--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/doc/FR-EFR-formats	Sun Apr 16 07:55:21 2023 +0000
@@ -0,0 +1,21 @@
+The two RTP analysis utilities rtp-gsmfr-dump and rtp-gsmfr-extr support the
+following RTP payload formats for FR/EFR codec frames and BFI markers:
+
+* Standard RFC 3551 (same as TS 101 318) format for good frames
+
+* TRAUlike RTP payload format defined here:
+
+https://www.freecalypso.org/hg/themwi-system-sw/file/tip/doc/RTP-TRAUlike-format
+
+* Old BFI marker format defined here:
+
+https://www.freecalypso.org/hg/themwi-system-sw/file/tip/doc/RTP-BFI-extension
+
+* Zero-length RTP payload serving as a BFI marker
+
+All of these formats are distinguished by rtp-gsmfr-dump in its human-oriented
+dump output, but not all of them can be represented losslessly in the gsmx
+binary file format defined in our Themyscira Wireless GSM codec libraries and
+utilities package.  The latter on-disk file format requires plain RFC 3551
+format for good frames and the old Themyscira 2-byte BFI marker format for bad
+frames; rtp-gsmfr-extr converts everything into this fixed format.