In a recent blog, we started to address some of the questions participants asked during a recent webinar we did with Infonetics on Diameter signaling in LTE networks. During the course of the webinar, we talked about some of the challenges that service providers are facing with LTE, and the topic of roaming garnered a lot of interest.

One roaming use case in particular involves the scenario in which a customer homed to a 3G network is roaming in a network where the operator has upgraded its mobile core to use Diameter interfaces. The process by which a subscriber’s device attaches to a network involves authentication of the subscriber from the visited network to its home network. In this scenario, the visited network is sending Diameter-based authentication messages over what’s referred to as the S6a interface, but the Home Location Register (HLR) in the home network is expecting to see the request via the Mobile Application Part or MAP protocol over a Gr interface.

This brings us to one of the questions that came up during the webinar as we talked about 3G to 4G roaming scenarios: “For security vector mapping between different radio technologies, do you need subscriber security key information on the Diameter Signaling Controller (DSC)  since normally these are only stored on the HLR/HSS?”

The 3rd Generation Partnership Project (3GPP), in technical specification 29.305, has standardized the mapping of Diameter to SS7 messages to enable the exchange of authentication information between a 3G and 4G network. A DSC is inserted between the visited and home network and uses its Interworking Functionality (IWF) to perform the translation of the Diameter message fields into the appropriate MAP fields. There is also a need to interwork the authentication vectors between networks, which may be in different formats depending on what 3GPP release the 3G network’s subscriber data repository currently supports.

There is a subscriber security key (K) that is the heart of the Authentication and Key Agreement strategy, and it is contained in two locations: the user’s Subscriber Identification Module or SIM card in their mobile device (UE) and the subscriber data management platform in the service provider’s mobile core. For 3G networks, this platform is referred to as the HLR, and for 4G networks it’s called the Home Subscriber Server (HSS).

When a 3G roaming subscriber tries to attach to the visited network, part of the process involves authentication of the user. Put very simply and leaving out a lot of the complex steps, in order to do this, the user’s phone sends out an authentication request to the visited network’s MME/SGSN or Mobility Management Entity.  The information contained in that request includes, among other things, the subscriber’s international mobile subscriber identity (IMSI). The MME/SGSN sends a Diameter-based Authentication Information Request (AIR) message to the subscriber’s home network, however, the home network is expecting a MAP/SS7-based SendAuthentication Info message. The DSC’s interworking capabilities map the fields in the Diameter AIR request to the corresponding fields in a MAP SendAuthenticationInfo request. The DSC then forwards the request onto the home network’s HLR using the MAP protocol.

The HLR then inspects the IMSI and computes an expected response (XRES) using the subscriber’s security key (K) and a random number (RAND) and generates two additional derived keys: the Cyphering Key (Ck), and Integrity Key (Ik).  It then sends back to the visited network’s MME/SGSN a response that includes the following information for the subscriber’s request: the RAND, an authentication token (AUTN), XRES, Ck, and Ik. The IWF translates the MAP response containing the authentication vectors into the appropriate Diameter response and sends it to the visited network’s MME/SGSN. What ultimately happens is that the MME/SGSN sends the UE a response that includes the RAND received from the HLR. The UE generates a response (RES) based on the RAND it receives from the network and the subscriber security key (K) stored on its SIM, which it sends back to the MME/SGNS for comparison. If the RES and XRES match, then all is good!

Here is the rub: there was a change in the authentication vectors in LTE (3GPP release 8 and later), so that instead of Ck and Ik being sent by the HLR/HSS, a derived key, KASME - also known as the Access Security Management Entity Key - is sent to the MME/SGSN, and this change was made in order to support the increased security enabled by LTE. The KASME is a key derived in the HSS/HLR from Ck and Ik and is needed to perform UMTS/EPS subscriber authentication.

So the visited network’s MME/SGSN is expecting an Authentication Information Answer (AIA) message containing the following authentication vectors: RAND, AUTN, XRES, and the KASME.  So if the HLR has not been upgraded to support E-UTRAN authentication vector requests, the IWF needs to provide a means to incorporate  KASME into the response messages from the HLR.

The subscriber key is not exposed, so the IWF needs to enhance the Diameter messages by inserting the proper authentication vectors into the Diameter responses to enable the MME/SGSN to determine if the visiting subscriber has the proper credentials to attach to the network. Clearly, interworking functionality is really important between different generations of mobile networks. If you want to find out more about interworking 3G and 4G networks for roaming applications, please get in touch with us. We can show you a demo so you can see firsthand how you can eliminate a lot of the interworking complexity in your network by using a next-generation DSC with IWF capabilities. In the next blog in this series, we’ll talk about using IT protocols to integrate with Business and Operational Support Systems (BSS/OSS) and leveraging data from other repositories to really start enhancing signaling messages.