Monday, 14 September 2015

4G LTE to 3G handover fail with cause code ranap-sccp-fail-triggered

Problem Description
According to the 3GPP standard, during handover from 4G network to 3G network, when SGSN receive forward relocation request from MME,
SGSN should send relocation request message to target RNC. However during this process,
notice sending relocation request message to target RNC fail with cause code ranap-sccp-fail-triggered(153).

3GPP standard for handover process:

User trace:

Handling Process
1) In the SGSN tracing, must first make sure that  the RNC reloacation request is sucessfully sent out to the RNC.

2) For this case, we try to capture all the IU interface, notice the RNC relocation request message in not sent out.
This is due to error in SPP process in SGSN that fail to recognize the signalling from 4G MME and
fail to trigger the ranap with RNC in SCTP layer.

3) From here, we can almost identified that this is due to SGSN internal process group problem, and it cannot recognize
relocation request signalling message from MME in the 4G network that used to trigger ranap with RNC and therefore handover fail.

4) This failure symptom most likely due to alike  SGSN did not enable the switch to recognize relocation request.
 Also this SGSN in 3G never tested handover from 4G service before.

5) Check through softparam table, notice softparam byte 128 controls whether the SGSN supports the Relocation Required
and Relocation Request messages and suspect this is the switch that enable SGSN to recognize and
process Relocation Request from 4G MME signalling.

6) Check back the softparam in SGSN, this byte 128 bit 1 is not enable.

7)  After softparameter  byte128 bit1=1 enable in SGSN, problem solve. Handover success.


After set this parameter, handover sucess. Please refer to trace attach and diagram below for futher explanation.

Budi Prasetyo

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