Some times S6a/S6d interfaces are treated as two separate interfaces but here we treat them as single because both have same application identifier(16777251).S6a interface is between MME-HSS and S6d interface between SGSN-HSS.
This section gives abstrace of s6a/s6d interface. In LTE network HSS (Home Subscriber Server) is a database that contains Authentication Information and Subscriber's Data such as Services associated, Location Information etc. Where MME shall take care of mobility of UE/Subscriber. This interface is used to Authenticate Subscriber, providing services to subscriber, to store location information of subscriber sent by MME.There are eight messages are exchanged between MME/SGSN and HSS. Four of them are invoked by MME and rest four are invoked by HSS.
1)AIR/AIA (Authentication-Information-Request/Answer):- MME fetches Authentication data from HSS to authenticate subscriber.
2)ULR/ULA (Update-Location-Request/Answer):- MME stores its own identity at HSS, and feteches subscription data from HSS.
3)NOR/NOA (Notification-Request/Answer):- MME stores PDN address and other attach information at HSS.
4)PUR/PUA (Purge Request/Answer):- MME informs the HSS that UE is inactive for a long period that why MME has deleted the Subscription Data received in previous ULR from its end.
5)IDR/IDA(Insert-Subscription-Data-Request/Answer):- It is invoked by HSS only when a subscriber is attached and there is change in subscriber profile at HSS end then same change to be reflected at Subscriber profile at MME (sent in ULA) end as well.
6)DSR/DSA (Delete-Subscriber-Data-Request/Answer):- It is invoked by HSS only when Subscriber is attached and some data is deleted at HSS. Now HSS informs MME with this message that some part of subscription data is deleted at HSS.
7)CLR/CLA(Cancel-Location-Request/Answer):- Invoked by HSS to detach the subscriber.
8)RSR/RSA(Reset-Request/Answer):- Invoked by HSS, to inform MME that HSS goes down for some time, kindly sync the data and send fresh location/PDN information at HSS.
Your Comments /Suggestions and Questions are always welcome. I would try to clear your doubts with best of my knowledge. So feel free to put Questions.
Detail behavior with example
2. ULR/ULA (Update-Location-Request/Answer)
3. NOR/NOA (Notification-Request/Answer)
6. DSR/DSA (Delete-Subscriber-Data-Request/Answer)