As noted in the Methodology section, there are several PDU types defined for GERAN RRC messages (3GPP TS 44.018). You must choose the correct PDU type to use based on the message format, message channel, and message direction. Having chosen the correct PDU type, you proceed with encoding/decoding in the same way as described in the sections on encoding and decoding messages "other than TS24301Msg_PDU". The following paragraphs give an overview of the PDU types for RRC and a few useful tips.