osmo_e1f: Allow to transmit AIS

AIS is an all-1 bit-pattern which is used by intermediate equipment
to indicate that the link somewhere upstream is lost.

Change-Id: Icc5a16ee44ecebe802c4cca0b6cc763a88b6cc05
diff --git a/software/obsolete/osmo_e1f.c b/software/obsolete/osmo_e1f.c
index 2c3fcfb..fa64e17 100644
--- a/software/obsolete/osmo_e1f.c
+++ b/software/obsolete/osmo_e1f.c
@@ -292,6 +292,11 @@
 {
 	int i;
 
+	if (e1i->tx.ais) {
+		memset(out_frame, 0xff, 32);
+		return 0;
+	}
+
 	/* generate TS0 */
 	out_frame[0] = e1_pull_ts0(e1i);
 
diff --git a/software/obsolete/osmo_e1f.h b/software/obsolete/osmo_e1f.h
index c9d4778..0b423d9 100644
--- a/software/obsolete/osmo_e1f.h
+++ b/software/obsolete/osmo_e1f.h
@@ -10,6 +10,7 @@
 struct osmo_e1f_tx_state {
 	bool remote_alarm;
 	bool crc4_error;
+	bool ais;
 	/* lower 5 bits: Sa4..Sa8 */
 	uint8_t sa4_sa8;
 	/* frame number 0..15 */