14 Identities

23.0603GPPGeneral Packet Radio Service (GPRS)Release 17Service descriptionStage 2TS

Tools: ARFCN - Frequency Conversion for 5G NR/LTE/UMTS/GSM

14.1 IMSI

A unique International Mobile Subscriber Identity (IMSI) shall be allocated to each packet-domain subscriber. This is also the case for GPRS-only subscribers. IMSI is defined in TS 23.003 [4].

14.2 Packet TMSI

A Packet Temporary Mobile Subscriber Identity shall be allocated to each GPRS-attached MS. P‑TMSI is defined in TS 23.003 [4].

14.2A GUTI

Globally Unique Temporary Identity (GUTI) is defined in TS 23.401 [89].

14.3 NSAPI and TLLI for A/Gb mode

The Network layer Service Access Point Identifier (NSAPI) and Temporary Logical Link Identity (TLLI) are used for network layer routeing. An NSAPI / TLLI pair is unambiguous within a routeing area.

In the MS, NSAPI identifies the PDP‑SAP. In the SGSN and GGSN, NSAPI identifies the PDP context associated with a PDP address. Between the MS and the SGSN, TLLI unambiguously identifies the logical link. The NSAPI may be related to either one IP address or two IP addresses (one IPv4 and one IPv6 if PDP Type IPv4v6 is supported and used).

When the MS requests the activation of a PDP context, the MS selects one of its unused NSAPIs.

For example (shown figuratively below), the MS receives an IP packet from a connected TE at the IP address A SAP. The IP PDU is encapsulated and NSAPI is initialised to NSAPI‑1. TLLI is set to the MS’s TLLI before the encapsulated IP packet is passed to the SNDC function. After the IP PDU is received, the SGSN analyses TLLI and NSAPI‑1 and determines that the IP PDU shall be sent to the GGSN associated with IP address A.

Figure 94: Use of NSAPI and TLLI

Within a routeing area, there is a one-to-one correspondence between TLLI and IMSI that is only known in the MS and SGSN. If it is not clear from the context which routeing area a TLLI belongs to, then TLLI is used together with RAI. TLLI is derived from a P‑TMSI, and does then provide user identity confidentiality as described in clause "User Identity Confidentiality (A/Gb mode)".

The TLLI address range is divided into four ranges: Local, Foreign, Random, and Auxiliary. The TLLI structure allows the MS and SGSN to deduce the range that a TLLI belongs to. A Local TLLI is derived from the P‑TMSI allocated by the SGSN, and is valid only in the RA associated with the P‑TMSI. A Foreign TLLI is derived from a P‑TMSI allocated in another RA. A Random TLLI is selected randomly by the MS, and is used when the MS does not have a valid P‑TMSI available. An Auxiliary TLLI is selected by the SGSN, but is not used in this release of the specifications.

If the MS has a valid P‑TMSI associated with the RA where the MS is currently located, the MS shall use a Local TLLI derived from its P‑TMSI, unless the MS performs a GPRS attach.

If the MS does not have a valid P‑TMSI associated with the current RA, or if the MS performs a GPRS attach, it shall derive a Foreign TLLI from its P‑TMSI, or allocate a Random TLLI if no valid P‑TMSI is available.

When a TLLI is exchanged between the MS and an SGSN, the TLLI is transmitted at the RLC/MAC layer within the Um protocol stack, and at the BSSGP layer within the Gb protocol stack. NSAPI is transmitted within the SNDCP layer in the user plane, and within the GMM/SM layer in the control plane. In some SM signalling messages, transaction identifier (TI) represents NSAPI . The TI is dynamically allocated by the MS for MS-requested PDP context activation, and by the network for network-requested PDP context activation. The TI is deallocated when a PDP context has been deactivated. TI usage is defined in TS 24.007 [12] and TS 24.008 [13].

By default, unless explicitly specified in the procedures, the TLLI transmitted at the RLC/MAC and BSSGP layers shall be used to identify the MS.

14.4 NSAPI, RB Identity, and RAB ID for Iu mode

The Network layer Service Access Point Identifier (NSAPI) and IMSI are used for network layer routeing. An NSAPI / IMSI pair is used to assign a Tunnel Endpoint Identifier (TEID).

In the MS, NSAPI identifies the PDP‑SAP. In the SGSN and GGSN, NSAPI identifies the PDP context associated with an MM context.

In the context of this specification, the term RNC refer also to a GERAN BSC when serving an MS in Iu mode.

In communication with the RNC across the Iu‑PS and Uu interfaces, the RAB ID is used to identify the radio access bearer and that information element shall be set identical to the NSAPI value. In the RNC, RAB ID identifies the RAB context. Radio Bearer Identity (RB Identity) is used to identify the Uu interface radio bearer(s) associated with the radio access bearer. The RAB ID shall uniquely identify the RAB for a specific CN domain and a particular MS.

There is a one-to-one relationship between NSAPI, Radio Access Bearer, and PDP context. In the packet domain, there is also a one-to-one relationship with Radio Bearer Identity. The NSAPI may be related to either one IP address or two IP addresses (one IPv4 and one IPv6 if PDP Type IPv4v6 is supported and used).

Figure 95: Use of NSAPI, RB Identity, and RAB ID

When the MS initiates activation of a PDP context, the MS selects one of its unused NSAPIs. When the SGSN initiates a RAB assignment procedure, the SGSN shall include the NSAPI(s) in the RAB ID information element(s).

