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 (single-link) or from 1 up to 4 (multi-link) physical links. However, the RLP multi-link version is designed to be able to support up to 8 physical links. If, in the call setup signalling, either end indicates that it cannot support multilink operation, neither end shall require usage of RLP-versions higher than 1. If the BC negotiation during call setup results in a possibility for multi-link operation during the call, both ends shall require and accept RLP version 2 only.
If the BC negotiation during call setup results in maximum number of traffic channels = 1 TCH and UIMI = not required/not allowed or up to 1 TCH/F allowed/may be requested, this is interpreted as if at least one end does not support multilink operation, and neither end shall require RLP version higher than 1.
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 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.
RLP frames are sent in strict alignment with the radio transmission. (For details, see GSM 04.21). RLP frames are of a fixed size of 240 (TCH/F9.6 channel coding) or 576 bits (TCH/F14.4 channel coding). 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 Mobile Station (MS) 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 MS may result in link-reset or even total loss of the connection.
The MS 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.