Enabling Data Roaming Between CDMA 1X and GPRS Networks

[April 29, 2004]

Enabling Data Roaming Between CDMA 1X and GPRS Networks

By Mark Denton, product manager for Bridgewater Systems

Over the past two years, major CDMA carriers in North America and Asia have rolled out packet data services based on the CDMA2000 1xRTT standard. The uptake of data services in consumer markets is now beginning to occur with the advent of services such as Multimedia Messaging Service (MMS) coupled with camera phones. Within corporate markets, new data services such as Push to Talk (PTT) are beginning to drive usage along with mobile data connectivity to the corporate office.

Demand for these services is such that one major CDMA wireless carrier reported quadruple growth in data service transactions in two successive months. As subscribers come to rely on these services, they require that they work anywhere, which means that carriers must work together to enable roaming. Roaming allows subscribers from one network to use their devices to access services on another network. This may be charged at a higher rate but since subscribers are used to this premium on their voice calls, they are likely to also expect it for their data services.

CDMA wireless technology is generally available worldwide but is non-existent in Western Europe. Subscribers of CDMA networks who are reliant on wireless connectivity for data services are demanding that they have ubiquitous access, even when in Europe. Without this ability, their only option is to change their CDMA device for a GPRS device or to carry two devices. Likewise, the reverse case exists for users from Europe with GPRS devices who want to access CDMA-based data services in North America.

While the demand for such services increases, there remain technical challenges in delivering wireless data services to subscribers regardless of where they are in the world. One major issue is the fact that to offer global roaming services, the devices require two radios, translating into a considerable increase in required power. There are also concerns about the network topology and how the subscriber is authenticated to use the network.

CDMA 1x to GPRS Roaming
Standards groups have begun working on the issue of enabling roaming between CDMA 1X and GPRS networks. The ETSI/3GPP working group has published �TS 29.061 Inter-working between the Public Land Mobile Network (PLMN)

One manner to address this is by managing the subscriber�s authentication to use data services on a GPRS network and the collection and delivery of the associated usage records into the CDMA 1x Carrier network. GPRS uses RADIUS to authenticate the subscriber to use a service defined by an Access Point Name (APN). This RADIUS authentication should be used to authenticate the subscriber for access to services. For simplicity in this solution, subscribers who have the CDMA 1x to GPRS roaming service enabled should use a special APN. This will allow the authentication, authorization and accounting (AAA) server within the GPRS network to easily identify that it has to contact the clearinghouse for authentication of the subscriber to access the service. Once the message is forwarded to the clearinghouse, it can be forwarded to the roaming partner for authentication by the home network.

Authentication in the home network can be done via the subscriber profiles that are available in some AAA systems. The accounting messages provided by the GGSN are used to bill the CDMA 1x subscriber. The Gateway GPRSN Support Node (GGSN) sends the accounting messages as normal to the local AAA server, which passes them on to the clearinghouse. If they are to be passed to the CDMA 1x network in real-time, it is necessary to manipulate the RADIUS accounting record within the clearinghouse so that it resembles a CDMA 1x record. This will allow the CDMA 1x network to bill the subscriber using a familiar system.

As an alternative option, this could be accomplished by passing the records into the clearinghouse, which then processes and passes them in batch mode to the CDMA 1x carrier�s billing system in a neutral format. The main issue with this option is determining the look of the neutral format.

GPRS to CDMA 1x Roaming
It is more difficult to envision how authentication and billing of GPRS subscribers on a CDMA 1x network can be handled. This difficulty arises because CDMA 1x is reliant on RADIUS for authentication and accounting and while RADIUS is present in GPRS networks, GPRS uses other technologies such at GPRS Tunneling Protocol (GTP) for accounting and the Home Location Register (HLR) for authentication.

Given this background, carriers should consider the use of a technology called EAP-SIM for the authentication of subscribers. EAP-SIM is a draft IETF standard described in �draft-haverinen-pppext-eap-sim-11.� EAP-SIM is recommended for use in Wi-Fi roaming but will also work in roaming on CDMA 1x networks because of the way it uses RADIUS. EAP-SIM uses subscribers� SIM cards as their credential to access the network and requires that a gateway be provided between RADIUS and the GSM MAP protocol.

Summary
Although not a trivial matter, carriers are now in the position to offer subscribers roaming between CDMA 1X and GPRS environments, making true mobility between environments a reality. Carriers interested in offering such services should work with their technology suppliers to devise an architecture that enables it.

Mark Denton is product manager for Bridgewater Systems (www.bridgewatersystems.com)

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