bts: bts_snd_dl_ass(): clarify the meaning of last_rts_fn + 21216

Change-Id: I8127e17607426443bb95b42873b580fc9f400af9
---
M src/bts.cpp
1 file changed, 5 insertions(+), 2 deletions(-)


git pull ssh://gerrit.osmocom.org:29418/osmo-pcu refs/changes/80/29880/1

diff --git a/src/bts.cpp b/src/bts.cpp
index df08cc6..3eb41d2 100644
--- a/src/bts.cpp
+++ b/src/bts.cpp
@@ -1083,8 +1083,11 @@
 	LOGPTBF(tbf, LOGL_INFO, "TX: START Immediate Assignment Downlink (PCH)\n");
 	bitvec *immediate_assignment = bitvec_alloc(22, tall_pcu_ctx); /* without plen */
 	bitvec_unhex(immediate_assignment, DUMMY_VEC); /* standard '2B'O padding */
-	/* use request reference that has maximum distance to current time,
-	 * so the assignment will not conflict with possible RACH requests. */
+	/* 3GPP TS 44.018, section 9.1.18.0d states that the network shall code the
+	 * Request Reference IE, e.g. by using a suitably offset frame number, such
+	 * that the resource reference cannot be confused with any CHANNEL REQUEST
+	 * message sent by a mobile station.  Use last_rts_fn + 21216 (16 TDMA
+	 * super-frame periods, or ~21.3 seconds) to achieve a decent distance. */
 	LOGP(DRLCMAC, LOGL_DEBUG, " - TRX=%d (%d) TS=%d TA=%d\n",
 		trx_no, tbf->trx->arfcn, ts_no, tbf->ta());
 	plen = Encoding::write_immediate_assignment(&bts->trx[trx_no].pdch[ts_no],

