Error Recovery Scheme for the IBM 3850 Mass Storage System

The IBM 3850 Mass Storage System (MSS) stores digital data onflexible magnetic tape media; however, it is different in many respects from the conventional multitrack tape machines. In particular, the use of a single-element rotary readwrite head imposes new demands in the areas of data encoding and error recovery. This paper presents a comprehensive scheme for error recovery for the 3850 MSS which features a new error-correction code in a serial, single-stripe data format. The recovery procedure is designed around resynchronizable sections of data which are rendered independent of each other in error modes through the use of zero-modulation encoding and self-contained error-detection pointers. These error-detection pointers and the resynchronization signals are utilized in conjunction with interleaved codewords of the error-correction code. The code is designed with a generating polynomial in which the roots are chosen from the set of elements of a 16-element subfield of the Galoisfield GF(2'). This choice provides the necessary code structurc>fiw desired code capabilities and facilitates fast decoding of errors with an economical implementation o f the decoder. The scheme provides correction capabilities for various combinations of mixed-mode short and long errors common to magnetic tape recording of digital data.