--- a/TCH-tap-modes	Mon Jul 22 22:36:29 2024 +0000
+++ b/TCH-tap-modes	Mon Jul 22 23:02:54 2024 +0000
@@ -351,6 +351,28 @@
 voting function output into the remaining bit positions, perhaps for loopback
 of TCH DL into TCH UL.  More study is needed in this area.
 
+Appearance of FACCH in TCH/HS downlink
+--------------------------------------
+
+All mentions of FACCH handling in the previous description refer only to TCH/F.
+In the case of TCH/HS, the following differences in FACCH handling are seen:
+
+* Bits [7:5] described earlier for the case of FACCH/F are never seen set in
+  TCH/HS DL captures.
+
+* In the first 20 ms frame position stolen for FACCH/H, status word 0 is seen
+  as 0xC007 (looks like regular BFI), status word 1 is set to 0 (instead of
+  whatever D_MACC is), and it is not clear whether or not status word 2 is
+  still filled with the bit error count from the convolutional decoding step.
+  (We've only seen 0 in that word.)  The payload bits are filled with what
+  looks like garbage, in contrast with FACCH/F behavior of retaining previous
+  buffer content.
+
+* In the second 20 ms frame position stolen for FACCH/H, status word 0 is seen
+  as 0, status word 1 looks like correct D_MACC after 456 rather than 228 bits
+  have been decoded, and status word 2 contains 0xFFFF.  The "garbage" in the
+  payload bits stays the same from the first half of FACCH/H.
+
 FreeCalypso file format for TCH DL captures
 ===========================================