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Content for  TS 23.003  Word version:  16.3.0

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2.8  Globally Unique Temporary UE Identity (GUTI) |R8|

2.8.1  Introduction

The purpose of the GUTI is to provide an unambiguous identification of the UE that does not reveal the UE or the user's permanent identity in the Evolved Packet System (EPS). It also allows the identification of the MME and network. It can be used by the network and the UE to establish the UE's identity during signalling between them in the EPS. See TS 23.401.
The GUTI has two main components:
  • one that uniquely identifies the MME which allocated the GUTI; and
  • one that uniquely identifies the UE within the MME that allocated the GUTI.
Within the MME, the mobile shall be identified by the M-TMSI.
The Globally Unique MME Identifier (GUMMEI) shall be constructed from the MCC, MNC and MME Identifier (MMEI).
The MMEI shall be constructed from an MME Group ID (MMEGI) and an MME Code (MMEC).
The GUTI shall be constructed from the GUMMEI and the M-TMSI.
For paging purposes, the mobile is paged with the S-TMSI. The S-TMSI shall be constructed from the MMEC and the M-TMSI.
The operator shall need to ensure that the MMEC is unique within the MME pool area and, if overlapping pool areas are in use, unique within the area of overlapping MME pools.
The GUTI shall be used to support subscriber identity confidentiality, and, in the shortened S-TMSI form, to enable more efficient radio signalling procedures (e.g. paging and Service Request).
The format and size of the GUTI is therefore the following:
  • <GUTI> = <GUMMEI><M-TMSI>,
    • where <GUMMEI> = <MCC><MNC><MME Identifier>
    • and <MME Identifier> = <MME Group ID><MME Code>
MCC and MNC shall have the same field size as in earlier 3GPP systems.
M-TMSI shall be of 32 bits length.
MME Group ID shall be of 16 bits length.
MME Code shall be of 8 bits length.
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2.8.2  Mapping between Temporary and Area Identities for the EUTRAN and the UTRAN/GERAN based systemsWord‑p. 26

2.8.2.0  Introduction

This clause provides information on the mapping of the temporary and location area identities, e.g. for the construction of the Routeing Area Update Request message in GERAN/UTRAN or Tracking Area Update Request message in E-UTRAN.
In GERAN and UTRAN:
  • <RAI> = <MCC><MNC><LAC><RAC>
  • <P-TMSI/TLLI> includes the mapped NRI
P-TMSI shall be of 32 bits length where the two topmost bits are reserved and always set to '11'. Hence, for a UE which may handover to GERAN/UTRAN (based on subscription and UE capabilities), the corresponding bits in the M-TMSI are set to '11' (see clause 2.8.2.1.3).
3GPP TS 23.236 specifies that the NRI field is of variable length and shall be mapped into the P-TMSI starting at bit 23 and down to bit 14. The most significant bit of the NRI is located at bit 23 of the P-TMSI regardless of the configured length of the NRI. To support mobility between GERAN/UTRAN and E-UTRAN, the NRI length is limited to a maximum of 8 bits to be compatible for the mapping to MME Code within GUTI (see clause 2.8.2.2).
The P-TMSI and NRI are defined elsewhere in this specification.
In the case of a combined MME-SGSN node, the NRI of the SGSN part and the MME code of the MME part, refer to the same combined node. RAN configuration allows NAS messages on GERAN/UTRAN and E-UTRAN to be routed to the same combined node. The same or different values of NRI and MME code may be used for a combined node.
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2.8.2.1  Mapping from GUTI to RAI, P-TMSI and P-TMSI signature

