The Diameter interfaces S6a and S6d are really the same interface at a functional level, both communicate with the 4G/LTE HSS relating to a specific subscriber, however the S6a interface originates from a MME (4G/LTE) node, where S6b implies the originating node is a SGSN (2G/3G) node.
The primary function of the interface is to communicate with the HSS (Home Subscriber Server), which is a centralized database for a specific subscriber hosted within it’s home network. The HSS maintains the subscription of the subscriber, along with location information of the subscriber to support mobility and terminating services.
Eight different messages are supported by S6a/S6d and these serve three primary purposes, which are:
- Firstly to support “authentication”, the protocol will transmit authentication vectors between a Subscriber/SIM and the HSS, to ensure that an encrypted challenge between the two elements is successfully performed.
- Secondly it facilitates “mobility”, once authenticated the MME will update the location of the subscriber on the HSS, so that inbound services are able to locate the subscriber at any given point in time. Similarly the HSS will update an MME should it be aware that the subscriber has moved away.
- Finally it manages the “subscription download”, during the process of updating the subscribers location following authentication the HSS will inform the MME what services the subscriber is permitted to access, and any specific restrictions or behavior that should be honored for these services.
Each message within the protocol provides an “Request” and “Answer” however the direction of the “Requests” and “Answer” between the MME/SGSN and HSS will depend on the message type.
The supported message types are below:
1) AIR /AIA (Authentication-Information-Request/Answer)
Using an AIR the MME will fetch Authentication data from the subscribers HSS to authenticate the SIM Card – Regardless of the MME location, the AIR request will always route to the home network of the subscriber, the routing of which is based on the MCC/MNC from the subscriber IMSI.
2) ULR / ULA (Update-Location-Request/Answer)
The MME of which the subscriber has been authenticated will inform the subscribers HSS that the authentication was successful, and that the subscriber is now attached to itself. In doing so it will request the subscription data from the HSS, which will be provided via the below IDR/IDA messages.
3) NOR / NOA (Notification-Request/Answer)
The NOR is used by the MME to store PDN address information and other details within the subscribers HSS.
4) PUR / PUA (Purge Request/Answer)
The MME sends a Purge Request to the HSS informing it that the subscriber has been detached for a long period of time, and therefore has purged the subscription data that it would have cached – Should the subscriber return to the MME it will go through a new attach process.
5) IDR / IDA (Insert-Subscription-Data-Request/Answer)
During the Location Update process the HSS will return the subscribers profile and subscription information to the MME by sending one or more IDRs to the MME.
6) DSR / DSA (Delete-Subscriber-Data-Request/Answer)
The DSR is sent from a HSS to MME for a specific subscriber when some subscription data has been modified or deleted on the HSS. The message ensuring that the MME has the most accurate subscription data for the subscriber.
7) CLR / CLA (Cancel-Location-Request/Answer)
The Cancel Location request would be sent by a HSS to the MME that the subscriber was last attached, and will reference the subscribers IMSI – Typically this is sent as the HSS has received a Location Update from another MME, and it will therefore inform the loosing MME of the change in subscriber status that it may not be aware of.
8) RSR / RSA (Reset-Request/Answer)
This message is seldom used, however in a state where the HSS has been out of service for a period of time, the HSS may send a RSR to a range of MME’s, informing them to perform fresh Location Updates to itself – The message will not address a specific IMSI, but imply this should occur for all the HSS subscribers attached to the target MME.