INTERNAL

RAU Reject When LTE UE Moving to 3G networkYin Xing 163982KeywordsRAU,MME, SGSN, TAI, NAS, Context Request, P-TMSI, Delay, CSFB.SymptomRouting area update failed every time when UE move from LTE netwok to 3G network. From UU interface monitoring, we found all the RAU failures were because UE received a RAU reject message from 3G network, further troubleshooting are required.

Cause AnalysisTo locate the problem, we checked Routing Area Update Reject meassage and found that the cause was ms-identity-cannot-be derived-by-the-network (9) as following:

As we know, after RAU request, The new SGSN uses the old RAI received from the UE to derive the old MME address, and sends a Context Request (PTMSI, old RAI, New SGSN Address, PTMSI Signature) message to the old MME to get the context for the UE. To validate the Context Request the old MME uses a NAS token mapped from the PTMSI Signature. So the only three reasons for ms-identity-cannot-be derived-by-the-network are:a) SGSN cant reach the old MMEb) Old RAI is not correctc) P-TMSI signature validate failure.From the RAU request message we can see the old RAI is correct:

So, SGSN cant reach the old MME and P-TMSI signature validate failure should be the cause.To pinpiont the problem, we contact the provider of MME and 3G SGSN to confirm if the routes between 3G SGSN and MME were defined correctly or not and the connection between 3G SGSN and MME is good or not. The feedback shows that MME and 3G SGSN are the same physical entity, so there should not be connection problem, but cooperating ware not defined in 3G side, which means all MME were not defined in all 3G SGSN. Due to this cooperating configuration missing, SGSN couldn't route the Context Request message to MME, and eventually cause the rejection because of SGSN couldnt get the UE information.Handling ProcessConfigure the following MME Group ID(LAC:0X8000) and MME code(RAC:0X1) in all 3G SGSN nodes:

After reconfiguration, problem solved.Suggestions and SummaryThis RAU rejection will not cause any service failure, but it will extend the delay to some of the services such as CSFB. Since every time after RAU rejection, UE have to initiate an attach procedure to re attach to the destination network, and this will introduce around 0.2 seconds latency to CSFB service according to field test, which is also not acceptable for this delay sensitive kind of services.

2015-04-01Huawei Proprietary - Restricted DistributionPage2, Total4