s6a interface

The diameter base protocol provides basic transport for diameter messages. There are many application defined by 3GPP for telecom communications. Applications are also called diameter interfaces. The s6a and S6d are the examples of diameter interfaces. The s6a interface is between MME and HSS in LTE network and s6d is between SGSN and HSS.  Both provides messages and procedures for phone attach. MME is the node in visiting 4G network. Which connects to the UE via eNodeB.  eNodeB have wireless interface with UE and wired interface with the MME. When an UE tries to register to network for mobile internet, MME sends signaling to the HSS and PGW. Both are located in home network.

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

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

Messages Over S6a/S6d interface:

First thing to send application specific messages, base transport is required.  For message transfer, 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 IP network. The choice is mobile operator specific.  The application is same for both type of transport protocols.

When a phone attaches to the visiting network, network procedures starts.  Following are the messages/procedures.

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

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

ULR/ULA :  As mentioned above, it is the next 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 subscription. If IMSI (mobile subscriber) subscription is found and subscriber is allowed to use data in visiting network. Then HSS sends ULA (update location answer) with subscription data , else an error is returned in ULA. Subscription data have list of APNs.

Following are the error codes for failures:

ErrorCodes   Description
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 list of RAT type.  This is defined in this 3GPP Spec

HSPA Evolution

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

After receiving success response, MME initiates a tunnel create procedure over GTPv2-C signaling.  This is called a create session request over  gtp protocol.  MME setup the session with the PGW located in home network via,  SGW located in visiting network.

Once the session create is successful, the user data flows from MME to PGW over 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 the roaming a Mobile phone changes the location, as a result of this,  serving MME changes or roaming location.  Phone attaches to new MME and HSS clears the data from older MME.  When HSS detects that now subscriber is under new 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 following cases.

  1. Subscriber is Inactive from very long time.
  2. GTP session creation failure as a part of update location procedure.
  3. Management Initiated. 

IDR/IDA (Insert Subscriber Data Request/Answer):

When a 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 about the change in UE. This may be because of change in terminal information, change in APN of any other cases.

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