The purpose of this Technical Specification is to present the possible architectures of the mobile system. Clause 3 of this specification contains a definition of the different functional entities needed to support the mobile service. In clause 4@ the configuration of a PLMN is described as well as the organisation of the functional entities; the configuration presented is the most general in order to cope with all the possible implementations which can be imagined in the different countries. To illustrate that purpose@ some examples of possible configurations are presented. Clause 5 of this specification contains a brief description of the interfaces involved which shows the principle of the organisation considered.