hi, i just found some valuable information this weekend at www.informatik.hu-berlin.de/~goeller/isdn/ an introduction into gsm tracing in german http://www.informatik.hu-berlin.de/~goeller/isdn/DieGSMDmKanaele.pdf and the same in english http://www.informatik.hu-berlin.de/~goeller/isdn/GSMDmChannels.pdf it gives a rough history of GSM development, GSM network structure, framing, encryption, coding, and services - in just 100 pages. for me it is usefull to verify my parsing of SYSTEM INFORMATION messages. it describes traces of the most importaint messages. andreas