8 Elements for layer-to-layer communication

25.3223GPPRadio Link Control (RLC) protocol specificationRelease 17TS

Tools: ARFCN - Frequency Conversion for 5G NR/LTE/UMTS/GSM

The interaction between the RLC sublayer and other layers are described in terms of primitives where the primitives represent the logical exchange of information and control between the RLC sublayer and other layers. The primitives shall not specify or constrain the implementation.

8.1 Primitives between RLC and upper layers

The primitives between RLC and upper layers are shown in table 8.1.

Table 8.1: Primitives between RLC and upper layers

Generic Name	Parameters
	Req.	Ind.	Resp.	Conf.
RLC-AM-DATA	Data, CNF, DiscardReq, MUI, UE-ID type indicator	Data, DiscardInfo	Not Defined	Status, MUI
RLC-UM-DATA	Data, UE-ID type indicator, DiscardReq, MUI	Data, Sequence_Number	Not Defined	MUI
RLC-TM-DATA	Data, UE-ID type indicator, DiscardReq, MUI	Data, Error_Indicator	Not Defined	MUI
CRLC-CONFIG	E/R, Stop (UM/AM only), Continue (UM/AM only), Ciphering Elements (UM/AM only), TM_parameters (TM only), UM_parameters (UM only), AM_parameters (AM only) , SN_Delivery (UM only)	Not Defined	Not Defined	Not Defined
CRLC-SUSPEND (UM/AM only)	N	Not Defined	Not Defined	VT(US) (UM only), VT(S) (AM only)
CRLC-RESUME (UM/AM only)	No Parameter	Not Defined	Not Defined	Not Defined
CRLC-STATUS	Not Defined	EVC	Not Defined	Not Defined

Each Primitive is defined as follows:

RLC-AM-DATA-Req/Ind/Conf

– RLC-AM-DATA-Req is used by upper layers to request transmission of an RLC SDU in acknowledged mode.

– RLC-AM-DATA-Ind is used by the AM RLC entity to deliver to upper layers an RLC SDU that has been transmitted in acknowledged mode and to indicate to upper layers of the discarded RLC SDU in the peer RLC AM entity.

– RLC-AM-DATA-Conf is used by the AM RLC entity to confirm to upper layers the reception of an RLC SDU by the peer-RLC AM entity or to inform the upper layers of a discarded SDU.

RLC-UM-DATA-Req/Ind/Conf

– RLC-UM-DATA-Req is used by upper layers to request transmission of an RLC SDU in unacknowledged mode.

– RLC-UM-DATA-Ind is used by the UM RLC entity to deliver to upper layers an RLC SDU that has been transmitted in unacknowledged mode. If SN_Delivery is configured, RLC-UM-DATA-Ind is used by the UM RLC entity to deliver to upper layers also Sequence_Number.

– RLC-UM-DATA-Conf is used by the UM RLC entity to inform the upper layers of a discarded SDU.

RLC-TM-DATA-Req/Ind/Conf

– RLC-TM-DATA-Req is used by upper layers to request transmission of an RLC SDU in transparent mode.

– RLC-TM-DATA-Ind is used by the TM RLC entity to deliver to upper layers an RLC SDU that has been transmitted in transparent mode.

– RLC-TM-DATA-Conf is used by the TM RLC entity to inform the upper layers of a discarded SDU.

CRLC-CONFIG-Req

This primitive is used by upper layers to establish, re-establish, release, stop, continue or modify the RLC. Ciphering elements are included for UM and AM operation.

CRLC-SUSPEND-Req/Conf

– CRLC-SUSPEND-Req is used by upper layers to suspend the UM or AM RLC entity.

– CRLC-SUSPEND-Conf is used by the UM or AM RLC entity to confirm that the entity is suspended.

CRLC-RESUME-Req

This primitive is used by upper layers to resume the UM or AM RLC entity after the UM or AM RLC entity has been suspended.

CRLC-STATUS-Ind

It is used by an RLC entity to send status information to upper layers.

8.2 Primitive parameters

Following parameters are used in the primitives:

1) The parameter Data is the RLC SDU that is mapped onto the Data field in RLC PDUs. When AM or UM RLC entities are used, the length of the Data parameter is a multiple of 8 bits, otherwise (TM RLC entity) the length of Data parameter is a bit-string whose length may not be a multiple of 8 bits.

2) The parameter Confirmation Request (CNF) indicates whether the transmitting side of the AM RLC entity needs to confirm the reception of the RLC SDU by the peer-RLC AM entity. If required, once all AMD PDUs that make up the RLC SDU are positively acknowledged by the receiving AM RLC entity, the transmitting AM RLC entity notifies upper layers.

3) The parameter Message Unit Identifier (MUI) is an identity of the RLC SDU, which is used to indicate which RLC SDU that is confirmed with the RLC-AM-DATA-Conf. primitive, or discarded with the RLC-AM/UM/TM-DATA-Conf. Primitive.

4) The parameter E/R indicates establishment, re-establishment, release or modification of an RLC entity, where re-establishment is applicable to AM and UM RLC entities only. If re-establishment is requested, the state variables and configurable parameters are initialised according to subclause 9.7.7. If release is requested, all protocol parameters, variables and timers are released and the RLC entity enters the NULL state. If modification is requested, the protocol parameters indicated by upper layers (e.g. ciphering parameters) are only modified, while keeping the other protocol parameters, such as the protocol variables, protocol timers and protocol state unchanged. AM RLC entities are always re-established if any of the uplink or downlink AMD PDU size is changed. The modification of other protocol parameters does not require a re-establishment.

