Sunday, 4 October 2015

4G handset with 3G SIM card try to enter LTE Network

Problem Description
DST Employee’s 4G handset with 3G SIM card try to enter DST LTE network. Those handset show no services with no signal bar and unable to reselect to 3G network.

Handling Process
For 4G handset, the UE will try to register into 4G as priority. When a UE with the IMSI range which are not define in MME enter the MME,
MME will send the authentication request to HSS. The HSS will send back the authentication response and reject the UE. 
Current setting in the network, UE with IMSI prefix 5281118 will send to HSS and other than that will be rejected from the MME with reject code Network Failure (17) 
For DST existing 3G user with MCC MNC 52811, MME will automatic generate a realm name according to 3GPP standard which is epc.mnc011. for DST network. MME will then match this realm name with its configuration and selected the DEA as gateway to HSS.
Since this user is not in the HSS database, and the HSS realm name is different from the IMSI generated realm name,
therefore the HSS reject with Diameter Link Unable to Deliver (3002). MME will send reject message Unspecific Protocol Error (111) to the UE.

Root Cause
To solve above both problems, non DST LTE user need to be restricted from entering the LTE network. Equipment Vendor provided a solution to allow only DST LTE user to enter the DST LTE Network.
Non DST LTE user will be rejected by the MME directly with reject code No Suitable Cells in TA (15).
This is in line with DST requirement that non DST LTE user should not be send to HSS for verification. With this reject code, the UE that had been rejected by MME, won’t reentry into this Tracking Area anymore.

Suggestions and Summary
This feature require Roaming Restriction license. Currently both MME for DST does not have this license.

Budi Prasetyo

About Budi Prasetyo

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