Radio Link Protocol

Description

The Radio Link Protocol (RLP) is a Layer 2 (Data Link Layer) protocol specified in GSM 04.22, designed to support transparent and non-transparent circuit-switched data (CSD) services over the GSM radio interface. It operates between the Mobile Station (MS) and the Interworking Function (IWF) in the network, typically located at the Mobile Switching Center (MSC). RLP's primary function is to transform the error-prone, raw radio channel into a virtual reliable data pipe. It achieves this through a frame-based structure and an Automatic Repeat Request (ARQ) mechanism. Data from the upper layer is segmented into RLP frames, each containing a header with sequence numbers and control information, and a payload. The protocol uses selective reject (SREJ) and reject (REJ) ARQ schemes for error recovery. If a frame is received in error or is missing, the receiver can request retransmission of specific frames, ensuring data integrity. RLP operates in three modes: Transparent Mode (where no ARQ is used, and errors are passed up), Non-Transparent Basic Mode (with ARQ), and Non-Transparent Extended Mode (with enhanced window sizes and features for higher throughput). The protocol manages flow control through its window mechanism, limiting the number of unacknowledged frames in transit. It also includes procedures for initialization, synchronization, and termination of the logical link. While primarily for circuit-switched bearers, RLP's concepts influenced later packet-switched protocols. Its operation is largely invisible to the end-user application, which interacts with higher-layer protocols like PPP that run over the RLP-established link.

Purpose & Motivation

RLP was created to enable reliable data communication over the GSM network's radio interface, which is susceptible to fading, interference, and high bit error rates (BER). Before digital cellular systems, data services were impractical. GSM's digital nature opened the possibility, but the raw radio channel was unsuitable for applications like fax, modem dial-up, and early mobile data terminals that required low error rates. The protocol solved this by implementing a robust ARQ mechanism at the link layer, shielding higher-layer protocols from radio-induced errors. It addressed the limitations of a purely transparent channel, which would have rendered many data applications unusable due to corruption. RLP was a cornerstone of GSM Phase 2+ data services, enabling the first wave of mobile data before the advent of GPRS and packet-switching. Its development was motivated by the need to provide a standardized, reliable data bearer service that could interwork with the existing PSTN/ISDN data world, facilitating services like circuit-switched fax and video telephony as defined in 3GPP releases.