Notify Request Message is sent by MME to the HSS at the end of the attach procedure.
The attach procedure is said to be complete when MME receives "Modify Bearer Response" from Serving gateway. i.e. the TEIDS for uplink & downlink have been updated & the UE is ready for both sending & receiving the packets(upink/downlink).
So when MME receives Modify Bearer Response from gateway, it checks if the current PDN gateway is different from the PDN Gateway Identity that HSS had sent it in the PDN subscription context.
If it is same then no issues. In case it is different, it needs to be updated & so MME sends Notify Request message to the HSS. This message contains the following info -
- PDN Gateway Identity.
The HSS updates the above info & acknowledges back to MME with a Notify Response Message.
Hope this helps!