diff --git a/openbsc/src/abis_rsl.c b/openbsc/src/abis_rsl.c
index 8cec2b6..7be1a6b 100644
--- a/openbsc/src/abis_rsl.c
+++ b/openbsc/src/abis_rsl.c
@@ -1085,6 +1085,7 @@
 	lchan->ms_power = ms_pwr_ctl_lvl(bts->band, bts->ms_max_power);
 	lchan->bs_power = 0; /* 0dB reduction, output power = Pn */
 	lchan->rsl_cmode = RSL_CMOD_SPD_SIGN;
+	lchan->tch_mode = GSM48_CMODE_SIGN;
 	rsl_chan_activate_lchan(lchan, 0x00, rqd_ta);
 
 	/* create IMMEDIATE ASSIGN 04.08 messge */