While experimenting with CSFB, it was observed that when the UE returns
to E-UTRAN after a CS call, the UE performs a Tracking Area Update
with a combined Tracking Area/Location Area update and IMSI attach.
Currently, Open5GS's MME simply responds with a TAU Accept message
but does not inform the MSC/VLR.
As a result, no further MT (Mobile Terminated) CS/SMS services are possible
in cases where the MSC/VLR only attempts paging on GERAN.
However, some MSC/VLR implementations with fast fallback may still attempt
paging on E-UTRAN, allowing MT CS/SMS services to function intermittently.
According to 3GPP TS 29.118 Section 5.2.2 Procedures in the MME,
specifically Section 5.2.2.2.1, if the timer Ts6-1 is not running,
the MME shall start the location update for non-EPS services procedure
upon receiving a combined Tracking Area Update Request indicating
combined TA/LA updating with IMSI attach. However, SGs timers are not
implemented in Open5GS, which is a separate issue.
To comply with the specification and ensure that the MSC/VLR is informed
when the UE becomes reachable via SGs, the following changes have been
implemented:
1. Delay UEContextReleaseCommand:
When the active_flag is set to 0, the UEContextReleaseCommand is now delayed
until the MME receives the TAU Complete message from the UE. This ensures
that the UE has acknowledged the new P-TMSI before the network releases
the context, maintaining proper synchronization between the UE and the network.
2. Include Mobile Identity Only When P-TMSI Changes:
The Mobile Identity is now included in the Attach/TAU Accept messages
only when the MSC/VLR updates the P-TMSI. This ensures that the UE receives
the Mobile Identity information solely when there is an actual change
in the P-TMSI, preventing unnecessary or incorrect handling
of TAU Complete messages.
3. Send SGsAP-REALLOCATION-COMPLETE Conditionally:
The SGsAP-REALLOCATION-COMPLETE message is now sent to the MSC/VLR
only upon receiving a Attach/TAU Complete message from the UE.
This confirmation indicates that the UE has successfully updated its P-TMSI,
ensuring that the MSC/VLR is accurately informed of the change.
4. Handle P-TMSI Confirmation:
When the MSC/VLR updates the P-TMSI, Open5GS stores the new P-TMSI
in the next field of the mme_ue structure. Upon receiving the TAU Complete
message from the UE, indicating acknowledgment of the new P-TMSI,
Open5GS confirms the update by transferring the P-TMSI from the next field
to the current field. This ensures that the MME maintains an accurate and
up-to-date record of the P-TMSI as confirmed by the UE.
commit 58d790f4916319b274c2c8a82b6226dc3f79258e
Author: Sukchan Lee <acetcom@gmail.com>
Date: Thu Aug 13 22:05:38 2020 +0000
udpate it
commit 5c06b8fa9114dc7391208d523cf850a3ca3f78d5
Author: Sukchan Lee <acetcom@gmail.com>
Date: Thu Aug 13 16:01:02 2020 +0000
update it
commit 19909f9d9e20f429bd06300e9ba2cfaef855c9b2
Author: Sukchan Lee <acetcom@gmail.com>
Date: Thu Aug 13 15:52:13 2020 +0000
update it
commit b7f2bb3681aa512dcb621e4662c90d7277863707
Author: Sukchan Lee <acetcom@gmail.com>
Date: Thu Aug 13 15:48:18 2020 +0000
update it
commit 53f3004467a81a0702d9c1ee3e29f4d4d29ff22c
Author: Sukchan Lee <acetcom@gmail.com>
Date: Thu Aug 13 15:32:30 2020 +0000
update it
commit e295ec4537c443cd6253592a9e31e151fc0bb827
Author: Sukchan Lee <acetcom@gmail.com>
Date: Thu Aug 13 14:50:23 2020 +0000
update it
commit 435edd22ba8118b31855b88d6974d9f757758da2
Author: Sukchan Lee <acetcom@gmail.com>
Date: Thu Aug 13 14:39:43 2020 +0000
update it
commit 49872a99f13476bd73a44150d18c66ad713ef699
Author: Sukchan Lee <acetcom@gmail.com>
Date: Thu Aug 13 14:36:30 2020 +0000
update it
