Overview of LTE-RLC
In 3GPP LTE System, Radio Link Control (RLC) layer is one of user plane (U-PLANE) protocol. RLC is symmetric protocol and is present in both UE and eNodeB. This protocol sends and receives packets to and from UE and eNB over Uu Interface.
eNB-RLC layer works on the top of MAC protocol and transmits SDUs received from MAC to PDCP in uplink, which in turn transmits packet to GTP.
RLC layer in downlink direction receives SDUs from PDCP and transmits it to MAC layer, which in turn transmits packets to eNB-PHY for further processing.
Below is the basic architecture of RLC.
LTE RLC works on three mode.
- RLC TM : Used only for common control channels (BCCH, DL/UL CCCH, PCCH and SBCCH).
- RLC UM : Used only for dedicated traffic channels and multicast channels (DL/UL DTCH,MCCH, MTCH, SC-MCCH, SC-MTCH or STCH).
- RLC AM : Used for dedicated control and data channels (DL/UL DCCH and DTCH).
Below is the high level model of two TM mode peer entity.
Below is the high level model of two UM mode peer entity.
Below is the high level model of two AM mode peer entity.
RLC changes for NB-IOT
For NB-IOT two level of retransmissions (i.e. HARQ at MAC and ARQ at RLC) are not required,
most of RLC AM related functionalities are removed for NB-IOT.
RLC UM mode is supported only for SC-MCCH and SC-MTCH channels.
Below are the high level modifications required in the RLC for NB-IOT
- For TX operation. counters like PDU_WITHOUT_POLL and BYTE_WITHOUT_POLL are no
more relevant, hence increment of these counters are not required.
- For RX operation, timer values of t-reordering and t-statusProhibit is mostly set as 0.
- For Status reporting, RRC configures whether or not status reporting due to link failure is to be
used for an AM RLC entity.
More information to be added...........
In 3GPP LTE System, Radio Link Control (RLC) layer is one of user plane (U-PLANE) protocol. RLC is symmetric protocol and is present in both UE and eNodeB. This protocol sends and receives packets to and from UE and eNB over Uu Interface.
eNB-RLC layer works on the top of MAC protocol and transmits SDUs received from MAC to PDCP in uplink, which in turn transmits packet to GTP.
RLC layer in downlink direction receives SDUs from PDCP and transmits it to MAC layer, which in turn transmits packets to eNB-PHY for further processing.
Below is the basic architecture of RLC.
- RLC TM : Used only for common control channels (BCCH, DL/UL CCCH, PCCH and SBCCH).
- RLC UM : Used only for dedicated traffic channels and multicast channels (DL/UL DTCH,MCCH, MTCH, SC-MCCH, SC-MTCH or STCH).
- RLC AM : Used for dedicated control and data channels (DL/UL DCCH and DTCH).
Below is the high level model of two TM mode peer entity.
Below is the high level model of two AM mode peer entity.
Detailed description about the each model of RLC mode is covered in 3GPP 36.322 specification.
Below are the basic functionalities of RLC mode.
- Transfer of PDCP and MAC SDUs.
- Error Correction through sliding window mechanism applicable for AM mode only.
- Segmentation, Concatenation and Reassembly of RLC SDUs except for TM mode.
- Re-segmentation of AM mode only.
- Duplicate detection and reordering except TM Mode.
- RLC SDU discard except TM Mode.
- RLC Re-establishment except TM Mode.
- Protocol error detection applicable for AM mode.
For NB-IOT two level of retransmissions (i.e. HARQ at MAC and ARQ at RLC) are not required,
most of RLC AM related functionalities are removed for NB-IOT.
RLC UM mode is supported only for SC-MCCH and SC-MTCH channels.
Below are the high level modifications required in the RLC for NB-IOT
- For TX operation. counters like PDU_WITHOUT_POLL and BYTE_WITHOUT_POLL are no
more relevant, hence increment of these counters are not required.
- For RX operation, timer values of t-reordering and t-statusProhibit is mostly set as 0.
- For Status reporting, RRC configures whether or not status reporting due to link failure is to be
used for an AM RLC entity.
More information to be added...........
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