diff --git a/include/osmocom/msc/msc_ifaces.h b/include/osmocom/msc/msc_ifaces.h
new file mode 100644
index 0000000..d46dfe9
--- /dev/null
+++ b/include/osmocom/msc/msc_ifaces.h
@@ -0,0 +1,42 @@
+#pragma once
+
+#include <osmocom/core/msgb.h>
+#include <osmocom/msc/gsm_data.h>
+
+/* These are the interfaces of the MSC layer towards (from?) the BSC and RNC,
+ * i.e. in the direction towards the mobile device (MS aka UE).
+ *
+ * 2G will use the A-interface,
+ * 3G aka UMTS will use the Iu-interface (for the MSC, it's IuCS).
+ *
+ * To allow linking parts of the MSC code without having to include entire
+ * infrastructures of external libraries, the core transmitting and receiving
+ * functions are left unimplemented. For example, a unit test does not need to
+ * link against external ASN1 libraries if it is never going to encode actual
+ * outgoing messages. It is up to each building scope to implement real world
+ * functions or to plug mere dummy implementations.
+ *
+ * For example, msc_tx_dtap(conn, msg), depending on conn->via_iface, will call
+ * either iu_tx() or a_tx() [note: at time of writing, the A-interface is not
+ * yet implemented]. When you try to link against libmsc, you will find that
+ * the compiler complains about an undefined reference to iu_tx(). If you,
+ * however, link against libiu as well as the osmo-iuh libs (etc.), iu_tx() is
+ * available. A unit test may instead simply implement a dummy iu_tx() function
+ * and not link against osmo-iuh, see tests/libiudummy/.
+ */
+
+/* Each main linkage must implement this function (see comment above). */
+extern int iu_tx(struct msgb *msg, uint8_t sapi);
+
+int msc_tx_dtap(struct gsm_subscriber_connection *conn,
+		struct msgb *msg);
+
+int msc_gsm48_tx_mm_serv_ack(struct gsm_subscriber_connection *conn);
+int msc_gsm48_tx_mm_serv_rej(struct gsm_subscriber_connection *conn,
+			     enum gsm48_reject_value value);
+
+int msc_tx_common_id(struct gsm_subscriber_connection *conn);
+int msc_call_assignment(struct gsm_trans *trans);
+int msc_call_bridge(struct gsm_trans *trans1, struct gsm_trans *trans2);
+void msc_call_release(struct gsm_trans *trans);
+int msc_call_connect(struct gsm_trans *trans, uint16_t port, uint32_t ip);