5) The parameter Event Code (EVC) indicates the reason for the CRLC-STATUS-Ind e.g., unrecoverable errors such as data link layer loss or recoverable status events such as reset.

6) The parameter Ciphering Elements are only applicable for UM and AM operations. These parameters are Ciphering Mode, Ciphering Key, Transmitting Activation Time (Sequence Number to activate a new ciphering configuration at the Sender), Receiving Activation Time (Sequence Number to activate a new ciphering configuration at the Receiver) and HFN (Hyper Frame Number).

7) The AM_parameters are only applicable for AM operation. These parameters are AMD PDU size, which can be either a fixed value or set to flexible size, largest UL AMD PDU size (only used when flexible PDU size is configured by upper layers, see subclause 9.2.2.8), Length Indicator Size, In-sequence Delivery Indication (indicating that RLC SDUs are delivered to upper layers in sequence or that they can be delivered out of sequence), Timer values (see subclause 9.5), Use of a special value of the HE field (see subclause 9.2.2.7), Protocol parameter values (see subclause 9.6), Polling triggers (see subclause 9.7.1), Status triggers (see subclause 9.7.2), Periodical Status blocking configuration (see subclause 9.7.2), SDU discard mode (see subclause 9.7.3), Minimum WSN (see subclause 9.2.2.11.3), and Send MRW. The Minimum WSN is always greater than or equal to the number of transport blocks in the smallest transport block set. The Send MRW indicates that the information of each discarded RLC SDU is sent to the Receiver, and the MRW SUFI is sent to the Receiver even if no segments of the RLC SDU to be discarded were submitted to a lower layer.

8) The parameter DiscardInfo indicates to upper layer the discarded RLC SDU in the peer-RLC AM entity. It is applicable only when in-sequence delivery is configured and it is to be used when upper layers require the reliable data transfer.

9) The Stop parameter is applicable to AM and UM RLC entities only and indicates to the RLC entity to (see subclause 9.7.6):

– not transmit nor receive any RLC PDUs.

10) The Continue parameter is applicable to AM and UM RLC entities only and indicates to the RLC entity to continue transmission and reception of RLC PDUs.

11) The UM_parameters are only applicable for UM operation. It contains Timer_Discard value (see subclause 9.5), use Alternative E-bit interpretation (see subclause 9.2.2.5), largest UL UMD PDU size (see subclause 9.2.2.8), DL RLC UM LI size (see subclause 9.2.2.8) and SN_Delivery. For a receiving UM RLC in a UE, an additional parameter indicating use/ no use of out of sequence SDU delivery is included (see subclause 11.2.3.2). If out of sequence SDU delivery is used, the parameters OSD_Window_Size (see subclause 9.6) and the timeout value of Timer_OSD (see subclause 9.5) are included. For a receiving UM RLC in a UE, an additional parameter indicating use/ no use of duplicate avoidance and reordering is included (see subclause 9.7.10). If duplicate avoidance and reordering is used, the parameters DAR_Window_Size (see subclause 9.6) and the timeout value of Timer_DAR (see subclause 9.5) are included. If out-of-sequence support is configured, the parameter Configured_Rx_Window_Size is included. If SN_Delivery parameter is configured, the receiving RLC entity delivers an RLC SDU with Sequence_Number to the upper layers. When SN_Delivery parameter is configured, the DL transmitting RLC entity should neither concatenate nor segment RLC SDUs, the UL transmitting RLC entity shall not concatenate RLC SDUs and may segment RLC SDUs.

12) The TM_parameters are only applicable for TM operation. It contains e.g. segmentation indication (see subclauses 9.2.2.9 and 11.1.2.1), Timer_Discard value (see subclause 9.5) and delivery of erroneous SDU indication (see subclause 11.1.3).

13) The N parameter indicates that an RLC entity will not send a PDU with "Sequence Number">=VT(S)+N for AM and "Sequence Number">=VT(US)+N for UM, where N is a non-negative integer.

14) The VT(S) parameter indicates the value of the Send State Variable for the case of the AM.

15) The VT(US) parameter indicates the value of the UM Data State Variable, for the case of the UM.

16) The Error_Indicator parameter indicates that the RLC SDU is erroneous (see subclause 11.1.3).

17) The parameter UE-ID type indicator indicates the RNTI type (U-RNTI or C-RNTI) to be used for the associated RLC SDU. This parameter is not required at the UE.

18) The parameter DiscardReq indicates whether the transmitting RLC entity needs to inform the upper layers of the discarded RLC SDU. If required, the transmitting RLC entity notifies upper layers when the SDU is discarded.

19) The parameter Status is only applicable for AM operation. This parameter indicates whether a RLC SDU is successfully transmitted or discarded.

20) The parameter Sequence_Number is the value that is mapped onto the Sequence Number (SN) field in an RLC PDU. It is applicable only for UM operation when SN_Delivery is configured for the UMD RLC entity.

21) The parameter Minimum UL PDU size is applicable for AM and UM operation in the uplink. The Minimum UL PDU size determines the smallest size of the RLC AMD or UMD PDU after segmentation or concatenation (see section 9.2.2.9). If data to be transmitted is not enough to create a PDU of the minimum size, it is allowed to create a PDU including all data to be transmitted, even if the resulting size is smaller than the Minimum UL RLC PDU size.