Interstandard, or cross-technology, roaming refers to roaming capabilities between two networks regardless of technology and standards. From business and user satisfaction points of view, interstandard roaming capabilities are required to further expand roaming services. The incompatibility in standards makes it difficult to enable roaming between networks. For example, second-generation networks are predominantly based on the GSM TDMA or the CDMA access technologies. GSM networks deploy GSM MAP and CDMA networks deploy IS-41 for internetwork communication. GSM uses the HLR for user authentication and CDMA uses HLR and AAA. Implementing roaming between these two islands of networks is not straightforward. Actually, the issues faced by the industry are more than technical. They include:
■ Interoperability issues related to access technologies, handsets, smart cards
■ Interoperability issues related to signaling protocols
■ End-to-end testing
■ Interoperator roaming agreements
■ Exchange of usage/billing information and format
■ Revenue assurance, e.g., prevention of fraud
■ Security
In addition to conventional mobile networks, one has to consider the convergence of the technologies such as WLAN and WiMax.
Most of the interstandard roaming solutions available today are based on SIM roaming. This allows subscribers to use their own SIM and an intelligent handset. For example, if a GSM subscriber wishes to roam in a CDMA network, he/she rents a special phone, which accepts GSM SIM. This enables a roamer to retain personal information and MSISDN number. The network provider or roaming hub deploys a protocol converter (generally known as roaming gateway) to convert IS-41 signaling to GSM MAP and vice versa.
The other solution requires a dual CDMA/GSM handset. These handsets are commercially available but are expensive. These handsets also support dual slots for smart cards, i.e., SIM and RUIM.
One of the important requirements laid down by ITU-T for 3G networks is the capability of seamless roaming. The standardization and harmonization efforts on access technologies ensure that the 3G subscribers are able to roam in any network on a global basis. The 3G mobile phones also support WCDMA/TDMA/CDMA access technologies. This means that 3G subscribers can roam in a GSM or a CDMA network when outside 3G geographical coverage.
