diff --git a/src/encoding.cpp b/src/encoding.cpp
index 881dc61..5d2434b 100644
--- a/src/encoding.cpp
+++ b/src/encoding.cpp
@@ -538,7 +538,7 @@
 void Encoding::write_packet_uplink_assignment(
 	bitvec * dest, uint8_t old_tfi,
 	uint8_t old_downlink, uint32_t tlli, uint8_t use_tlli,
-	struct gprs_rlcmac_ul_tbf *tbf, uint8_t poll, uint8_t rrbp, uint8_t alpha,
+	const struct gprs_rlcmac_ul_tbf *tbf, uint8_t poll, uint8_t rrbp, uint8_t alpha,
 	uint8_t gamma, int8_t ta_idx, int8_t use_egprs)
 {
 	// TODO We should use our implementation of encode RLC/MAC Control messages.
@@ -629,7 +629,7 @@
 /* generate downlink assignment */
 void Encoding::write_packet_downlink_assignment(RlcMacDownlink_t * block,
 	bool old_tfi_is_valid, uint8_t old_tfi, uint8_t old_downlink,
-	struct gprs_rlcmac_dl_tbf *tbf, uint8_t poll, uint8_t rrbp,
+	const struct gprs_rlcmac_dl_tbf *tbf, uint8_t poll, uint8_t rrbp,
 	uint8_t alpha, uint8_t gamma, int8_t ta_idx,
 	uint8_t ta_ts, bool use_egprs)
 {
diff --git a/src/encoding.h b/src/encoding.h
index 2f6b273..525f8e6 100644
--- a/src/encoding.h
+++ b/src/encoding.h
@@ -59,13 +59,13 @@
 	static void write_packet_uplink_assignment(
 			bitvec * dest, uint8_t old_tfi,
 			uint8_t old_downlink, uint32_t tlli, uint8_t use_tlli,
-			struct gprs_rlcmac_ul_tbf *tbf, uint8_t poll, uint8_t rrbp,
+			const struct gprs_rlcmac_ul_tbf *tbf, uint8_t poll, uint8_t rrbp,
 			uint8_t alpha, uint8_t gamma, int8_t ta_idx,
 			int8_t use_egprs);
 
 	static void write_packet_downlink_assignment(RlcMacDownlink_t * block,
 			bool old_tfi_is_valid, uint8_t old_tfi, uint8_t old_downlink,
-			struct gprs_rlcmac_dl_tbf *tbf, uint8_t poll, uint8_t rrbp,
+			const struct gprs_rlcmac_dl_tbf *tbf, uint8_t poll, uint8_t rrbp,
 			uint8_t alpha, uint8_t gamma,
 			int8_t ta_idx, uint8_t ta_ts, bool use_egprs);