Hi!
I've been experimenting for the last couple of hours with two BS-11 in a multi-drop configuration.
The connection looks like this:
HFC-E1 connects to BTS A Y01/Y02, just like normal. BTS-A Y03/Y04 connect to BTS-B Y01/Y02.
BTS-A LI object is configured as multi-drop BTS-B LI object is configured normal (star)
BTA-A is configured to use TEI 25 on TS 1 for OML (normal) BTS-B is configured to use TEI 26 on TS 10 for OML (differnt)
If I put the MA-10 A-bis signal analyzer between BTS A and BTS, I can see * that the E1 physical layer is up and there are no alarms * BTS-B sends TEI requests on timeslot 10, as requested
however, there is no response. Looking at TS 10 between BTS-A and BSC, I can see that there are no TEI requests or any other data.
GSM 12.21 specifies a "CONNECT MULTI-DROP LINK" message, using which you can map two E1 timeslots on two interfaces to each other. I've done this (mapping port 0, TS 10 to port 1, TS 10) - but it seems the BS-11 does not support this message. The MA-10 also calls it an "unknown message".
I'm Sending this message to object class 0xa5 (SiemensHW) and to the LI object in that class (object instance 7, 0, 0). The BS-11 does not respond with ACK and not with NACK. It seems it is simply ignored silently.
Does anyone have an idea how to proceed? Or does anyone have an A-bis trace from a working multi-drop configuration?
Thanks.
Hi all,
On Tue, Aug 04, 2009 at 12:53:38AM +0200, Harald Welte wrote:
If I put the MA-10 A-bis signal analyzer between BTS A and BTS, I can see
- that the E1 physical layer is up and there are no alarms
- BTS-B sends TEI requests on timeslot 10, as requested
however, there is no response. Looking at TS 10 between BTS-A and BSC, I can see that there are no TEI requests or any other data.
The problem is now solved. It seems that we need to create the BPORT1 object on BTS-A, and configure both BPORT0 and BPORT1 into multi-drop mode. Thanks to Dieter for his help with that.
I can now see the TEI requests from BTS-B on the E1 link between BSC and BTS-A.
Karsten Keil of mISDN has offeres his help to provide a quick solution in the hfcmulti driver where we would have two virtual mISDN cards on one physical HFC-E1 device. OpenBSC's mISDN input module then needs to be instantiated twice, it simply looks exactly like the two BTS are connected to two different E1 cards.
This is actually quite nice, since then the OpenBSC configuration does not change at all, whether you have two BTS in star topology, using two HFC-E1 boards - or two BTS in multi-drop configuration using one HFC-E1 board.
GSM 12.21 specifies a "CONNECT MULTI-DROP LINK" message, using which you can
This is not required at all. It seems the BTS just passes all E1 frames from BPORT1 to BPORT0, unless that very timeslot is used by the BTS itself.
So if BTS-A is the multi-drop BTS and it uses Timeslot 1 for signalling, all data from BPORT1 (BTS-B) on Timeslot 1 is discarded.
Regards,
-
Harald Welte