2.8.2.1.1  Introduction
This clause addresses the case when a UE moves from an MME to an SGSN. The SGSN may be either an S4 SGSN or a Gn/Gp SGSN.
2.8.2.1.2  Mapping in the UE
When a UE moves from an E-UTRAN to a GERAN/UTRAN, the UE needs to map the GUTI to an RAI, a P-TMSI and a P-TMSI Signature, for them to be sent to the SGSN. For GERAN, the TLLI is derived from the P-TMSI by the UE and is a foreign TLLI (see clause 2.6).
The mapping of the GUTI shall be done to the combination of RAI of GERAN / UTRAN and the P-TMSI:
    E-UTRAN <MCC> maps to GERAN/UTRAN <MCC>
    E-UTRAN <MNC> maps to GERAN/UTRAN <MNC>
    E-UTRAN <MME Group ID> maps to GERAN/UTRAN <LAC>
    E-UTRAN <MME Code> maps to GERAN/UTRAN <RAC> and is also copied into the 8 Most Significant Bits of the NRI field within the P-TMSI;
    E-UTRAN <M-TMSI> maps as follows:
    • 6 bits of the E-UTRAN <M-TMSI> starting at bit 29 and down to bit 24 are mapped into bit 29 and down to bit 24 of the GERAN/UTRAN <P-TMSI>;
    • 16 bits of the E-UTRAN <M-TMSI> starting at bit 15 and down to bit 0 are mapped into bit 15 and down to bit 0 of the GERAN/UTRAN <P-TMSI>;
    • and the remaining 8 bits of the E-UTRAN <M-TMSI> are mapped into the 8 Most Significant Bits of the <P-TMSI signature> field.
      The UE shall fill the remaining 2 octets of the <P-TMSI signature> according to clauses 9.1.1, 9.4.1, 10.2.1, or 10.5.1 of 3GPP TS.33.401 [89] , as appropriate, for RAU/Attach procedures.
For UTRAN, the 10-bit long NRI bits are masked out from the P-TMSI and are also supplied by the UE to the RAN node as IDNNS (Intra Domain NAS Node Selector) (see TS 23.236). However, the RAN configured NRI length should not exceed 8 bits.
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2.8.2.1.3  Mapping in the old MMEWord‑p. 27
A new SGSN attempts to retrieve information regarding the UE, e.g. the IMSI, from the old MME. In order to find the UE context, the MME needs to map the RAI, P-TMSI (or TLLI) and the P-TMSI Signature (sent by the SGSN) to create the GUTI and compare it with the stored GUTI.
The MME shall perform a reverse mapping to the mapping procedure specified in clause 2.8.2.1.2 "Mapping in the UE" (see TS 29.060 and TS 29.274 for specifics of the messaging). For the reverse mapping, the E-UTRAN <MME Code> within the GUTI shall be set either to bits 23 to 16 of the GERAN/UTRAN <P-TMSI> (i.e., the NRI field) or to the GERAN/UTRAN <RAC>. For GERAN TLLI, the old MME replaces the two topmost bits of TLLI, received from new SGSN via GTPv1, with '11' before mapping the TLLI to the GUTI used for looking up the "UE Context".
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2.8.2.2  Mapping from RAI and P-TMSI to GUTI

