Location Area Updates missed during calls

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Ralph A. Schmid, dk5ras ralph at schmid.xxx
Thu Mar 20 14:20:42 UTC 2014


Not exactly your issue, and mainly a testbed situation, nothing for the real
world, but at least with OpenBTS I ran into similar issues when I stopped
for some reason the BTS and restarted it immediately without keeping the
TMSI assignment list. In this case the mobiles were not yet on extensive
network search, so no registration attempts on other networks, registration
flag still valid in the ME, no expired T3212. This leads to strange effects
with GPRS and with incoming calls and short messages. I can imagine it may
be similar with the openbsc family of BTSs. But as I have said, more an
academic issue in lab tests, a real BTS runs all the time and is not
restarted every few minutes :)

 

Under what circumstances does the network initiate a LU procedure? Up to now
I was not aware of this possibility. 


Ralph.

 

 

From: openbsc-bounces at lists.osmocom.org
[mailto:openbsc-bounces at lists.osmocom.org] On Behalf Of Luca Valtulina
Sent: Thursday, March 20, 2014 2:40 PM
To: openbsc at lists.osmocom.org
Subject: Location Area Updates missed during calls

 

Hi list,

 

when a MS is in a call it misses the network-initiated Location Area Update
procedure(s). This inconvenience can lead (if the call ends after the
expire_lu time) to having an attached (and active) subscriber marked as
expired in the HLR. Furthermore if at this point the MS initiates a new
call, the call will be dropped and (as expected) a new Location Area Update
procedure will be initiated by the BSC.

 

I find this behavior annoying mostly in scenarios where the T3212 (LAU
timer) has been setup to a low value.

 

To tackle this issue I thought of triggering a subscriber update when a call
get released (either by the network or by the MS) for each of the MS
involved in the call.

The code can also be optimized by checking whether the subscriber is expired
before performing the update.

 

I can provide a patch if you also consider the above as a misbehavior of
OpenBSC.

 

cheers

luca

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