The Tracking Area Code (TAC) is a fixed length code (of 2 octets) identifying a Tracking Area within a PLMN. This
part of the tracking area identification shall be coded using a full hexadecimal representation. The hexadecimal values 0000 and FFFE are reserved.
The Tracking Area Identity (TAI) consists of a Mobile Country Code (MCC), Mobile Network Code (MNC), and
Tracking Area Code (TAC).
A subdomain name can be derived from the TAI. This shall be done by adding the label "tac" to the beginning of the
Home Network Realm/Domain and encoding the TAC as a sub-domain. This is called the TAI FQDN.
The TAI FQDN shall be constructed as follows:
The TAC is a 16‑bit integer. The <TAC‑hb> is the hexadecimal string of the most significant byte in the TAC
and the <TAC‑lb> is the hexadecimal string of the least significant byte. If there are less than 2 significant digits
in <TAC‑hb> or <TAC‑lb>, "0" digit(s) shall be inserted at the left side to fill the 2 digit coding.
For addressing on resources used for GPRS, a Temporary Logical Link Identity (TLLI) is used. The TLLI to use is built
by the MS either on the basis of the P‑TMSI (local or foreign TLLI), or directly (random TLLI).
In order to support the subscriber identity confidentiality service, the VLRs, SGSNs and MME may allocate Temporary
Mobile Subscriber Identities (TMSI) to visiting mobile subscribers. The VLR, SGSN and MME must be capable of
correlating an allocated TMSI with the IMSI of the MS to which it is allocated.
An MS may be allocated three TMSIs, one for services provided through the MSC, one for services provided through
the SGSN (P-TMSI) and one for the services provided via the MME (M-TMSI).
Since the TMSI has only local significance (i.e. within a VLR and the area controlled by a VLR, or within an SGSN and
the area controlled by an SGSN, or within an MME and the area controlled by an MME), the structure and coding of it
can be chosen by agreement between operator and manufacturer in order to meet local needs.
The TMSI consists of 4 octets. It can be coded using a full hexadecimal representation.