let's have a look at all the layers which are available in E-UTRAN Protocol Stack. The functions of the different protocol layers in LTE stack are explained below.

  • Physical Layer (Layer 1)

Physical Layer carries all information from the MAC transport channels over the air interface. Takes care of the link adaptation (AMC), power control, cell search (for initial synchronization and handover purposes) and other measurements (inside the LTE system and between systems) for the RRC layer.

 

  • Medium Access Layer (MAC)

MAC layer is responsible for Mapping between logical channels and transport channels, Multiplexing of MAC SDUs from one or different logical channels onto transport blocks (TB) to be delivered to the physical layer on transport channels, de multiplexing of MAC SDUs from one or different logical channels from transport blocks (TB) delivered from the physical layer on transport channels, Scheduling information reporting, Error correction through HARQ, Priority handling between UEs by means of dynamic scheduling, Priority handling between logical channels of one UE.

LTE Protocol Stack Layers

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  • Radio Link Control (RLC)

RLC operates in 3 modes of operation: Transparent Mode (TM), Unacknowledged Mode (UM), and Acknowledged Mode (AM).

RLC Layer is responsible for transfer of upper layer PDUs, error correction through ARQ (Only for AM data transfer), Concatenation, segmentation and reassembly of RLC SDUs (Only for UM and AM data transfer).

RLC is also responsible for re-segmentation of RLC data PDUs (Only for AM data transfer), reordering of RLC data PDUs (Only for UM and AM data transfer), duplicate detection (Only for UM and AM data transfer), RLC SDU discard (Only for UM and AM data transfer), RLC re-establishment, and protocol error detection (Only for AM data transfer).

 

  • Radio Resource Control (RRC)

The main services and functions of the RRC sublayer include broadcast of System Information related to the non-access stratum (NAS), broadcast of System Information related to the access stratum (AS), Paging, establishment, maintenance and release of an RRC connection between the UE and E-UTRAN, Security functions including key management, establishment, configuration, maintenance and release of point to point Radio Bearers. RRC has only 2 states (RRC_IDLE and RRC_CONNECTED).

​​RRC_IDLE:

When no RRC connection is established, the UE is in RRC_IDLE state.

 

  1. A UE specific DRX may be configured by upper layers.

  2. UE controlled mobility;

  3. The UE:

  • Monitors a Paging channel to detect incoming calls, system information change, for ETWS capable UEs, ETWS notification, and for CMAS capable UEs, CMAS notification;

  • Performs neighbouring cell measurements and cell (re-)selection;

  • Acquires system information.

  • Performs logging of available measurements together with location and time for logged measurement configured UEs.


RRC_CONNECTED:

A UE is in RRC_CONNECTED when an RRC connection has been established.

  1. Transfer of unicast data to/from UE.

  2. At lower layers, the UE may be configured with a UE specific DRX.

  3. For UEs supporting CA, use of one or more SCells, aggregated with the PCell, for increased bandwidth;

  4. Network controlled mobility, i.e. handover and cell change order with optional network assistance (NACC)

  5. to GERAN;

  6. The UE:

  • Monitors a Paging channel and/ or System Information Block Type 1 contents to detect system information change, for ETWS capable UEs, ETWS notification, and for CMAS capable UEs, CMAS notification;

  • Monitors control channels associated with the shared data channel to determine if data is scheduled for it;

  • Provides channel quality and feedback information;

  • Performs neighbouring cell measurements and measurement reporting;

  • Acquires system information.

The below figure not only provides an overview of the RRC states in E-UTRA, but also illustrates the mobility support between E-UTRAN, UTRAN and GERAN.

  • Packet Data Convergence Control (PDCP)

PDCP Layer is responsible for Header compression and decompression of IP data, Transfer of data (user plane or control plane), Maintenance of PDCP Sequence Numbers (SNs), In-sequence delivery of upper layer PDUs at re-establishment of lower layers, Duplicate elimination of lower layer SDUs at re-establishment of lower layers for radio bearers mapped on RLC AM, Ciphering and deciphering of user plane data and control plane data, Integrity protection and integrity verification of control plane data, Timer based discard, duplicate discarding, PDCP is used for SRBs and DRBs mapped on DCCH and DTCH type of logical channels.

 

  • Non Access Stratum (NAS) Protocols

The non-access stratum (NAS) protocols form the highest stratum of the control plane between the user equipment (UE) and MME. NAS protocols support the mobility of the UE and the session management procedures to establish and maintain IP connectivity between the UE and a PDN GW.

Please refer to 3GPP spec 36.331 for detailed information.