Thanks: 
Re: umts to lte fast reselection
We are using Huawei LMT for traces. We noticed that inside the Initial_UE_Message for LAU there are some fields which are not decoded which refer to CSFB MO/MT. Nevertheless, this message is transparent for RNC so it is not this one.Originally Posted by lq182
Regarding the other IEs mentioned above it seems they are indeed not sent by the network (upgrade is necessary).
In the end we solve it together with Huawei suport. They are using a smart trick to recognize the CSFB calls.
For us it seemed reasonable. We also re-tested and it seems that the phone does not send the pre-redirection IE if it is forced in UMTS. If it is not forced in UMTS, it actually sends the pre-redirection IE.
"Pre-redirection" is a optional IE, it is firstly imported in 3GPP R8.6.0,3GPP R9.4.0 protocol describes it as follows:
-The way that recognize the CSFB users through this "Pre-redirection" IE should satisfy three conditions :
- The UE supports LTE
- UE sends RRC CONNECTION REQUEST with V860 Structure, but without "pre-redirection" IE.（we can think the user is redirected from LTE to UMTS ,if without "pre-redirection" IE ）
- UE can build CS service in 10s after building RRC connection successfully
Because the Pre-redirection is a optional IE, so some CSFB UEs don't support this way to be recognized, which results in CSFB identify failure. UL fast return is ineffective to the UE.
In future releases (and after network upgrade) there are other IE which could easily recognize the CSFB calls. Please see other posts for details.
I hope this helps everybody.
ninjafine, thanks for the explanation, I'll look into IWF details these days. We actually found the problem, an incorrect mapping between LAC and TA in MME.
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