The invention relates to a communication system and a method for providing a mobile telecommunication service. A communication network transmits messages on the basis of at least one internet protocol. A network connection computer connects an access network for mobile computers and the communication network. The access network is equipped with several access points comprising corresponding access point connection computers, which are configured to create a communication connection between variable access points and the mobile computers. An authorization verification computer establishes and manages trusted relationships between several communication elements. The network connection computer and the access point connection computer are configured to execute packet filtering during the receipt and transmission of messages for the secure protection of the communication system.