The present invention relates to a telecommunication network structure and in particular to a mobile telephone network structure of the kind which includes a first number of base station control units, each being intended to control a limited number of individual base stations.
A telecommunication network contains a the telephone network (PSTN), a service specific digital network for telephony and data (ISDN), a telex network, a teletext network or a data packet network and combinations thereof.
A mobile station MS, as used in this document, is defined as a mobile telephone, normally a vehicle installed telephone, and also portable telephones which utilize the mobile telephone network. The mobile telephones are able to transmit speech and in some cases also digital data information. The term mobile station shall not be confused with a mobile telephone station, which is a switch for mobile telephone calls. In this document, a mobile telephone station, i.e. a switch for mobile telephone calls, is referred to as a mobile service telephone station.
A network structure of the aforesaid kind is available as a pan-European standard for mobile telephone networks. This standard is referred to popularly as the GSM-system. The GSM-system represents a step forward in the art in comparison with earlier known mobile telephone systems, since signalling between the mobile telephone network and the mobile telephone stations is effected by digital transmission via radio. The GSM-system has a number of base station control units, BSC, subordinated a central mobile service telephone station MSC which is connected to the public telephone network via an extension terminal. The mobile service telephone station MSC includes a switch which enables physical telephone connections to be established, so-called originating and terminating calls.
In addition to establishing telephone connections, the mobile service telephone station MSC also performs other services, such as a billing or charging service for instance, and also incorporates the hand-over and paging functions unique to mobile telephony. The GSM-network structure also includes a so-called home location register in the form of a database in which the subscriber of the mobile station MS and subscriber data are stored. The home location register HLR also stores information relating to the location of the mobile station in the network structure, i.e. the location of the subscriber. The home location register HLR also contains information for establishing a connection to the mobile station MS, so-called routing information.
Also included in the GSM-network structure is a visitor location register VLR, which has the form of a database in which identification data relating to the mobile station which temporarily visits the area served by the mobile service telephone station is normally stored. In addition to this identification data, there is also stored in the visitor location register a copy of the subscriber data which is stored in the home location register for the mobile station. The visitor location register VLR takes information from the home location register HLR when the visitor location register discovers that a new mobile station MS is located in the area covered by the mobile service telephone station. The network structure of the GSM-system also includes other units, such as an authorization-control-database AC which contains secret keys for checking the authorization of each mobile station. The system also includes a database EIR which stores information relating to accepted mobile telephones, black-listed mobile telephones, etc.
A base station BTS comprises a base station transceiver having transmitting and receiving means, antennas, and operating and maintenance means.
The base stations BTS are distributed over a geographical area served by one base station control unit BSC. The base stations BTS are each connected to a main base station control unit BSC by trunk lines.
Permanent connections in the form of so-called trunk lines are also found between each base station control unit BSC and mobile service telephone station MSC, these trunk lines typically consisting of link connections and/or underground cables.
FIG. 1 illustrates a typical example, in which a mobile service telephone station MSC is situated in a first town, called town-X, a first base station control unit BSC is situated in a second town, called town-Y which is situated, e.g., 70 km from town-X for instance, a second base station control unit BSC is located in a third town, called town-Z which is situated, e.g., 70 km from town X and at a distance from town-Y. The majority of the mobile telephone traffic in town-Y, however, is local traffic, i.e. traffic passing between a mobile station MS located in town-Y and a stationary subscriber Ab also located in town-Y. The majority of the mobile telephone traffic in town-Z is also local traffic. All of the telephone connections between the parties in local calls within town-Y are connected through the trunk lines T1 from town-Y to the mobile service telephone station MSC in town-X and back to town-Y via trunk lines. Sound information is thus transmitted through lines over a distance of 140 km, just to provide a connection between two parties who are separated geographically by only some few kilometers. This is highly unsatisfactory with view to the cost of the-trunk lines and the cost of their installation.
It is known in the art to use remote subscriber selector stages in telephony and mobile telephony stations. A remote subscriber selector stage is located geographically close to the area served by the selector stage. Remote subscriber selector stages are used for the purpose of reducing the length of the links. Remote subscriber selector stages are controlled by system specific signals. The system specific signals are transmitted on a dedicated signalling connection between the telephone station and the selector station. Remote subscriber selector stages form an integral part of the telecommunication network and are installed when the telecommunication network is expanded and require common maintenance with the telecommunication network. European Patent No. EP-B1-0 113 662 describes an remote subscriber selector stage but has nothing in common with the present invention, since this patent specification describes a method of multiplexing transceiver control signals so as to enable a central network unit to control several transceiver units.