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. TS 27.002 for the asynchronous case and TS 27.003 for the synchronous case. Care must be taken that that material also applies to Interworking Functions; see TS 29.007. 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.

The following documents contain provisions which, through reference in this text, constitute provisions of the present document.

• References are either specific (identified by date of publication, edition number, version number, etc.) or non-specific.
• For a specific reference, subsequent revisions do not apply.
• For a non-specific reference, the latest version applies. In the case of a reference to a 3GPP document (including a GSM document), a non-specific reference implicitly refers to the latest version of that document in the same Release as the present document.

Three versions of RLP are defined:

• RLP version 0: single-link basic version;
• RLP version 1: single-link extended version (e.g. extended by data compression);
• RLP version 2: multi-link version.

RLP uses one substream on one physical link (single-link) or from 1 up to 4 (multi-link) substreams on one or more physical links. However, the RLP multi-link version is designed to be able to support up to 8 substreams. In A/Gb and GERAN Iu mode:

• If in the call set-up signalling, either end indicates that it cannot support multi-link operation, neither end shall require usage of RLP-versions higher than 1. If the BC negotiation during call set-up results in a possibility for multi-link operation during the call, both ends shall require and accept RLP-version 2 only;
• If the BC-IE sent by the UE in the SETUP or CALL CONFIRM message indicates "maximum number of traffic channels" = "1 TCH" and WAIUR ≤ 14,4 kbit/s and the BC-IE sent by the UE in the CALL CONFIRM message (MT case) or by the MSC in the CALL PROCEEDING message (MO case) indicates UIMI = "User initiated modification not allowed/required/applicable" or "User initiated modification up to 1 TCH/F allowed/may be requested", this shall be interpreted as if at least one end does not support multi-link operation, and neither end shall require an RLP version higher than 1.

In UTRAN Iu mode, either end is allowed to request the usage of any RLP-version. RLP makes use of an underlying FEC (Forward Error Correction) mechanism. For RLP to perform adequately it is assumed that the basic radio channel together with FEC provides for a block error rate of less than 10 %, where a block consists of 240 bits or 576 bits (Further study on the BLER for 576-bit blocks is needed). Furthermore, it is assumed that in case of multi-link RLP the difference of the delay between all physical links is less than timer T4. In A/Gb mode and in GERAN Iu mode, RLP frames are of a fixed size of 240 (TCH/F4,8 and TCH/F9,6 channel codings) or 576 bits (TCH/F14,4, TCH/F28,8 and TCH/F43,2 channel codings). In UTRAN Iu mode, the RLP frame size does not depend on the channel coding, only 576 bit frames are used. RLP entities running only in an UTRAN Iu mode environment need only to support the 576 bit frame length. The REMAP function is not necessary. RLP entities running in both of the systems have to support the REMAP function. In a handover from UTRAN Iu mode to A/Gb mode or GERAN Iu mode the frame either stays 576 bits long or changes from 576 bits to 240 bits incurring a REMAP. In a handover from A/Gb mode or GERAN Iu mode to UTRAN Iu mode the frame either stays 576 bits long or changes from 240 bits to 576 bits incurring a REMAP. In A/Gb mode, RLP frames are sent in strict alignment with the radio transmission. (For details, see TS 44.021. Whenever a frame is to be sent, the RLP entity has to provide the necessary protocol information to be contained in it. Provision is made for Discontinuous Transmission (DTX). RLP spans from the User Equipment (UE) to the interworking function (IWF), located at the nearest Mobile Switching Centre (MSC), or beyond. Depending on the exact location of the IWF, handover of the UE may result in link-reset or even total loss of the connection. The UE shall initiate the RLP link. In addition the MSC/IWF may initiate the RLP link. In the terminology of HDLC, RLP is used in a balanced configuration, employing asynchronous operation, i.e. either station has the right to set-up, reset, or disconnect a link at any time. Procedural means are provided for to deal with contentious situations, should they ever occur. RLP is full-duplex in the sense that it allows for information to be transferred in both directions simultaneously.