CSFB MO gets through but MT not when redirection carrier is not present

[erraviesh]

Scenario : Operator has 4G, 3G & 2G network. CSFB fallback redirection is to 3G. No 3G coverage present at testing location.

Vendor : 4G - Samsung, 2G,3G - Huawei

Testing result : All MO CSFB calls get mature in 2G , though there is no 2G details in redirection message. While all MT calls get blocked with announcement " Customer is not reachable".

Drive test observation (TEMS):
MO CSFB Call : UE searches 3G for 6 secs, CSFB call gets block and at the same automatically call attempted to 2G and get mature. (OSS stats shows call is )
MT CSFB Call : UE searches 3G for 6 secs, CSFB call gets block and UE returns to LTE.

Query : After CSFB MO get blocked when 3G not found for 6 sec, call gets automatically attempted in case of CSFB MO and gets mature in 2G, but why not this happening in case of MT CSFB.

[wolverine]

For a MT call to be successful the UE needs to send a paging response in the LAC where the call has been directed. This is where the MSC has terminated the signalling path. If the UE appears in 2G (and I imagine a different LAC) the paging response procedure cannot be successful. For MO this is not a problem as there is no paging and the MSC just sees it as a normal MO call. There are some CN solutions like MTRF for the first case but they are quite signalling intensive. Google it if you need more info.

[erraviesh]

For a MT call to be successful the UE needs to send a paging response in the LAC where the call has been directed. This is where the MSC has terminated the signalling path. If the UE appears in 2G (and I imagine a different LAC) the paging response procedure cannot be successful. For MO this is not a problem as there is no paging and the MSC just sees it as a normal MO call. There are some CN solutions like MTRF for the first case but they are quite signalling intensive. Google it if you need more info.

Thanks wolverine for your response,

MTRF comes in picture if there is different MSCs or TA-LA mapping is with one RAT and redirection is with other RAT. So this we could be ruled out.

For MT, UE searches 3G (its redirected RAT), if it not found 3G it directly moves back to LTE. It never moves to 2G, while in MO, when it don't find 3G it moves to 2G and call attempted.

[marcengo]

That`s a very interesting scenario, I guess you may be entering the famous "OEM-dependent" area. Not sure if 3GPP specifies that UE should search for another RAT if CSFB call fails and the second RAT is not present in. So those would be my first two points of verification: both the standard and if GSM is sent in the redirection message in LTE.