Tuesday, 22 September 2015

LTE TDD to GSM HO preparation failure

Problem Description
Customer was  trying to get the TDD v5 eNodeB to redirect the UE to GSM.  They could see the GSM neighbors in SIB7,
and also could see the measurement and A2 but redirect did not happen. Customer wanted the following sequence:
1.UE connects to LTE cell and starts data call;
2.LTE signal level drops below A2 threshold leading the UE to report Event A2;
3.On receiving event A2, we want the LTE eNB to redirect UE to TDS/GSM neighbor.
All the inter-rat parameter values were checked and they were all ok. Customer was asked to take MME traces.





N/A
Handling Process
Picture 1- Handover preparation

Picture 2 - Handover preparation failure.

From the above pictures we can see that the “Handover_Preparation_Failure” cause is  cause is the “Unknown Target ID”. 
The missing config for the target LAC/RAC ID in MME appears to be the main reason and we told the cusotmer to add related RAC/LAC config to MME.
Customer added the following configuration to their MME:
ADD IPV4DNSH: HSINDEX=12, HOSTNAME="RNC08AE.RNC.EPC.MNC000.MCC310.3GPPNETWORK.ORG",
ADDRSECTION=SECTION1, IPV4ADDR1="172.20.135.99", IPV4ADDR2="172.20.135.100";
ADD IPV4DNSH: HSINDEX=13, HOSTNAME="RNC08AE.MNC000.MCC310.GPRS", ADDRSECTION=SECTION1, IPV4ADDR1="172.20.135.99", IPV4ADDR2="172.20.135.100";
ADD DNSN: FQDN="RNC08AE.MNC000.MCC310.GPRS", HSINDEX=13, ENTITY=SGSN, INTYPE=S4;
The redirect worked fine and also there were no handover preparation failures.
Root Cause
N/A
Solution
I started looking at all the Inter - RAT and handover parameters to see if they were ok. Then we asked the customer to do the traces.
In the traces we could see that in the HO preparation failure we could see 'unknown target ID'.
The configuration of MME was not complete and did not have the target RAC/LAC in the host file name.
ADD IPV4DNSH: HSINDEX=12, HOSTNAME="RNC08AE.RNC.EPC.MNC000.MCC310.3GPPNETWORK.ORG",
ADDRSECTION=SECTION1, IPV4ADDR1="172.20.135.99", IPV4ADDR2="172.20.135.100";

ADD IPV4DNSH: HSINDEX=13, HOSTNAME="RNC08AE.MNC000.MCC310.GPRS", ADDRSECTION=SECTION1,
IPV4ADDR1="172.20.135.99", IPV4ADDR2="172.20.135.100";
ADD DNSN: FQDN="RNC08AE.MNC000.MCC310.GPRS", HSINDEX=13, ENTITY=SGSN, INTYPE=S4;
Suggestions and Summary
MME needs to have the target LAC/RAC in the configuration otherwise it would result in the handover preparation failure message.

Budi Prasetyo

About Budi Prasetyo

All About LTE

Subscribe to this Blog via Email :