Once MME receives attach request from UE, it retrieves authentication and security keys from HSS by exchanging Diameter based messages such as Authentication-Information-Request and Authentication-Information-Answer messages.
Based the received authentication parameters, MME executes authentication procedure with UE and if authentication procedures gets executed successfully then it enables security between itself and UE.
After successful authentication and security, MME has to initiate "create session" procedure. For that it needs couple of other information such as QoS, APN etc. To get all these details for an UE, MME executes "Update Location Procedure" towards HSS.
MME sends "Update Location Request" to HSS and in response it gets "Update Location Answer" message. The Update Location Answer message contains UE subscription details.
In essence,after authentication and security procedure, MME fetches UE subscription details from HSS.