2.8.2.2.1  Introduction
This clause addresses the case when a UE moves from an SGSN to an MME (i.e. during a TAU or an Attach to MME). The SGSN may be either an S4 SGSN or a Gn/Gp SGSN.
2.8.2.2.2  Mapping in the UE
When the UE moves from the GERAN/UTRAN to the E-UTRAN, the UE needs to map the RAI and P-TMSI to a GUTI to be sent to the MME. The P-TMSI signature is sent intact to the MME.
The mapping of P-TMSI (TLLI) and RAI in GERAN/UTRAN to GUTI in E-UTRAN shall be performed as follows:
  • GERAN/UTRAN <MCC> maps to E-UTRAN <MCC>
  • GERAN/UTRAN <MNC> maps to E-UTRAN <MNC>
  • GERAN/UTRAN <LAC> maps to E-UTRAN <MME Group ID>
  • GERAN/UTRAN <RAC> maps into bit 23 and down to bit 16 of the M-TMSI
The 8 most significant bits of GERAN/UTRAN <NRI> map to the MME code.
  • GERAN/UTRAN <P-TMSI> maps as follows:
    • 6 bits of the GERAN/UTRAN <P-TMSI> starting at bit 29 and down to bit 24 are mapped into bit 29 and down to bit 24 of the E-UTRAN <M-TMSI>;
    • 16 bits of the GERAN/UTRAN <P-TMSI> starting at bit 15 and down to bit 0 are mapped into bit 15 and down to bit 0 of the E-UTRAN <M-TMSI>.
The values of <LAC> and <MME group id> shall be disjoint, so that they can be differentiated.
The most significant bit of the <LAC> shall be set to zero; and the most significant bit of <MME group id> shall be set to one. Based on this definition, the most significant bit of the <MME group id> can be used to distinguish the node type, i.e. whether it is an MME or SGSN. The UE copies the received old SGSN's <LAC> into the <MME Group ID> when sending a message to an MME, regardless of the value of the most significant bit of the <LAC>.
In networks where this definition is not applied (e.g. in networks already configured with LAC with the most significant bit set to 1 before LTE deployment), the information in the TAU/RAU request indicating whether the provided GUTI/P-TMSI is "native" (i.e. no system change) or "mapped" (i.e. system change) can be used to distinguish the node type for UEs implemented according to this release of the specification (see TS 24.301 and TS 24.008). Specific network implementations still satisfying 3GPP standard interfaces can be used for pre-Rel-10 UEs to distinguish the node type.
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2.8.2.2.3  Mapping in the new MMEWord‑p. 28
In order to retrieve the UE's information, e.g. the IMSI, from the old SGSN, the new MME extracts only the RAI and P-TMSI from the GUTI via the reverse mapping procedure to that specified in clause 2.8.2.2.2. This is done in order to be able to include the mapped RAI and P-TMSI, along with the P-TMSI Signature received by the MME from the UE, in the corresponding message sent to the old SGSN (see TS 29.060 and TS 29.274 for specifics of the messaging). The old SGSN compares the received RAI, P-TMSI and P-TMSI Signature with the stored values for identifying the UE.
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2.9  Structure of the S-Temporary Mobile Subscriber Identity (S-TMSI) |R8|

The S-TMSI is the shortened form of the GUTI to enable more efficient radio signalling procedures (e.g. paging and Service Request). For paging purposes, the mobile is paged with the S-TMSI. The S-TMSI shall be constructed from the MMEC and the M-TMSI:
  • <S-TMSI> = <MMEC><M-TMSI>
See clause 2.8 for these definitions and the mapping.

