Combined Attached
TAC-LAC Mismatch Problems
MTRR/MTRF & ISR
Methods to Decrease Call setup time in CSFB
SGs: It is the reference point between the MME and MSC server. The SGs reference point is used for the mobility management and paging procedures between
EPS and CS domain, and is based on the Gs interface procedures. The SGs reference point is also used for the delivery of both mobile originating and mobile
terminating SMS. Additional procedures for alignment with the Gs reference point are not precluded
S3: It is defined
in TS 23.401 [2] with the additional functionality to support ISR for CS fallback/SMS over SGs as defined in this
specificationfor alignment with the Gs reference point are not precluded
From this procedure, MSC/VLR stores LAI that
Identify certain MME.
3) If the Attach Request message includes an Attach Type indicating that the UE requests a combined EPS/IMSI attach, the MME allocates a LAI for the
UE. The MME derives a VLR number based on the allocated LAI and on an IMSI hash function defined in TS 23.236 [23]. The MME starts the location
update procedure towards the new MSC/VLR upon receipt of the subscriber data from the HSS in step 2). This operation marks the MS as EPS-attached in
the VLR
4) The MME sends a Location Update Request (new LAI, IMSI, MME name, Location Update Type) message to the VLR. MME name is a FQDN string
5) The VLR creates an association with the MME by storing MME name.
6) The VLR performs the normal subscription checks for CS and if all checks are successful performs Location Updating procedure in CS domain
7) The VLR responds with Location Update Accept (VLR TMSI) to the MME.
Combined Attached
TAC-LAC Mismatch Problems
MTRR/MTRF & ISR
Methods to Decrease Call setup time in CSFB
Possible TAC-LAC Scenarios – Two Cases
TAC5
TAC Mismatch
MOD CNOPERATORTA: TrackingAreaId=0, CnOperatorId=0, Tac=5;
LAC Splitting
LAC4
MOD UCELLACINFO: CELLID=1, LAC=4, RAC=H'00;
Different LAC than obtaining in Combined A/LA Attached:
 In MO CSFB : Call will Success but UE will perform LA Update and
This prolong call setup time by timing of LA update/ LA Accept duration
TAC exist in the
network
and have
 In MT CSFB
:
LAC
mapping
 If target
LAC
located on same MSC/VLR serving Area, the call w
Success and UE follow same procedure as mentioned above.
 If target LAC located on border of two different MSCs/VLRs ,
 so if MTRR/MTRF is enabled then UE will request LAC Upd
With Flag “CSMT” so another delay will add and the call success.
 If MTRR/MTRF not enabled , so UE will perform normal LA
On 3G Side the Call initiation will stop as no “CSMT” flag in LA
So MT CSFB Will fail
TAC-LAC Mismatch Problems
Configured @ MME: TAC-LAC Mapping
Multiple LAC per one TAC
Current LAC
2546
7246
Current TAC
37246
37246
Action
change LAC to be 7246
It is OK
Current LAC
1049
5550
Current TAC
51049
51049
Action
It is Ok
Change TAC to be 15550
In the field : TAC-LAC Mapping
LAC Splitting
TAC Mismatch
MOD CNOPERATORTA: TrackingAreaId=0, CnOperatorId=0, Tac=
MOD UCELLACINFO: CELLID=1, LAC=H'2501, RAC=H'00;
3rd
The possible scenario :
 UE will fail in Combined Attached to get LAC value as there is not
TAC-LAC mapping on MME after that failure , UR retry to make TA Update
Request for 4 times with duration T3411 (10 sec)then camp on 3G to
select so UE not camp on LTE from the beginning.
Case
Mail and Mobile SNaps
TAC not exist in the
network and have not
LAC mapping
D CNOPERATORTA: TrackingAreaId=0, CnOperatorId=0, Tac=20;
Combined Attached
TAC-LAC Mismatch Problems
MTRR/MTRF & ISR
Methods to Decrease Call setup time in CSFB
CSFB Setup time decreasing
• Correct TAC-LAC Mapping
• From 3G Side : Use Flash CSFB/ RIM CSFB
• From 3G Side: Use DMCR (Deferred Measurement Control Reading)