射：
LTE和NR的MAC的PDU/HEADERS比较：
