Roaming is one of the most popular features offered by wireless networks today. For mobile users, it offers the ability to use the mobile services outside their service provider’s coverage area with the same phone. For service providers, roaming offers an opportunity to serve visitors from foreign networks as well as their own subscribers anywhere and anytime. it is also a most profitable revenue stream for the wireless service providers.
Roaming was introduced in the very first generation of cellular networks but was not available on a global basis till recent years. The early standards for cellular networks were focused in standardizing the Common Air interface (CAi). There was not much work done in standardizing internetwork communication, resulting in a variety of vendor-dependent proprietary protocols. This means that roaming was possible only between two networks supplied by the same vendor. As the demand for roaming increased, the need for standards for communication between home and visited network was felt. The IS-41 standard was introduced as a standard protocol for internetwork communication to enable roaming in AMPS-based networks. Later, as part of GSM standardization, Mobile Application Part (MAP) was developed. Both IS-41 and GSM MAP were enhanced several times to ensure seamless roaming for the next generation of networks.
Today, with multimode mobile phones supporting GSM 900/1800/1900, it is possible to roam in a visited network with different radio frequencies. New UMTS phones are backward-compatible with GSM/GPRS networks. This allows 3G subscribers to roam in GSM/GPRS networks when they are outside 3G coverage. This is a very important feature, as initial deployment of 3G networks is unlikely to cover the entire nation because of cost constraints.
During the last few years, GPRS and 3G networks were deployed. Roaming in a GPRS/3G network is not an automatic extension of GSM voice roaming. The service providers are progressively building necessary infrastructure and services to offer true seamless global roaming as envisioned in 3G specifications. It may take a few years before GPRS/3G roaming can reach a level comparable to GSM roaming.
National and International Roaming
Roaming is the ability for a mobile subscriber to make/receive voice calls, send/receive data, and use other value-added services in a visited network, outside the geographical coverage area of the home network. The home and visited networks are referred to as the Home Public Land Mobile Network (HPLMN) and the Visited Public Land Mobile Network (VPLMN) respectively. In the case of international roaming, the HPLMN and the VPLMN belong to two different countries.
Not all wireless service providers offer their mobile services across national boundaries. This constraint may be because of licensing, technical, or commercial reasons. For example in India, a license to run mobile networks was initially based on "circles," each circle consisting of one or more states or provinces. In order to offer a nationwide service, a wireless service provider offers roaming within national boundaries. in the case of national roaming, the HPLMN and the VPLMN belong to the same country.
