[DFDL-WG] Asterix format - not compelling examples of bitOrder leastSignificantBitFirst but byteOrder bigEndian

[Mike Beckerle]

http://www.eurocontrol.int/sites/default/files/content/documents/nm/asterix/20141201-asterix-sdps-cat062-part9-v1.17.pdf

The above is part of the Asterix spec.

Some interesting data layouts are on pages:

Page 28 - shows integer fields that span bytes. These are big-endian
integers, however the bits are numbered in the prose LSBF.  But DFDL
doesn't use bit positions, it uses lengths. So that prose isn't needed.
Page 30 - simple example of two single bit fields, followed by a 14-bit
big-endian integer
Page 35 - simple example of a single bit followed by a big-endian 15-bit
integer
Page 41 - String of 8 characters represented as 6-bit code points.
(motivates the proposal for dfdl:character(...) function)

In every case I see, it's very easy to model this data using bitOrder
mostSignificantBitFirst and byteOrder bigEndian, and simply specifying the
length (in bits) of the elements.

 It is just funny to look at the way they number the bits, and talk about
the bit positions. Their numbering scheme uses bit 1 as the
least-significant bit, but the order they present things starts first with
the most significant bit.

So in a 16 bit word, they'll start by describing "bit 16" first, which is
the most significant bit first. So their bit numbers are assigned LSBF,
but the "first" bit is the most-significant bit, first byte is the most
significant byte (bigEndian).

Now bit position numbers really don't play a role in a DFDL schema, so I'd
say Asterix is really just bigEndian and mostSignificantBitFirst. There's
nothing compelling here about the need for the additional combination of
bigEndian with leastSignificantBitFirst bit order.

So I guess I withdraw the suggestion that this combination is really needed
by this Asterix format.


Mike Beckerle | OGF DFDL Workgroup Co-Chair | Tresys Technology |
www.tresys.com
Please note: Contributions to the DFDL Workgroup's email discussions are
subject to the OGF Intellectual Property Policy
<http://www.ogf.org/About/abt_policies.php>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://www.ogf.org/pipermail/dfdl-wg/attachments/20160510/b216aae6/attachment.html>