FX.25 Forward Error Correction

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FX.25 is the protocol extension for AX.25 Link Layer Protocol. FX.25 provides Forward Error Correction (FEC) capability while maintaining inherited compatibility with non-FEC equipment. FX.25 was created by Stensat Group in 2005, and presented as a technical paper at the 2006 TAPR Digital Communications Conference in Tucson, AZ.

Video FX.25 Forward Error Correction

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FX.25 is intended to supplement the AX.25 protocol instead of replacing it. It provides an encapsulation mechanism that does not alter data or function AX.25. Error correction capabilities are introduced at the bottom of Layer 2 in the OSI model.

The AX.25 Link Layer Protocol is widely used in amateur radio communications. These packets are validated by a 16-bit CRC, and are discarded if one or more errors are detected. In many cases, such as space-to-earth telemetry, the packets are broadcasted indirectly. No back-channel available to request re-transmission of incorrect elements. As a result, the AX.25 link is basically intolerant of error.

The FX.25 protocol extension provides "wrapper" error correction around the AX.25 packet, allowing for deletion of errors on the final recipient. The data field has been carefully selected to allow receipt of AX.25 packet data in frame FX.25 by non-FEC decoders.

Maps FX.25 Forward Error Correction

Technical Implementation

A composite FX.25 entity is called a "frame," which distinguishes it from the "AX25" packet contained therein. Frame FX.25 contains the following elements:
- Site Opening - Tag Correlation - - AX.25 Packages
- - AX.25 Start Package
- - AX.25 Packet Body
- - AX.25 Packet Frame Check Sequence (FCS)
- - AX.25 Final Package
- Pad for bit-to-byte alignment - FEC Check Symbol - - Postamble

"Codeblock FEC" contains all elements except Preamble, Correlation Tag, and Postamble. These three elements are outside the correction room for the FEC algorithm. The Opening Block and Postamble are variable lengths, and put into account for delays commonly found in radio links - "key" transmitters for stable operation, latency of lazy recipients, etc. The Correlation Tag is a Gold code, and it contains an inherent fault tolerance. This is required to provide a "start frame" marker without requiring dependence on FEC capabilities.

The current FEC frame implements the Reed Solomon error correction algorithm, but is not limited to this.

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Performance

Performance improvements will be a function of packet size AX.25 combined with the noise characteristics of the transmission line. Initial performance testing involves transmitting 61 frames of FX.25 over an interval of about 15 minutes - 9 frames received without error - 19 frames received with errors that can be fixed - 33 frames received with errors that can not be fixed

15% of AX.25 [9/61] packages can be decoded without FEC capability - 46% of the AX.25 package [(9 19)/61] has been decoded with FEC capability

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References

• "Encoding FEC for AO-40 Telemetry from Phil Karn" Ã,
• "FX.25 On-Air Performance" Ã,

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External links

• 2006 TAPR DCC web page
• Specification FX.25 (pdf)
• FX.25 Slide Presentation from 2006 TAPR DCC (pdf)
• FX.25 Google Discussion Groups

Source of the article : Wikipedia