IMSI in the clear on Um. The TMSI is assigned by the BSC and is only meaningful within specific network. The TMSI is assigned by the network with the MM TMSI Reallocation Command, a message that is normally not sent until after ciphering is started, so as to hide the TMSI/IMSI relationship. Once the TMSI is established, it can be used to anonymize future transactions. Note that the subscriber identity must be established before authentication or encryption, so the first transaction in a new network must be initiated by transmitting the IMSI in the clear. See also OpenBTS GSM 03.40 Further reading External links 3GPP - The current standardization body for GSM with free standards available. General Packet Radio Service GPRS: Architecture, Protocols, and Air Interface. GSM standard