We (Themyscira Wireless) define our own binary file format for testing of GSM
06.10 (FR) and EFR codec functions; this format of ours is an extension of
classic .gsm format from libgsm/toast.  The original libgsm file format is a
directly abutted sequence of 33-byte libgsm frames, equivalent to RTP frames
for GSM FR, with the upper nibble of the first byte in each frame equal to 0xD,
serving as a signature.  We simply extend this idea: our version is still a
directly abutted sequence of binary records, but each record is now one of 3
possibilities:

- a 33-byte GSM FR frame in libgsm/RTP format, 0xD signature
- a 31-byte GSM EFR frame in RTP format (ETSI TS 101 318), 0xC signature
- a 2-byte Themyscira-extension BFI marker, 0xBF signature, see below

File reading functions begin by reading only one byte; this byte, once decoded,
tells us how many more bytes need to be read, and frame synchronization is thus
maintained.

The recommended filename suffix for extended-libgsm binary files in the present
format is .gsmx; of course dot-separated filename suffixes hold absolutely no
special meaning on Unix systems, but many developers still strongly prefer to
have them for psychological comfort.

Any gsmx file (FR or EFR) can be dumped in human-readable form with our
gsmrec-dump utility.  This utility turns every read frame from bytes into codec
parameters with gsm_explode() or EFR_frame2params(), and then displays those
parameters in a sensible manner, with a per-frame header line followed by 4
lines of subframe parameters.

FR and EFR frames are not expected to be mixed in the same stream recording;
our low-level binary file reading function and gsmrec-dump will grok such mixing
just fine, but each higher-level test program (beyond gsmrec-dump) is expected
to be written for only one codec, either FR or EFR.

BFI marker format
=================

Every 20 ms frame in our gsmx files is either a good FR/EFR frame or a BFI (Bad
Frame Indication) marker.  The BFI marker format used in our gsmx file format is
the same format which we (Themyscira Wireless) previously used in our GSM RAN
RTP transport, before switching to our current TRAUlike RTP format.  This BFI
marker format is quite simple:

byte 0: 0xBF signature;
byte 1: least-significant bit encoding TAF per GSM 06.31 or GSM 06.81,
        section 6.1.1 in both documents; other bits are reserved.