Hello Experts.
A query related to Core flows.
Whenever a user location needs to be fetched, platform sends ATI request to HLR which subsequently sends out PSI request to MSC/VLR for obtaining location related information.
On the other hand, for call/SMS termination SRI request is sent to HLR which again subsequently sends out PSI request to MSC/VLR for location related information.
Since in both scenarios PSI is being trigerred by HLR, why isn’t SRI used for obtaining user location and why is ATI used 90% of the time?
Is there any advantages of ATI over SRI or is it a mere practice adopted by Telco operators?
Short Answer: yes!
Only ATI should be used to get Location information, not the SRI, though both of them trigger PSI internally.
Detailed Answer: SRI is designed specifically for Incoming call purposes. If a subscriber has a T-CSI (it will be predominantly applicable only when Roaming or any MT CAMEL services are applicable, which is rare), then only SRI initiates a PSI. In the absence of a T-CSI, the HLR initiates a PRN ( to the serving MSC) instead of PSI. In such a case, the MSC reserves an MSRN for the call to get matured. Since applicability of T-CSI is quite rare for an incoming call, the SRI procedure will waste an MSRN in the MSC and soon the Visiting MSCs might run out of MSRNs.
You cannot be sure that SRI will always get the location for you. So, you should only use ATI to get the location information.
If you send it via the international or national SS7 link, do not use ATI. ATI is supposed to be used only inside one operator network, not via the interconnect. A halfway decent signalling firewall will block ATIs and for good reason (to much unauthorized tracking of VIPs).
SRI_SM on the other hand can not be easily blocked as it is required for delivery of SMS.
PSI is also a message that has been heavily misused over interconnect to track people, so also not recommended.