Hi Neels,
On Thu, Mar 17, 2016 at 02:05:32AM +0100, Neels Hofmeyr wrote:
about our difficulty in maintaining a prolonged link with the UE, I found a bit of discrepancy concerning the TMSIs used.
- UE sends TMSI in "(GMM) Attach Request" (PS)
- SGSN sends TMSI in "(GMM) Attach Accept" (PS)
PS uses P-TMSI
- CSCN sends TMSI in "Location Update Accept" (CS)
CS uses TMSI
P-TMSI and TMSI are in completely different namespaces.
All these TMSIs differ.
1+2 should be the same. '3' should be different.
For 3), according to the RRC log, it seems that the UE expects to see the very same TMSI in CS that it sent to PS -- that'd be a problem. On the other hand, when noting that, in the Location Updating Request, the UE identified itself using the IMSI and not a TMSI, it may be indeed the correct choice to disable TMSI use for CS?
It might be the case that our network does something that implies we support a 'combined CS+PS attach', which we don't.
In an (unsupported) combind CS+PS attach, the SGSN would send an ATTACH ACCEPT with IEs for both TMSI and P-TMSI.
If you're looking for more information on TMSI/IMSI/P-TMSI, 04.08 is the wrong place. Check 23.003, it contains thins like
"In areas where both MSC-based services and SGSN-based services are provided, some discrimination is needed between the allocation of TMSIs for MSC-based services and the allocation of TMSIs for SGSN-based services. The discrimination shall be done on the 2 most significant bits, with values 00, 01, and 10 being used by the VLR, and 11 being used by the SGSN."
Regards, Harald