817   Fixed-length GOOSE float encoding

Created: 03 Feb 2012

Status: In Force (green)

Part: Part 8-1 (2011; Edition 2)

Links:

Page: 139

Clause: n/a

Paragraph: n/a

Category: Issue may impact interoperability of implementations of Edition 2

Issue

The fixed-length GOOSE encoding for 32-bit floating point numbers specifies a length of 4 bytes, in IEEE 754 format. It is also claimed that fixed-length encoding should be backwards-compatible with existing GOOSE decoders. Existing GOOSE implementations encode 32-bit floats in 5 bytes, with the first byte being used to indicate the number of bits used for the exponent. Therefore, there is no guarantee that 4-byte fixed-length floating-point values will be decoded correctly.

Proposal

Possible solutions:

1) Do nothing, because this encoding format is optional, i.e., subscribers are responsible for only subscribing to fixed-length encoded datasets if they can decode them.

2) State in IEC 61850-8-1 that datasets containing fixed-length floating-point values may not be decoded correctly.

3) Change the fixed-length encoding of 32-bit floating point values to match existing implementations.

4) Create a non-backwards-compatible fixed-length encoding. At the same time, change the byte length for integers to native sizes, e.g., use 4 bytes for 32-bit signed integers, rather than 5 bytes (as needed for backwards-compatibility).