Diameter SWx Interface explained.

The SWx interface is a diameter protocol-based interface between the AAA server and HSS. AAA means Authentication, Authorization, and accounting. The standard protocol is defined in 3GPP Spec.   This interface is used when UE does non-3GPP access. We have discussed in the s6a interface that MME is the node in EPC that does the authentication with the HSS over the s6a interface.

But why do we need another interface for authentication, e.g., SWx? This is because LTE provides a way to connect UE from a network other than MME. This may be a non-3GPP network. One example is WiFi calling. Where a home WiFi antenna works as an LTE tower. The following diagram shows the SWx interface and other interfaces in EPC.

 Procedures/messages over the SWx interface:

This section describes the protocol messages over the SWx interface. The base protocol is the diameter. Over the base, the application uses 3GPP application id 16777265.

SWx Interface
SWx Interface

Multimedia Authentication Request (MAR)/Answer(MAA) :

AAA server sends this command to the HSS for accessing security information. The command code value is 303. In the message, there is a diameter AVP called User Name. This AVP may have a subscriber IMSI for identifying a subscriber on HSS. If the subscriber is found and allowed to use non-3GPP access, HSS returns the security information in the answer command; else, an error is returned, and authentication fails.

Push Profile Request (PPP)/Answer(PPA): 

The push profile request command has command code 505. HSS sends this command to the AAA server if any subscription data has been changed on the HSS for a subscriber. The data may be related to the call or other.

E.g., if a mobile user’s validity has expired, then HSS will push the subscriber data, where the subscriber will not be allowed to make calls. AAA server successfully updated the data. Then the answer message has success. Else has an error code.

Server Assignment Request (SAR)/ Answer (SAA):

The command code for the server assignment request is 301. AAA server initiates the request towards the HSS. This can be a register, a user profile download, and use a de-registration request. If all is fine on HSS, there is a success in the answer else error code.

Registration termination request (RTR)/Answer(RTA) :

The command code is 304. The multimedia server sends this message to a multimedia client over the Swx interface for the de-registration of a client.