Slashdot reports an open-source camp-wide GSM network was operated at HAR2009, a Dutch hacker conference. The network uses two 100mW antennas, SMS was used for authentication and on the second day of the event a total of 391 users had subscribed to the network.
Photographs and a description and of the setup, run under license of the Dutch regulatory authority, are provided; essentially the setup consisted of a pair of BTS' (Base Transceiver Stations) running at 100mW transmit power each and tied to a tree. In turn these provided access to the Base Station Controller (BSC), in this case a Linux server in a tent running OpenBSC. The system authenticated users with a token sent via SMS; in total 391 users subscribed to the service and were able to use their phones as if they were on any other network.