S6a interface in LTE

The Diameter base protocol provides essential transport for diameter messages. There are many applications defined by 3GPP for telecom communications. Applications are also called diameter interfaces. The s6a and S6d are two examples of diameter interfaces. The s6a interface is between MME and HSS in the LTE network, and the s6d is between SGSN and HSS. Both provide messages and procedures for the phone to attach for 4G services.

MME is the visiting node in the 4G network, which connects to the UE via eNodeB. eNodeB has a wireless interface with UE and a wired interface with the MME. When a UE tries to register to network for mobile internet, MME sends signaling to the HSS and PGW. Both are located inside the home network of an MNO (Mobile Network Operator).

The application id for the s6a and s6d interface is 16777251, allocated by IANA. The 3GPP application specification is given in 3GPP TS 29.272

The s6d interface is also specified in the same specification as the s6a specification. In 4G, the SGSN also uses the diameter protocol, while in 3G, it uses the GSM MAP protocol over SS7 or Sigtran.

Messages Over S6a/S6d interface:

The first thing to send application-specific messages is base transport is required. For message transfer, a diameter base protocol is used. Base protocol setups Diameter connection with remote proxy or Diameter Routing Agent (DRA). It may use TCP or SCTP as a transport protocol over the IP network. The choice depends on the mobile operator, as the application remains the same for both types of transport protocols.

Network procedures start between roaming and home network over diameter protocol when a phone attaches to the visiting network. The followings are the messages/procedures.

AIR/AIA: Authentication information retrieval is the first message MME initiates towards the HSS for a UE (User equipment, phone device in 4G) to attach to the visiting network for high-speed data services. The procedure is the Authentication for a 4G SIM card. AIR message has IMSI and other information. HSS returns the answer message (AIA) on successful Authentication, with authentication vectors in case of success. Else, HSS sends a diameter error in response.

In case of a successful response, UE verifies the authentication information in response. Suppose Authentication is successful on UE too. MME initiates the location update request (ULR) towards the HSS, else. UE keeps trying Authentication after some intervals.

ULR/ULA: As mentioned above, it is the following message after successful Authentication. ULR sends the identity and roaming information to the home network. The primary identity is IMSI. Location information is visiting MCC, MNC, and cell id. Upon receiving ULR, HSS checks for the subscription. If IMSI (mobile subscriber) subscription is found, the subscriber can use data in the visiting network. Then HSS sends ULA (update location answer) with subscription data, else an error is returned in ULA. Subscription data have a list of APNs.

Following are the error codes for failures:

ErrorCodes	Description
DIAMETER_ERROR_USER_UNKNOWN	IMSI is not configured.
DIAMETER_ERROR_UNKNOWN_EPS_SUBSCRIPTION	No APN is configured.
DIAMETER_ERROR_ROAMING_NOT_ALLOWED	UE is not allowed in the roaming network.
DIAMETER_ERROR_RAT_NOT_ALLOWED	The selected Radio technology is not allowed. Following is a list of RAT types. This is defined in this 3GPP Spec. UTRAN GERAN WLAN GAN HSPA Evolution EUTRAN

In case of success, the ULR has the error code DIAMETER_SUCCESS and the Subscription profile AVP.

After receiving a successful response, MME initiates a tunnel creation procedure over GTPv2-C signaling. This is called a create session request over the GTP protocol. MME set up the session with the PGW in-home network via SGW in the visiting network.

Once the session is created successfully, the user data flow from MME to PGW over the GTP session.

If session creation is failed, MME sends the Purge Request (PUR) to the HSS to delete the roaming data from HSS.

CLR/CLA (Cancel Location Request/Answer):

During roaming, a Mobile phone changes its location, resulting in serving MME changes or roaming location. The phone attaches to the new MME, and HSS clears the data from the older MME. When HSS detects that now subscriber is under a newer MME, it sends the CLR to the previous MME. As a response, MME sends the CLA to the HSS.

PUR/PUA (Purge UE Request/Answer):

When MME needs to delete the subscriber record, MME initiates a PUR towards the HSS. Records may be deleted in the following cases.

Subscriber is Inactive from very long time.

GTP session creation failure as a part of update location procedure.

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Management Initiated. 
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IDR/IDA (Insert Subscriber Data Request/Answer):

When information or full subscription data needs to change on MME by the mobile operator, HSS sends the IDR to the visiting MME.

DSR/DSA(Delete Subscriber Data Request/Answer):

When subscriber data needs to remove from MME.

NOR/NOA (Notify Request/Answer):

This procedure is used when HSS needs to modify the change in UE. This may be because of changes in terminal information, changes in APN, or other cases.