A method for setting up a data link including the steps of requesting an identity of an IP network node by a registration module; transmitting an International Mobile Subscriber Identity (IMSI) stored on a Subscriber Identity Module (SIM) card of the IP network node to the registration module, storing the received IMSI in a user database of the registration module, complementing an IP data channel to form signal and data channels of a GSM network by a signaling gateway module, using the complemented IP data channel having GSM data as a basis for generating the necessary SS7/MAP functions for authenticating the IP network node, authenticating the IP network node by using the IMSI of the SIM card of the IP network node, receiving an appropriate entry in the user database of the registration module upon successful authentication, and setting up a data link to the at least some of the network nodes by the control module of the central connecting unit.