A rather cool paper from GSM hacker (OpenBSC and OsmocomBB etc) Harald Welte:

http://laforge.gnumonks.org/weblog/2010/04/14/#20100414-anatomy_of_gsm_cellp...

Will perhaps be good reading for anyone planning to attend the Open Source in Telecomms event on the 22nd that will have an OpenBTS presentation:

http://ossg.bcs.org/2010/03/22/open-source-in-telecommunications-london-2204...

Cheers,

Andrew