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The LTE and 5G NR air interface RLC layers provide the capability to exchange information between a UE and the LTE E‐UTRAN or between a UE and the 5G NG‐RAN in terms of the PDU. An RLC layer PDU facilitates transfer of higher‐layer data in Transparent (TM), Unacknowledged (UNACK) or Acknowledged (ACK) mode. A PDU consists of a header part that is further followed by the data part of the PDU.

 PDU Description

An RLC PDU, ACK, and UNACK mode header consist of several fields with different lengths in bits. Thus, the encoding and decoding of each field are different. Nevertheless, the protocol header and the data part of RLC PDU are octet aligned and is described in a tabular format. The TM PDU of the RLC layer does not contain the header part and is used to transfer messages such as paging and system information messages. Neither the sending RLC nor the receiving RLC layer performs any operations on a TM PDU. There is another PDU called Control PDU, which is used by the receiving RLC layer to inform the sending RLC layer on the status, i.e. lost or successfully decoded, of a PDU being received.

 Encoding of RLC PDU

Though the RLC PDU is described in a tabular format, the header and data part is encoded as bit strings where the leftmost bit of the first line of the table is considered as the most significant bit and the rightmost bit of the last line of the table is considered as the least significant bit. Depending on the length of Sequence Number (SN) used in an RLC header, the length of the RLC header may take 1 or 2 octets at the beginning of the table and is different for the ACK mode and UNACK mode of data transfers. The 5G NR air interface RLC layer and its PDU formats are described later in Chapter 19. For more information on the RLC layer protocol header, its different parameters, and their encoding requirements, refer to TS 38.322 [114] for 5G NR.