Multiple BTS's on one one E1?

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Gus Bourg gus at bourg.net
Tue Aug 23 17:56:51 UTC 2011


Hello all,

I'm attempting to connect multiple BTS's to one E1 card.  Each BTS is a
single TRX and I'm using a dahdi card plugged into a DCS.  The DCS maps the
channels as follows:

Name                Src                   Dest                  Speed
Type
--------------------------------------------------------------------------------
bscnok1             01.T1E1-A.02.01       01.T1E1-A.03.01       0256k D
FDX
bscnok2             01.T1E1-A.02.05       01.T1E1-A.04.01       0256k D
FDX
bscnok3             01.T1E1-A.01.09       01.T1E1-B.01.01       0256k D
FDX

So essentially:

TS 1,2,3,4 from OpenBSC (T1E1-A-02) are mapped to nok1 1,2,3,4 (T1E1-A-03)
TS 5,6,7,8 from OpenBSC (T1E1-A-02) are mapped to nok2 1,2,3,4 (T1E1-A-04)
TS 9,10,11,12 from OpenBSC (T1E1-A-02) are mapped to nok3 1,2,3,4
(T1E1-B-01)

This all works fine.  The BTS operates on TS1,2,3,4 on any of the ports I
have configured.  I just swap timeslots in openbsc.cfg from 1,2,3,4 to
5,6,7,8 or 9,10,11,12 and everything is happy.

Where I run into a problem is when I try to actually run multiple BTS's at
once.  So I take everything defined in "bts 0" and duplicate it as "bts 1".
I change the time slots for oml, rsl, and traffic channels to match the
appropriate channels for bts 1/2.  But when I go to start osmo-nitb I get an
error that it can't open /dev/dahdi/1 because it's already in use.  It
appears for some reason that it's attempting to open /dev/dahdi/1 for both
BTS's even though only bts 0 uses /dev/dahdi/1 for OML - bts 1 should use
/dev/dahdi/5 and bts 2 should use /dev/dahdi/9.

My /etc/dahdi/system.conf is defined as:
span=1,1,0,ccs,hdb3,crc4
bchan=2-4,6-8,10-12
dchan=1,5,9


I'm at a loss.  Has anyone tried to run multiple BTS's with dahdi on a
single E1 or T1 span?  Is there something special I need to specify so that
OpenBSC doesn't grab the first dchan for each BTS?

Thanks,
Gus
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