diff --git a/openbsc/src/abis_rsl.c b/openbsc/src/abis_rsl.c
index 6b328bb..a4764f5 100644
--- a/openbsc/src/abis_rsl.c
+++ b/openbsc/src/abis_rsl.c
@@ -239,6 +239,7 @@
 	return lchan;
 }
 
+/* See Table 10.5.25 of GSM04.08 */
 u_int8_t lchan2chan_nr(struct gsm_lchan *lchan)
 {
 	struct gsm_bts_trx_ts *ts = lchan->ts;