diff --git a/include/osmocom/coding/gsm0503_coding.h b/include/osmocom/coding/gsm0503_coding.h
index b8eb205..f6591f5 100644
--- a/include/osmocom/coding/gsm0503_coding.h
+++ b/include/osmocom/coding/gsm0503_coding.h
@@ -21,43 +21,43 @@
 	EGPRS_NUM_MCS,
 };
 
-int gsm0503_xcch_encode(ubit_t *bursts, uint8_t *l2_data);
-int gsm0503_xcch_decode(uint8_t *l2_data, sbit_t *bursts,
+int gsm0503_xcch_encode(ubit_t *bursts, const uint8_t *l2_data);
+int gsm0503_xcch_decode(uint8_t *l2_data, const sbit_t *bursts,
 	int *n_errors, int *n_bits_total);
 
-int gsm0503_pdtch_encode(ubit_t *bursts, uint8_t *l2_data, uint8_t l2_len);
-int gsm0503_pdtch_decode(uint8_t *l2_data, sbit_t *bursts, uint8_t *usf_p,
+int gsm0503_pdtch_encode(ubit_t *bursts, const uint8_t *l2_data, uint8_t l2_len);
+int gsm0503_pdtch_decode(uint8_t *l2_data, const sbit_t *bursts, uint8_t *usf_p,
 	int *n_errors, int *n_bits_total);
 
-int gsm0503_pdtch_egprs_encode(ubit_t *bursts, uint8_t *l2_data,
+int gsm0503_pdtch_egprs_encode(ubit_t *bursts, const uint8_t *l2_data,
 	uint8_t l2_len);
-int gsm0503_pdtch_egprs_decode(uint8_t *l2_data, sbit_t *bursts,
+int gsm0503_pdtch_egprs_decode(uint8_t *l2_data, const sbit_t *bursts,
 	uint16_t nbits, uint8_t *usf_p, int *n_errors, int *n_bits_total);
 
-int gsm0503_tch_fr_encode(ubit_t *bursts, uint8_t *tch_data, int len,
+int gsm0503_tch_fr_encode(ubit_t *bursts, const uint8_t *tch_data, int len,
 	int net_order);
-int gsm0503_tch_fr_decode(uint8_t *tch_data, sbit_t *bursts, int net_order,
+int gsm0503_tch_fr_decode(uint8_t *tch_data, const sbit_t *bursts, int net_order,
 	int efr, int *n_errors, int *n_bits_total);
 
-int gsm0503_tch_hr_encode(ubit_t *bursts, uint8_t *tch_data, int len);
-int gsm0503_tch_hr_decode(uint8_t *tch_data, sbit_t *bursts, int odd,
+int gsm0503_tch_hr_encode(ubit_t *bursts, const uint8_t *tch_data, int len);
+int gsm0503_tch_hr_decode(uint8_t *tch_data, const sbit_t *bursts, int odd,
 	int *n_errors, int *n_bits_total);
 
-int gsm0503_tch_afs_encode(ubit_t *bursts, uint8_t *tch_data, int len,
+int gsm0503_tch_afs_encode(ubit_t *bursts, const uint8_t *tch_data, int len,
 	int codec_mode_req, uint8_t *codec, int codecs, uint8_t ft,
 	uint8_t cmr);
-int gsm0503_tch_afs_decode(uint8_t *tch_data, sbit_t *bursts,
+int gsm0503_tch_afs_decode(uint8_t *tch_data, const sbit_t *bursts,
 	int codec_mode_req, uint8_t *codec, int codecs, uint8_t *ft,
 	uint8_t *cmr, int *n_errors, int *n_bits_total);
 
-int gsm0503_tch_ahs_encode(ubit_t *bursts, uint8_t *tch_data, int len,
+int gsm0503_tch_ahs_encode(ubit_t *bursts, const uint8_t *tch_data, int len,
 	int codec_mode_req, uint8_t *codec, int codecs, uint8_t ft, uint8_t cmr);
-int gsm0503_tch_ahs_decode(uint8_t *tch_data, sbit_t *bursts, int odd,
+int gsm0503_tch_ahs_decode(uint8_t *tch_data, const sbit_t *bursts, int odd,
 	int codec_mode_req, uint8_t *codec, int codecs, uint8_t *ft,
 	uint8_t *cmr, int *n_errors, int *n_bits_total);
 
-int gsm0503_rach_encode(ubit_t *burst, uint8_t *ra, uint8_t bsic);
-int gsm0503_rach_decode(uint8_t *ra, sbit_t *burst, uint8_t bsic);
+int gsm0503_rach_encode(ubit_t *burst, const uint8_t *ra, uint8_t bsic);
+int gsm0503_rach_decode(uint8_t *ra, const sbit_t *burst, uint8_t bsic);
 
-int gsm0503_sch_encode(ubit_t *burst, uint8_t *sb_info);
-int gsm0503_sch_decode(uint8_t *sb_info, sbit_t *burst);
+int gsm0503_sch_encode(ubit_t *burst, const uint8_t *sb_info);
+int gsm0503_sch_decode(uint8_t *sb_info, const sbit_t *burst);