Monday, 21 September 2015

UE can not camp on LTE network after reselection from UMTS TO FDD LTE problem analysis

 Incident Description (Incident Phenomena)
When test UMTS and LTE inter-RAT reselection and Handover feature, UE can not camp on LTE network after reselection from UMTS to LTE, and immediately rollback to UMTS network, according to the strategy UE should be stay at LTE network instead of UMTS network because LTE absolute priority is 7 high and UMTS network absolute priority is 1 low,this phenomenence is abnormal.
 Networking Environment
UMTS network have been comissioning and LTE network is still in feature test. And LTE network can not support fast return feature because the SGs interface between MME and MSC server is not working.
 Problem Cause Analysis
Use QXDM testing and analysis the logfile and it was found the message "MSC temporarily not reachable in Attach accept Message.It indicate that MSC can not reach from MME, it is ture that in current CN  the MSC didn't configure the SGs interface between MME and MSC server.
       So continue to analyse what is the root cause for UE cann't camp on the LTE network. According to 3gpp ts 24.301 there are 4 operation mode:
A UE attached for EPS services shall operate in one of the following operation modes:
PS mode 1 of operation: the UE registers only to EPS services, and UE's usage setting is "voice centric";
PS mode 2 of operation: the UE registers only to EPS services, and UE's usage setting is "data centric";
CS/PS mode 1 of operation: the UE registers to both EPS and non-EPS services, and UE's usage setting is "voice centric"; and
CS/PS mode 2 of operation: the UE registers to both EPS and non-EPS services, and UE's usage setting is "data centric".
And also according to the 3gpp there is one TAU type combined TA/LA updateing with IMSI attach, which requrie the IMSI register in both CS domain and PS domain.
 


And in the UE log file we find that the TAU type is  combined TA/LA updateing with IMSI attach, but the MSC and MME didn't configure SGs interface, so the TAU fail.
 Solution
It is clear that the reason is Ue trigger the combined TA/LA updateing with IMSI attach, but untill now the MSC and MME didn't configure SGs interface, so the TAU fail.Then caaording the 3gpp when TAU fail, the UE shall stop timer T3430 if still running. The tracking area updating attempt counter shall be incremented, unless it was already set to 5, If the tracking area updating attempt counter is less than 5.the UE shall start timer T3411, shall set the EPS update status to EU1 UPDATED and shall enter state EMM-REGISTERED.ATTEMPTING-TO-UPDATE-MM. When timer T3411 expires the combined tracking area updating procedure indicating "combined TA/LA updating with IMSI attach" is triggered again.
 If the tracking area updating attempt counter is equal to 5:
- a UE operating in CS/PS mode 2 of operation shall start timer T3402, shall set the EPS update status to EU1 UPDATED and shall enter state EMM-REGISTERED.ATTEMPTING-TO-UPDATE-MM. When timer T3402 expires the combined tracking area updating procedure indicating "combined TA/LA updating with IMSI attach" is triggered again;
- a UE operating in CS/PS mode 1 of operation with "IMS voice not available" shall attempt to select GERAN or UTRAN radio access technology and proceed with appropriate MM or GMM specific procedures. The UE shall disable the E-UTRA capability (see subclause 4.5). If the UE is in the EMM-CONNECTED mode, the UE shall locally release the established NAS signalling connection and enter the EMM-IDLE mode before selecting GERAN or UTRAN radio access technology.
 Summary and Notes
When perform the 3G and 4G operation it it necessary to configure the interface between CS CN and EPS, If we don't configure the SGs interface between MSC and MME, the UE can't stay in LTE network in CS/PS mode, but UE can camp on LTE when setting only LTE mode.

Budi Prasetyo

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