diff --git a/src/cnetz/cnetz.c b/src/cnetz/cnetz.c
index 97c8795..e09a52b 100644
--- a/src/cnetz/cnetz.c
+++ b/src/cnetz/cnetz.c
@@ -1300,9 +1300,10 @@ const telegramm_t *cnetz_transmit_telegramm_spk_k(cnetz_t *cnetz)
 	transaction_t *trans = cnetz->trans_list;
 	cnetz_t *ogk;
 
-	memset(&telegramm, 0, sizeof(telegramm));
 	if (!trans)
-		return &telegramm;
+		return NULL;
+
+	memset(&telegramm, 0, sizeof(telegramm));
 
 	telegramm.max_sendeleistung = cnetz_power2bits(cnetz->ms_power);
 	telegramm.sendeleistungsanpassung = (cnetz->ms_power < 8) ? 1 : 0;
@@ -1623,9 +1624,10 @@ const telegramm_t *cnetz_transmit_telegramm_spk_v(cnetz_t *cnetz)
 	transaction_t *trans = cnetz->trans_list;
 	int meter = 0;
 
-	memset(&telegramm, 0, sizeof(telegramm));
 	if (!trans)
-		return &telegramm;
+		return NULL;
+
+	memset(&telegramm, 0, sizeof(telegramm));
 
 	if (cnetz->metering) {
 		double now = get_time();
diff --git a/src/cnetz/dsp.c b/src/cnetz/dsp.c
index 1c03b6d..493c515 100644
--- a/src/cnetz/dsp.c
+++ b/src/cnetz/dsp.c
@@ -686,25 +686,38 @@ again:
 					/* set last time slot, so we know to which time slot the message from mobile station belongs to */
 					cnetz->sched_last_ts = cnetz->sched_ts;
 					bits = cnetz_encode_telegramm(cnetz);
-					PDEBUG_CHAN(DDSP, DEBUG_DEBUG, "Transmitting 'Rufblock' at timeslot %d\n", cnetz->sched_ts);
+					if (bits) {
+						PDEBUG_CHAN(DDSP, DEBUG_DEBUG, "Transmitting 'Rufblock' at timeslot %d\n", cnetz->sched_ts);
+						fsk_block_encode(cnetz, bits, 1);
+					} else
+						fsk_nothing_encode(cnetz);
 				} else {
 					bits = cnetz_encode_telegramm(cnetz);
-					PDEBUG_CHAN(DDSP, DEBUG_DEBUG, "Transmitting 'Meldeblock' at timeslot %d\n", cnetz->sched_ts);
+					if (bits) {
+						PDEBUG_CHAN(DDSP, DEBUG_DEBUG, "Transmitting 'Meldeblock' at timeslot %d\n", cnetz->sched_ts);
+						fsk_block_encode(cnetz, bits, 1);
+					} else
+						fsk_nothing_encode(cnetz);
 				}
-				fsk_block_encode(cnetz, bits, 1);
 			} else {
 				fsk_nothing_encode(cnetz);
 			}
 			break;
 		case DSP_MODE_SPK_K:
 			bits = cnetz_encode_telegramm(cnetz);
-			fsk_block_encode(cnetz, bits, 0);
-			PDEBUG_CHAN(DDSP, DEBUG_DEBUG, "Transmitting 'Konzentrierte Signalisierung' at timeslot %d.%d\n", cnetz->sched_ts, cnetz->sched_r_m * 5);
+			if (bits) {
+				PDEBUG_CHAN(DDSP, DEBUG_DEBUG, "Transmitting 'Konzentrierte Signalisierung' at timeslot %d.%d\n", cnetz->sched_ts, cnetz->sched_r_m * 5);
+				fsk_block_encode(cnetz, bits, 0);
+			} else
+				fsk_nothing_encode(cnetz);
 			break;
 		case DSP_MODE_SPK_V:
 			bits = cnetz_encode_telegramm(cnetz);
-			fsk_distributed_encode(cnetz, bits);
-			PDEBUG_CHAN(DDSP, DEBUG_DEBUG, "Transmitting 'Verteilte Signalisierung' starting at timeslot %d\n", cnetz->sched_ts);
+			if (bits) {
+				PDEBUG_CHAN(DDSP, DEBUG_DEBUG, "Transmitting 'Verteilte Signalisierung' starting at timeslot %d\n", cnetz->sched_ts);
+				fsk_distributed_encode(cnetz, bits);
+			} else
+				fsk_nothing_encode(cnetz);
 			break;
 		case DSP_MODE_OFF:
 		default:
@@ -879,10 +892,12 @@ void cnetz_set_dsp_mode(cnetz_t *cnetz, enum dsp_mode mode)
 
 void cnetz_set_sched_dsp_mode(cnetz_t *cnetz, enum dsp_mode mode, int timeslot)
 {
-	if (mode != cnetz->dsp_mode) {
-		PDEBUG_CHAN(DDSP, DEBUG_INFO, "Schedule DSP mode %s -> %s at timeslot %d\n", cnetz_dsp_mode_name(cnetz->dsp_mode), cnetz_dsp_mode_name(mode), timeslot);
-		cnetz->sched_dsp_mode = mode;
-		cnetz->sched_dsp_mode_ts = timeslot;
+	if (cnetz->sched_dsp_mode_ts < 0 && mode == cnetz->dsp_mode) {
+		PDEBUG_CHAN(DDSP, DEBUG_INFO, "Schedule DSP mode %s not required, we are already in that mode\n", cnetz_dsp_mode_name(mode));
+		return;
 	}
+	PDEBUG_CHAN(DDSP, DEBUG_INFO, "Schedule DSP mode %s -> %s at timeslot %d\n", cnetz_dsp_mode_name(cnetz->dsp_mode), cnetz_dsp_mode_name(mode), timeslot);
+	cnetz->sched_dsp_mode = mode;
+	cnetz->sched_dsp_mode_ts = timeslot;
 }
 
diff --git a/src/cnetz/telegramm.c b/src/cnetz/telegramm.c
index e6f8a5d..f74fffd 100644
--- a/src/cnetz/telegramm.c
+++ b/src/cnetz/telegramm.c
@@ -1597,6 +1597,9 @@ const char *cnetz_encode_telegramm(cnetz_t *cnetz)
 		;
 	}
 
+	if (!telegramm)
+		return NULL;
+
 	opcode = telegramm->opcode;
 	bits = assemble_telegramm(telegramm, (opcode != OPCODE_LR_R) && (opcode != OPCODE_MLR_M));
 	bits = encode(bits);