Frame Reject

Description

Frame Reject (FRMR) is a specific supervisory frame defined in the Link Access Protocol on the Dm channel (LAPDm), which is the data link layer (Layer 2) protocol used for signaling and data transfer on the radio interface in GSM (and its evolution). LAPDm is an adaptation of the ISDN LAPD protocol for the unique constraints of the mobile radio path. The FRMR frame is a key mechanism for error recovery when a protocol violation occurs that cannot be corrected by standard cyclic redundancy check (CRC) failure or sequence number errors handled by REJ (Reject) frames.

FRMR operates as follows: When a receiver (either the Mobile Station/MS or the Base Station System/BSS) receives a frame that is syntactically correct (passes the Frame Check Sequence) but contains a protocol error, it sends an FRMR frame to the transmitter. Protocol errors include receiving an undefined or unimplemented frame type, a frame with an information field longer than the maximum allowed, or an invalid control field parameter (e.g., a poll/final bit set incorrectly for the context). The FRMR frame itself contains an information field that includes a copy of the offending frame's control field and, often, a diagnostic code explaining the specific reason for the rejection. This allows the transmitting side to diagnose the exact protocol mismatch.

Upon receiving an FRMR, the transmitter must take specific recovery actions as defined by the LAPDm state machine. Typically, this involves resetting the data link layer connection for the affected logical channel. This may mean re-establishing the link, which ensures both ends re-synchronize their state variables (send and receive sequence numbers, V(S) and V(R)). The FRMR mechanism is a last-resort error handling procedure, as it indicates a breakdown in the agreed protocol operation between the two peers. It is crucial for maintaining the integrity and consistency of signaling messages (e.g., for call control, mobility management) and user data packets (in GPRS/EDGE) over an unreliable radio link, ensuring that persistent protocol errors do not cause silent failures or undefined behavior.

Purpose & Motivation

The Frame Reject mechanism exists to handle a specific class of errors in data link layer communication: protocol violations. In any reliable data link protocol, simple bit errors are detected by the Frame Check Sequence (FCS/CRC), and frames with CRC errors are silently discarded, relying on higher-layer timers and sequence numbers (like in REJ frames) to trigger retransmission. However, some errors result in a frame that is technically valid according to the FCS but logically invalid according to the protocol state machine or rules.

Without FRMR, such protocol error frames could cause the two communicating ends to become unsynchronized or enter an undefined state. For example, if one side starts sending a new, longer type of information frame that the other side's software does not understand, the receiver might simply discard it. The transmitter, not receiving an acknowledgment, might retransmit it endlessly, causing a deadlock. The FRMR frame provides a formal, in-band notification mechanism to break this deadlock. It informs the sender, 'I received your frame, it passed the CRC check, but I cannot process it due to a protocol rule violation; we need to reset our conversation.'

This capability was particularly important in the design of GSM's LAPDm, which needed to be robust for operation over a harsh, error-prone radio channel while supporting multiple logical channels (SDCCH, SACCH, FACCH) for critical network control signaling. It ensures that any fundamental incompatibility or software bug causing protocol misuse can be detected and a recovery procedure (link reset) can be initiated automatically, maintaining overall system stability and preventing calls from being stuck in an unrecoverable signaling error state. This principle of using FRMR (or its equivalent, like LAPD's FRMR) is a classic data communications concept adapted for the mobile environment.

Key Features

• LAPDm supervisory frame type for reporting unrecoverable protocol errors
• Contains a diagnostic field with the rejected frame's control field and error cause
• Triggers a data link layer reset procedure to re-synchronize communication peers
• Handles errors such as undefined frame types or invalid control field parameters
• Operates on the Um interface between MS and BSS for GSM signaling and data
• A last-resort error recovery mechanism when standard ARQ fails

Evolution Across Releases

Defining Specifications