General considerations In the land mobile telephony service@ the Home Public Land Mobile Network (HPLMN) will know the location of all operational mobile stations for which it is the home registered network@ when the station is operating within its HPLMN or has roamed in accordance with the principles contained in ITU-T Recommendations of the Q.1000 series. The Visited Public Land Mobile Network (VPLMN) will be able to access and record the necessary details of any mobile station which has roamed and registered with it@ such that it can forward all necessary billing information to the HPLMN. Mobile stations will only be able to roam to another Public Land Mobile Network (PLMN): ? upon the conclusion of a bilateral agreement between PLMNs; ? upon the granting by the HPLMN of the option to roam to the mobile station. As part of the bilateral agreement@ the HPLMN should undertake to pay the VPLMN the charges incurred by the HPLMN registered mobile subscribers. Examples of the application of the charging and accounting principles@ set out below@ for various call routing scenarios are given in Annex A. Definitions home PLMN The PLMN in which a mobile station is permanently registered. home location register The location register to which a mobile station is assigned for record purposes such as subscriber information visited PLMN The PLMN@ other than the home PLMN@ in which a roaming subscriber is currently located. visitor location register The location register@ other than the home location register used by an MSC to retrieve information for@ for instance@ handling of calls to or from a roaming mobile station@ currently located in its area. mobile station roaming number The network internal number used for routing of calls to the mobile station. See Recommendation ITU-T E.213.