This European Telecommunication Standard (ETS) specifies the Radio Lin k Protocol (RLP) for data transmission over the GSM PLMN. RLP covers t he Layer 2 functionality of the ISO OSI Reference Model (IS 7498). It is based on ideas contained in IS 3309, IS 4335 and IS 7809 (HDLC of I SO) as well as CCITT X.25 and Q.92x (LAP-B and LAP-D of CCITT, respect ively.) RLP has been tailored to the special needs of digital radio tr ansmission. RLP provides to its users the OSI Data Link Service (IS 88 86). RLP is intended for use with non-transparent data-transfer. Protocol c onversion may be provided for a variety of protocol configurations. Th ose foreseen immediately are: - Character-mode protocols using start-stop transmission (IA5); - X.25 LAP-B. For reasons of better presentation, material about protocol conversion has been placed within those Specifications concerned with the releva nt Terminal Adaptors, i.e. GSM 07.02 for the asynchronous case and GSM 07.03 for the synchronous case. Care must be taken that that material also applies to Interworking Functions; see GSM 09.04 - 09.07.