bsc: Start MGCP transaction ID sequence based on media_nr

This allows different MscConnHandlers to concurrently submit MGCP
messages with unique transaction ID through the shared MGCP-over-IPA
multiplex BSC<->MSC.

Without this, 2 MscConnHandlers would both send a
CRCX using trans_id=0, and the ConnectionTable in RAN_Emulation.cpp
would lookup and return the first MscConnHandler in the table during the
2nd CRCX ACK which was targeted at the second MscConnHandler.

Change-Id: Icb09d67a1f0207dc8501dd2b47c9162175b691b1
---
M bsc/BSC_Tests.ttcn
M bsc/MSC_ConnectionHandler.ttcn
2 files changed, 20 insertions(+), 0 deletions(-)

diff --git a/bsc/BSC_Tests.ttcn b/bsc/BSC_Tests.ttcn
index b4d6e02..426109a 100644
--- a/bsc/BSC_Tests.ttcn
+++ b/bsc/BSC_Tests.ttcn
@@ -12230,6 +12230,7 @@
 	var TestHdlrParams pars2 := f_gen_test_hdlr_pars();
 	var MSC_ConnHdlr vc_conn2;
 	pars2.mgwpool_idx := 1;
+	pars2.media_nr := 2;
 
 	f_init(1, true, nr_mgw := 2);
 	f_sleep(1.0);
diff --git a/bsc/MSC_ConnectionHandler.ttcn b/bsc/MSC_ConnectionHandler.ttcn
index 7a5e55e..b499244 100644
--- a/bsc/MSC_ConnectionHandler.ttcn
+++ b/bsc/MSC_ConnectionHandler.ttcn
@@ -581,6 +581,7 @@
 
 /* initialize all parameters */
 function f_MscConnHdlr_init(integer i, HostName bts, HostName mgw, BSSMAP_FIELD_CodecType codecType) runs on MSC_ConnHdlr {
+	g_trans_id := i * 1000; /* Avoid different MscConnHdlr submitting same trans_id over MGCP-IPA */
 	f_MediaState_init(g_media, i, bts, mgw, codecType);
 	f_MscConnHdlr_init_vty();
 }

