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RRC (L3) – Radio resource control protocol exists at IP level (layer 3) between UE and eNB.
- The major functions of the RRC protocol include connection establishment and release functions, broadcast of system information, radio bearer establishment, reconfiguration and release, RRC connection mobility procedures, paging notification and release and outer loop power control.
- The operation of the RRC is guided by a state machine which defines certain specific states that a UE may be present in.
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MAC (L2) – Medium Access Control is responsible for data transfer and logical channel multiplexing, hybrid automatic repeat request (HARQ) retransmissions and uplink/downlink scheduling, random access procedure, and maintenance of uplink timing. It is also responsible for multiplexing/demultiplexing data across multiple component carriers when carrier aggregation is used. From a physical layer (PHY) perspective, the MAC sublayer provides and receives services in the form of transport channels.
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RLC (L2)– Radio Link Control acts as an interface and buffer between the higher layers of the protocol stack (usually PDCP in the user plane) and the MAC layer, which has almost no buffering capability and acts more as a router.
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PDCP (L2) - Packet data convergence protocol provides its services to upper layers. The following services are provided by PDCP to upper layers such as transfer of user plane & control plane data, header compression, ciphering & integrity protection.
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Physical layer (L1) contains different channels are signals present in downlink such as Physical data control channel (PDCCH) to conveys Downlink Control Information (DCI); Physical Broadcast Channel (PBCH), which carries part of the system information (MIB) required by the UE to access the network; Primary and Secondary Synchronization Signals (PSS & SSS) for cell-search procedures and cell identification; Physical control format indicator channel (PCFICH) to carry the number of OFDM symbols used for transmission of PDCCHs in a subframe; Physical Hybrid Automatic request (HARQ) indicator (PHICH) to send ACK/NACK feedback to the UE for the uplink blocks received by eNB; Physical data shared channel (PDSCH) to carry user data, paging info & SIB msgs.
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