2.10  5G Globally Unique Temporary UE Identity (5G-GUTI) |R15|

2.10.1  Introduction

The purpose of the 5G-GUTI is to provide an unambiguous identification of the UE that does not reveal the UE or the user's permanent identity in the 5G System (5GS). It also allows the identification of the Access and Mobility Management Function (AMF) and network. It can be used by the network and the UE to establish the UE's identity during signalling between them in the 5GS. See TS 23.501.
The 5G-GUTI has two main components:
  • one that identifies the AMF(s) which allocated the 5G-GUTI; and
  • one that uniquely identifies the UE within the AMF(s) that allocated the 5G-GUTI.
Within the AMF(s), the mobile shall be identified by the 5G-TMSI.
The Globally Unique AMF Identifier (GUAMI) shall be constructed from the MCC, MNC and AMF Identifier (AMFI).
The AMFI shall be constructed from an AMF Region ID, an AMF Set ID and an AMF Pointer. The AMF Region ID identifies the region, the AMF Set ID uniquely identifies the AMF Set within the AMF Region, and the AMF Pointer identifies one or more AMFs within the AMF Set.
The 5G-GUTI shall be constructed from the GUAMI and the 5G-TMSI.
For paging purposes, the mobile is paged with the 5G-S-TMSI. The 5G-S-TMSI shall be constructed from the AMF Set ID, the AMF Pointer and the 5G-TMSI.
The operator shall need to ensure that the combination of the AMF Set ID and AMF Pointer is unique within the AMF Region and, if overlapping AMF Regions are in use, unique within the area of overlapping AMF Regions.
The 5G-GUTI shall be used to support subscriber identity confidentiality, and, in the shortened 5G-S-TMSI form, to enable more efficient radio signalling procedures (e.g. paging and Service Request).
The format and size of the 5G-GUTI is therefore the following:
  • <5G-GUTI> = <GUAMI><5G-TMSI>,
    • where <GUAMI> = <MCC><MNC><AMF Identifier>
    • and <AMF Identifier> = <AMF Region ID><AMF Set ID><AMF Pointer>
MCC and MNC shall have the same field size as in earlier 3GPP systems.
5G-TMSI shall be of 32 bits length.
AMF Region ID shall be of 8 bits length.
AMF Set ID shall be of 10 bits length.
AMF Pointer shall be of 6 bits length.
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2.10.2  Mapping between Temporary Identities for the 5GS and the E-UTRANWord‑p. 29

2.10.2.0  Introduction

This clause provides information on the mapping of the temporary identities, e.g. for the construction of the Tracking Area Update Request message in E-UTRAN.
In E-UTRAN:
  • <GUTI> = <MCC><MNC><MME Group ID><MME Code><M-TMSI>

2.10.2.1  Mapping from 5G-GUTI to GUTI

2.10.2.1.1  Introduction
This clause addresses the case when a UE moves from an AMF to an MME.
2.10.2.1.2  Mapping in the UE
When a UE moves from 5GS to an E-UTRAN, the UE needs to map the 5G-GUTI to a GUTI.
The mapping of the 5G-GUTI to a GUTI shall be done as follows:
  • 5GS <MCC> maps to E-UTRAN <MCC>
  • 5GS <MNC> maps to E-UTRAN <MNC>
  • 5GS <AMF Region ID> and 5GS <AMF Set ID> map to E-UTRAN <MME Group ID> and part of E-UTRAN <MME Code> as follows:
    • 8 bits of the 5GS <AMF Region ID> starting at bit 7 and down to bit 0 are mapped into bit 15 and down to bit 8 of the E-UTRAN <MME Group ID>;
    • 8 bits of the 5GS <AMF Set ID> starting at bit 9 and down to bit 2 are mapped into bit 7 and down to bit 0 of the E-UTRAN <MME Group ID>;
    • 2 bits of the 5GS <AMF Set ID> starting at bit 1 and down to bit 0 are mapped into bit 7 and down to bit 6 of the E-UTRAN <MME Code>;
  • 5GS <AMF Pointer> maps to part of E-UTRAN <MME Code> as follows:
    • 6 bits of the 5GS <AMF Pointer> starting at bit 5 and down to bit 0 are mapped into bit 5 and down to bit 0 of the E-UTRAN <MME Code>.
  • 5GS <5G-TMSI> maps to to E-UTRAN <M-TMSI>
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2.10.2.1.3  Mapping in the old AMFWord‑p. 30
A new MME attempts to retrieve information regarding the UE, e.g. the IMSI, from the old AMF. In order to find the UE context, the AMF needs to map the GUTI (sent by the MME) to create the 5G-GUTI and compare it with the stored 5G-GUTI.
The AMF shall perform a reverse mapping to the mapping procedure specified in clause 2.10.2.1.2 "Mapping in the UE".

