Overview#Temporary Mobile Subscriber Identity (TMSI) is an Pairwise Pseudonymous Identifier with the International Mobile Subscriber Identity (IMSI) assigned by the Visitor Location Register (VLR) to every Mobile Device in the area when it joins
Temporary Mobile Subscriber Identity is local to a location area and has to be updated each time the mobile moves to a new geographical (Visitor Location Register) area.
Temporary Mobile Subscriber Identity can be changed by the Mobile Network at any time and typically would to avoid the subscriber from being identified.
Temporary Mobile Subscriber Identity is a part of the ability for "Paging" or Peer-to-peer communication between the Mobile Device and another Mobile Network Operator. The most important use of broadcast information is to set up channels for "paging". Every cellular system has a broadcast mechanism to distribute such information to a plurality of mobiles.
Size of TMSI is 4 octet with full hex digits and can't be all FF because the SIM uses 4 octets with all bits equal to 1 to indicate that no valid TMSI is available.