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.
In order to avoid double allocation of TMSIs after a restart of an allocating node, some part of the TMSI may be related to the time when it was allocated or contain a bit field which is changed when the allocating node has recovered from the restart.
In areas where both MSC-based services and SGSN-based services are provided, some discrimination is needed between the allocation of TMSIs for MSC-based services and the allocation of TMSIs for SGSN-based services. The discrimination shall be done on the 2 most significant bits, with values 00, 01, and 10 being used by the VLR, and 11 being used by the SGSN.
If intra domain connection of RAN nodes to multiple CN nodes as described in
TS 23.236 is applied in the MSC/VLR or SGSN, then the NRI shall be part of the TMSI. The NRI has a configurable length of 0 to 10 bits. A configurable length of 0 bits indicates that the NRI is not used and this feature is not applied in the MSC/VLR or SGSN. The NRI shall be coded in bits 23 to 14. An NRI shorter than 10 bits shall be encoded with the most significant bit of the NRI field in bit 23.
The TMSI shall be allocated only in ciphered form. See also
TS 43.020 and
TS 33.102.
The network shall not allocate a TMSI with all 32 bits equal to 1 (this is because the TMSI must be stored in the SIM, and the SIM uses 4 octets with all bits equal to 1 to indicate that no valid TMSI is available).
To allow for eventual modifications of the management of the TMSI code space management, MSs shall not check if an allocated TMSI belongs to the range allocated to the allocating node. MSs shall use an allocated TMSI according to the specifications, whatever its value.
The LMSI consists of 4 octets and may be allocated by the VLR. The VLR shall not allocate the value zero. The value zero is reserved to indicate that an LMSI parameter sent from the HLR to the VLR shall not be interpreted.
A TLLI is built by the MS or by the SGSN either on the basis of the P-TMSI (local or foreign TLLI), or directly (random or auxiliary TLLI), according to the following rules.
The TLLI consists of 32 bits, numbered from 0 to 31 by order of significance, with bit 0 being the LSB.
A local TLLI is built by an MS which has a valid P-TMSI as follows:
-
bits 31 down to 30 are set to 1; and
-
bits 29 down to 0 are set equal to bits 29 to 0 of the P-TMSI.
A foreign TLLI is built by an MS which has a valid P-TMSI as follows:
-
bit 31 is set to 1 and bit 30 is set to 0; and
-
bits 29 down to 0 are set equal to bits 29 to 0 of the P-TMSI.
A random TLLI is built by an MS as follows:
-
bit 31 is set to 0;
-
bits 30 down to 27 are set to 1; and
-
bits 0 to 26 are chosen randomly.
An auxiliary TLLI is built by the SGSN as follows:
-
bit 31 is set to 0;
-
bits 30 down to 28 are set to 1;
-
bit 27 is set to 0; and
-
bits 0 to 26 can be assigned independently.
Other types of TLLI may be introduced in the future.
Part of the TLLI codespace is re-used in GERAN to allow for the inclusion of the GERAN Radio Network Temporary Identifier in RLC/MAC messages. The G-RNTI is defined in
TS 44.118.
The structure of the TLLI is summarised in table 1.
31 |
30 |
29 |
28 |
27 |
26 to 0 |
Type of TLLI |
1 | 1 | T |
T | T | T |
Local TLLI |
1 | 0 | T |
T | T | T |
Foreign TLLI |
0 | 1 | 1 |
1 | 1 | R |
Random TLLI |
0 | 1 | 1 |
1 | 0 | A |
Auxiliary TLLI |
0 | 1 | 1 |
0 | X | X |
Reserved |
0 | 1 | 0 |
X | X | X |
Reserved |
0 | 0 | 0 |
0 | G | G |
Part of the assigned G-RNTI |
0 | 0 | 0 |
1 | R | R |
Random G-RNTI |
'T', 'R', 'A' and 'X' indicate bits which can take any value for the type of TLLI. More precisely, 'T' indicates bits derived from a P-TMSI, 'R' indicates bits chosen randomly, 'A' indicates bits chosen by the SGSN, 'G' indicates bits derived from the assigned G-RNTI and 'X' indicates bits in reserved ranges.