The present document specifies the Radio Link Protocol (RLP) for circuit switched data transmission within a PLMN. RLP covers the Layer 2 functionality of the ISO OSI Reference Model (ISO/IEC 7498 [22]). It is based on ideas contained in ISO/IEC 3309 [21], ISO/IEC 4335 [20] and ISO/IEC 7809 [26] (HDLC of ISO) as well as ITU-T Recommendation X.25 [30] and Q.92x (LAP-B and LAP-D of ITU, respectively.) RLP has been tailored to the special needs of digital radio transmission. RLP provides to its users the OSI Data Link Service (ISO/IEC 8886 [24]).

RLP is intended for use with non-transparent data-transfer. Protocol conversion may be provided for a variety of protocol configurations. Those 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 relevant Terminal Adapters, i.e. 3GPP TS 27.002 [10] for the asynchronous case and 3GPP TS 27.003 [11] for the synchronous case. Care must be taken that that material also applies to Interworking Functions; see 3GPP TS 29.007 [13].

The present document is valid for a PLMN in A/Gb mode as well as in Iu mode. If text applies only for one of these systems it is explicitly mentioned by using the terms "A/Gb mode" and "Iu mode". Please note, that the Gb interface does not play any role in the scope of the present document although the term "A/Gb mode" is used.