tbf: constify the IMSI argument for the downlink assignment
diff --git a/src/pcu_l1_if.cpp b/src/pcu_l1_if.cpp
index 43bd36e..218dc23 100644
--- a/src/pcu_l1_if.cpp
+++ b/src/pcu_l1_if.cpp
@@ -174,7 +174,7 @@
 	pcu_tx_data_req(0, 0, PCU_IF_SAPI_AGCH, 0, 0, 0, data, 23);
 }
 
-void pcu_l1if_tx_pch(bitvec * block, int plen, char *imsi)
+void pcu_l1if_tx_pch(bitvec * block, int plen, const char *imsi)
 {
 	uint8_t data[23+3]; /* prefix PLEN */