Description
Backward Error Correction (BEC) is an error control mechanism defined in 3GPP TS 43.064 for the GSM/EDGE Radio Access Network (GERAN). It operates at the Radio Link Control (RLC) layer and is a key component of the data link layer protocol stack for packet-switched services like GPRS and EDGE. BEC is fundamentally an Automatic Repeat Request (ARQ) protocol, specifically a selective-repeat ARQ mechanism. Its primary function is to ensure the reliable delivery of data blocks across the air interface by detecting and correcting transmission errors that occur due to factors like interference, fading, and noise.
The protocol works through a cyclical process of transmission, acknowledgment, and potential retransmission. The sender transmits data blocks, each tagged with a sequence number. The receiver checks each block for errors using a Frame Check Sequence (FCS). If a block is received correctly, the receiver sends a positive acknowledgment (ACK) back to the sender. If an error is detected, the receiver discards the corrupted block and sends a negative acknowledgment (NACK) or, in some implementations, simply withholds an ACK. The sender maintains a retransmission buffer. Upon receiving a NACK or after a timeout period without an ACK, it retransmits the specific data block identified by its sequence number. This selective retransmission is more efficient than go-back-N protocols, as it only resends the faulty blocks rather than all subsequent ones.
Key architectural components include the RLC transmitter entity, the RLC receiver entity, the sequence numbering scheme, the acknowledgment signaling (ACK/NACK), and the retransmission timer. BEC operates in conjunction with Forward Error Correction (FEC) at the physical layer. While FEC adds redundant bits to correct a limited number of errors automatically, BEC handles error cases beyond FEC's capability. This hybrid approach provides a robust defense against varying channel conditions. The protocol's parameters, such as the window size (governing the number of unacknowledged blocks allowed) and timer values, are configurable to balance throughput and delay based on network conditions.
In the overall GERAN architecture, BEC is crucial for the Radio Link Control/Medium Access Control (RLC/MAC) protocol. It ensures that the unreliable physical radio link appears as a reliable data pipe to the upper layers (LLC and above). This reliability is essential for TCP/IP-based applications running over GPRS/EDGE, as it prevents the TCP layer from misinterpreting radio-layer packet loss as network congestion, which would trigger unnecessary and detrimental congestion control mechanisms. BEC thus plays a vital role in maintaining efficient and predictable data service quality.
Purpose & Motivation
BEC was introduced to address the fundamental challenge of providing reliable data services over the inherently error-prone GSM radio interface. Early circuit-switched voice services used FEC, but for packet data (GPRS), a more efficient and robust error control mechanism was needed. The primary problem BEC solves is the reliable transfer of data packets despite high and variable Bit Error Rates (BER) on the wireless channel. Without it, upper-layer protocols would be burdened with excessive packet loss, leading to poor application performance and inefficient use of network resources.
The motivation for BEC's creation stemmed from the evolution of GSM into a data-capable network with GPRS. Pure FEC schemes require adding significant redundancy to every transmission, which consumes valuable bandwidth even when channel conditions are good. For bursty data traffic, this is inefficient. BEC, as an ARQ scheme, only consumes extra bandwidth (for retransmissions) when errors actually occur. This makes it spectrally more efficient for typical data patterns. It was designed to work in tandem with the existing FEC, creating a layered defense: FEC corrects frequent, small errors, while BEC recovers from less frequent but larger error bursts.
Historically, BEC provided a standardized, optimized method for error recovery that was tailored to the timing and frame structure of the GSM/GERAN system. It addressed limitations of simpler ARQ schemes by implementing selective repeat, which improves throughput over links with higher latency or error rates compared to stop-and-wait or go-back-N ARQ. Its design was crucial for enabling the commercial viability of mobile internet and messaging services on 2G and 2.5G networks, laying groundwork for concepts later refined in 3G UMTS and 4G LTE.
Key Features
- Selective Repeat ARQ mechanism for efficient retransmissions
- Operates at the RLC layer in the GERAN protocol stack
- Uses sequence numbers and ACK/NACK signaling for error control
- Works in conjunction with Physical Layer Forward Error Correction (FEC)
- Configurable parameters like window size and timers for performance tuning
- Ensures reliable data pipe for upper-layer protocols like TCP/IP
Evolution Across Releases
Introduced Backward Error Correction as defined in TS 43.064 for GERAN. The initial architecture established the selective-repeat ARQ protocol at the RLC layer, integrating it with the GPRS/EDGE data service framework. It defined the core procedures for block sequencing, error detection via FCS, acknowledgment mechanisms (ACK/NACK), and timer-based retransmissions to ensure reliable data transfer over the GSM radio interface.
Defining Specifications
| Specification | Title |
|---|---|
| TS 43.064 | 3GPP TR 43.064 |