Description
The REMAP frame is a fundamental component of the Radio Link Protocol (RLP) as specified in 3GPP TS 24.022. RLP operates at the data link layer (Layer 2) and is responsible for providing a reliable, in-sequence delivery service for user data over the inherently error-prone radio interface in GSM and UMTS circuit-switched data services, such as fax or non-transparent data calls. The REMAP frame is one of several RLP frame types, specifically designed for control and signaling functions rather than carrying user data payloads.
Architecturally, RLP sits between the physical layer and higher-layer protocols. It uses a frame-based structure to segment and reassemble data blocks, manage acknowledgments, and perform error recovery through retransmissions. The REMAP frame contains control information essential for RLP operation. This includes fields for sequence numbers, frame type identification, and control parameters that govern the protocol's state machine, such as reset commands or window management instructions. The frame structure includes a header with the necessary control bits and may include an optional information field for specific commands.
In operation, when an RLP entity needs to signal a change in protocol state, initiate recovery procedures, or exchange control parameters with its peer, it generates and transmits a REMAP frame. For instance, if excessive errors are detected, a REMAP frame might be used to command a reset of the sequence numbering or to adjust the transmission window size. The reception of a REMAP frame triggers specific actions in the receiving RLP entity, ensuring both ends of the link remain synchronized and can recover from errors. Its role is critical for maintaining the logical link, managing flow control, and ensuring the reliable data transfer service that higher-layer applications depend on, even as radio conditions fluctuate.
Purpose & Motivation
The REMAP frame was introduced to address the need for robust in-band signaling within the Radio Link Protocol for GSM circuit-switched data services. Early mobile data services required a mechanism to manage the data link reliably over the unreliable radio channel. Simple data frames alone were insufficient; a dedicated control frame was necessary to coordinate the two RLP peers, handle error scenarios, and manage protocol parameters without corrupting user data streams.
Prior to standardized protocols like RLP, data transmission over radio was prone to high error rates and required complex, proprietary link-layer solutions. The creation of RLP, and within it the REMAP frame, provided a standardized, reliable method for data transfer. The REMAP frame specifically solves the problem of protocol coordination. It allows peers to exchange control information—like reset requests or window updates—seamlessly within the same logical channel used for data, ensuring the link can adapt to changing conditions and recover from failures without requiring higher-layer intervention. This was essential for supporting fax, dial-up data, and other circuit-switched bearer services expected to work reliably over cellular networks.
Key Features
- Defined control frame type within the Radio Link Protocol (RLP)
- Carries signaling and management information between RLP entities
- Used for protocol state synchronization and error recovery procedures
- Contains fields for sequence numbers and control commands
- Operates over the Um (radio) interface in GSM/UMTS
- Essential for reliable circuit-switched data service operation
Evolution Across Releases
Defining Specifications
| Specification | Title |
|---|---|
| TS 24.022 | 3GPP TS 24.022 |