USSD works only when IuCS is released
Using the ip.access nano3g, I observe some unreliability when issuing USSD requests
from the UE towards the CN:
- When a registered UE's IuCS is in release, a USSD request succeeds very quickly:
an InitialUE message with a CM Service Request "wakes up" IuCS,
authentication and ciphering is established, then the USSD goes through.
- When the IuCS is not released yet but still open, e.g. a USSD is issued
right after a call or another USSD request, no message at all is received
from the nano3G. The phone shows a USSD processing dialog that "hangs".
I expect a Supplementary Services message to show on the Iuh to be
forwarded to IuCS, but the nano3G stays quiet. The USSD times out.
Waiting for another IuCS release and sending the same USSD again is then
I assume this is a problem with the nano3G specifically.
Check how the SysmoCell behaves in this scenario.
#1 Updated by neels about 4 years ago
- Priority changed from Normal to Low
Note that since reporting of this, OsmoCSCN changed its behavior to release IuCS
more aggressively, i.e. right away when nothing is pending for a subscriber.
So in effect, this issue should "never" occur anymore.
See commit "IuCS: rapidly release connections" on openbsc branch sysmocom/iu
(likely to be rebased, so omitting a git hash here).
For curiosity's sake, I would still take a look at how sysmocell 5000 handles this
and then just close the issue, we're not going to fix the nano3G firmware anyway.
#2 Updated by neels about 4 years ago
- Status changed from New to Rejected
Tested with the sysmocell 5k: the same problem exists (when not aggressively releasing Iu).
The first USSD request establishes IuCS with auth+ciph, and a second USSD in close succession
just idles around and then fails. This way I can even reach a state where no Iu Release seems to
happen at all, and CM Service Requests don't work either.
So, to aggressively release the Iu connections from the CN is verified to be a good idea.
It is conceivable that we can mess things up by requesting USSD while the authentication
is still in progress or somesuch. That would be a separate issue though.