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The roaming capability of a mobile communications network makes it possible to offer seamless voice and data services by a network operator to its subscribers while traveling outside of their home network and location and enter into a network that is run by the same or different operator in a different location. Roaming services are delivered to subscribers through the interoperation of networks operated by the same or another operator. Interoperation, in turn, is achieved through the interworking of network elements and interfaces as described in the previous sections.

In the previous Sections 6.2 and 6.3, we have presented the interworking of the GSM, GPRS UMTS, and LTE/EPS networks, through the enhanced as well as legacy network elements, run by an operator. In this section, we further present the interoperations and interworking of mobile communications networks operated by different operators in different regions to provide roaming services to subscribers.

A mobile communications network operated by an operator to provide communications services in a particular area/location is known as the Public Land Mobile Network (PLMN). A PLMN is uniquely identified, also see Section 5.2, by the combination of an MCC and MNC of an operator. Within a PLMN, an operator may provide both the voice (CS) and data services to subscribers. The PLMN in which an MS/UE is currently subscribed and registered in an LTE/EPS network is known as the home PLMN (HPLMN). The PLMN of either the same or different operator in which an MS/UE is currently roaming into and accessed the visited LTE/EPS network is known as the visiting PLMN (VPLMN).