Hi all,
Doing some testing recently with osmo-sip-connector, I noticed a few
things that seem to be important regarding the user experience of a
network with osmo-sip-connector.
* The osmo-sip-connector is an external MNCC handler interface for one
MSC. The MSC doesn't have any information about what is in the HLR.
When the osmo-sip-connector sends a call towards the MSC, the msc does a
VLR lookup: vlr_subscr_find_by_msisdn() in gsm_04_08_cc.c around line 1860.
if that returns false we are doing mncc_release_ind() with
GSM48_CC_CAUSE_UNASSIGNED_NR. That is getting translated into a SIP
404, which informs the caller that the number was not found. Logical but
it's not correct if the subscriber does actually exist in the HLR.
Actually the same problem exists when using the internal MNCC, If you
call a subscriber that has just switched off their phone, you'll get an
Un Assigned Number error cause. That would be incredibly confusing for
the users. In rhizomatica's case, the audio played back to the caller
would be "The number you have dialed does not exist, please verify it",
while it should obviously be "The number you have dialed is temporarily
unavailable, please try again later."
What we have done there is send the caller to go physically ask ask the
callee why they canceled their phone service, when all they did was
switch off the phone.
SO:
There was some discussion about this at the devcon, I want to outline
some points here, hopefully I understand it right.
* The osmo-sip-connector should ask the HLR if the number exists before
forwarding the call to MNCC.
* There may be more than one MSC, in which case the HLR would inform
osmo-sip-connector about which MSC to use.
* The osmo-sip-connector could either be SIP associated, or MSC
associated, that is to say:
a) one osmo-sip-connector per SIP UA, sip-connector talks to HLR, and
has multiple connections to MSCs (UDP then, not Unix Socket?)
or
b) one osmo-sip-connector per MSC, in which case the SIP UA would have
to somehow query the HLR to ask where to send the call.
Does that all sound correct?
and then.. still a doubt:
IIUC, in the traditional core network. When an MSC receives a MO SETUP
from a local MS, and it does not have this number in the VLR, it should
be querying HLR for routing info, (which may subsequently query another
HLR) no? why are we only doing VLR lookup in the MSC? Is it simply "To
be implemented" ?
thanks!
k/