2.10.2.2  Mapping from GUTI to 5G-GUTI

2.10.2.2.1  Introduction
This clause addresses the case when a UE moves from an MME to an AMF (i.e. during a Registration Update or an Initial Registration to an AMF).
2.10.2.2.2  Mapping in the UE
When the UE moves from the E-UTRAN to 5GS, the UE needs to map the GUTI to a 5G-GUTI to be sent to the AMF.
The mapping of the GUTI to a 5G-GUTI shall be performed as follows:
  • E-UTRAN <MCC> maps to 5GS <MCC>
  • E-UTRAN <MNC> maps to 5GS <MNC>
  • E-UTRAN <MME Group ID> maps to 5GS <AMF Region ID> and part of 5GS <AMF Set ID> as follows:
    • 8 bits of the E-UTRAN <MME Group ID> starting at bit 15 and down to bit 8 are mapped into bit 7 and down to bit 0 of the 5GS <AMF Region ID>;
    • 8 bits of the E-UTRAN <MME Group ID> starting at bit 7 and down to bit 0 are mapped into bit 9 and down to bit 2 of the 5GS <AMF Set ID>;E-UTRAN <MME Code> maps to 5GS <AMF Set ID> and 5GS <AMF Pointer> as follows:
    • 2 bits of the E-UTRAN <MME Code> starting at bit 7 and down to bit 6 are mapped into bit 1 and down to bit 0 of the 5GS <AMF Set ID>;
    • 6 bits of the E-UTRAN <MMEC Code> starting at bit 5 and down to bit 0 are mapped into bit 5 and down to bit 0 of the 5GS <AMF Pointer >;
  • E-UTRAN <M-TMSI> maps to 5GS <5G-TMSI>
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2.10.2.2.3  Mapping in the new AMFWord‑p. 31
In order to retrieve the UE's information, e.g. the IMSI, from the old MME, the new AMF shall perform a reverse mapping to the mapping procedure specified in clause 2.10.2.2.2 "Mapping in the UE". This is done in order to be able to include the mapped GUTI in the corresponding message sent to the old MME. The old MME compares the received GUTI with the stored values for identifying the UE.

2.11  Structure of the 5G-S-Temporary Mobile Subscriber Identity (5G-S-TMSI) |R15|

The 5G-S-TMSI is the shortened form of the 5G-GUTI to enable more efficient radio signalling procedures (e.g. paging and Service Request). For paging purposes, the mobile is paged with the 5G-S-TMSI. The 5G-S-TMSI shall be constructed from the AMF Set ID, the AMF Pointer and the 5G-TMSI:
  • <5G-S-TMSI> = <AMF Set ID><AMF Pointer><5G-TMSI>
See clause 2.10.1 for these definitions and clause 2.10.2 for the mapping.
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2.12  Structure of the Truncated 5G-S-Temporary Mobile Subscriber Identity (Truncated 5G-S-TMSI) |R16|

The Truncated 5G-S-TMSI is a 40 bit UE identifier constructed from the 5G-S-TMSI. It is used in RRC Connection Re-Establishment for the control plane for NB-IoT as described in TS 36.300. The Truncated 5G-S-TMSI shall be constructed from the Truncated AMF set ID, the Truncated AMF Pointer and the Truncated 5G-TMSI:
  • <Truncated 5G-S-TMSI> = <Truncated AMF set ID><Truncated AMF Pointer><Truncated 5G-TMSI>
Truncated AMF set ID is n least significant bits of AMF Set ID, where n is no greater than 10 bits.
Truncated AMF Pointer is m least significant bits of AMF Pointer, where m is no greater than 6 bits.
Truncated 5G-TMSI is (40-n-m) least significant bits of 5G-TMSI.
T The values n and m are configurable based on network deployment. The value n+m shall be larger or equal to 8 bits.
The NG-RAN is configured with the values n and m, and it is configured with how to recreate AMF Set ID from Truncated AMF Set ID, AMF Pointer from Truncated AMF Pointer, and 5G-TMSI from Truncated 5G-TMSI. The configuration of these parameters are specific to each PLMN.
The NG-RAN configures the UE with n and m during RRC connection reconfiguration as described in TS 36.331. The configuration applies only to the registered PLMN.
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