14.4A EPS Bearer Identity

An EPS bearer identity uniquely identifies an EPS bearer for one UE. When there is a mapping between an EPS bearer and a PDP context, the same identity value is used for the EPS bearer ID and the NSAPI/RAB ID. The EPS Bearer Identity is defined in TS 23.401 [89].

14.5 PDP Address

A packet-domain subscriber identified by an IMSI, shall have one or more network layer addresses, i.e. PDP addresses, temporarily and/or permanently associated with it that conforms to the standard addressing scheme of the respective network layer service used, e.g.:

– an IP version 4 address; or

– an IP version 6 prefix; or

– an IP version 4 address and an IP version 6 prefix.

PDP addresses are activated and deactivated through MM procedures described in clause "PDP Context Activation, Modification, Deactivation, and Preservation Functions".

A corresponding identity "PDN Address" is used over S-based interfaces using GTP.

14.6 TEID

A Tunnel Endpoint Identifier (TEID) is used by the GPRS tunnelling protocol between GSNs, between SGSNs and S‑GWs and P‑GWs and between RNCs/BSCs and SGSNs, to identify a tunnel endpoint in the receiving GTP‑C or GTP‑U protocol entity and to identify an EPS Bearer and/or a PDP context (or in the Iu case a Radio Access Bearer). The receiving end side of a GTP-U tunnel locally assigns the TEID value that the transmitting side has to use. The TEID values are exchanged between tunnel endpoints using GTP‑C (or RANAP in the Iu case) messages.

The TEID is a unique identifier within one IP address of a logical node, i.e. RNC, BSC, SGSN, S‑GW, P‑GW or GGSN, which has meaning only within the GTP protocol. For the user plane, i.e. GTP-U, each PDP context has a one-to-one relationship between the TEID on one hand and NSAPI and IMSI on the other hand, or in the Iu reference point case, between the TEID and RAB ID and IMSI. When a node releases an EPS Bearer/PDP context, the corresponding TEID shall not be re-used within a significant period of time to ensure a low probability of the TEID being still assigned to an existing EPS Bearer/PDP context in a peer node. However, the algorithm for computing the value of the TEID and the period of time until the re-use of a TEID are implementation dependent.

The TEID is forwarded to the S‑GW and P‑GW or GGSN upon PDP Context Activation and it is used in subsequent tunnelling of user data between the GGSN or S‑GW and P‑GW and the SGSN to identify the MS’s PDP contexts in the SGSN and GGSN or S‑GW and P‑GW. The TEID is also used to forward N‑PDUs from the old SGSN to the new SGSN at and after an inter-SGSN routeing area update. In Iu mode, the TEID is also forwarded to the RNC upon RAB assignment and it is used in subsequent tunnelling of user data between the SGSN and the RNC/BSC in order to identify the MS’s PDP contexts in the SGSN and the MS’s RAB contexts in the RNC/BSC. It is also used to forward N‑PDUs from the SRNC/SBSC to the target RNC/BSC at SRNS/SBSS relocation.

14.7 Routeing Area Identity

Routeing Area Identity (RAI), defined by an operator, identifies one or several cells.

In A/Gb mode, RAI is broadcast as system information.

In Iu mode, RAI is broadcast to MSs in RRC Idle mode, and is notified to MSs in RRC Connected mode on established RRC connections as MM system information.

The location of an MS in STANDBY or PMM‑IDLE state is known in the SGSN on an RA level. Cells that do not support packet-domain services within an LA are grouped into a null RA. The MS is paged for packet services in the RA where the MS is located when mobile-terminated traffic arrives in the SGSN. The MS is paged for circuit-switched services by the SGSN in the last known RA plus in the null RA.

NOTE: Cells not supporting GPRS and served by a BSC without a Gb interface should not be included in the same location area as cells not supporting GPRS and served by a BSC with a Gb interface.

A Routeing Area is a subset of one, and only one, Location Area (LA), meaning that an RA cannot span more than one LA. An RA is served by only one SGSN.

The following rules apply for the Routeing Area Identity:

– RAC is only unique when presented together with LAI.

– CI is only unique when presented together with LAI or RAI (A/Gb mode only).

– LAI = MCC + MNC + LAC.

– RAI = MCC + MNC + LAC + RAC.

– CGI = LAI + CI (A/Gb mode only).

14.8 RAN Registration Area Identity (Iu mode)

The UTRAN/GERAN Registration Area Identity (URA/GRA Id) identifies a UTRAN/GERAN Registration Area (URA/GRA) and is defined in TS 25.331 [52]. URA/GRA Id can be used to indicate to the MS which URA/GRA it shall use in case of overlapping URA/GRAs.

14.9 Cell Identity

In A/Gb mode, Cell Identity (CI) identifies one cell. In Iu mode, Cell Identifier (C‑Id) uniquely identifies a cell within an RNS. CI and C‑Id are defined in TS 23.003 [4].

14.10 Service Area Identity (Iu mode)

The Service Area Identity (SAI) is used to uniquely identify an area consisting of one or more cells belonging to the same location area. Such an area is called a Service Area and can be used for indicating the location of an MS to the CN.

The Service Area Code (SAC) together with the PLMN identity and the LAC constitutes the Service Area Identity:

– SAI = MCC + MNC + LAC + SAC.

14.11 CN Node Addresses

14.11.1 CN Node Address

Each SGSN, S‑GW, P‑GW and GGSN shall have one or more IP addresses of type IPv4, and optionally of type IPv6, for inter-communication over the backbone network. When an SGSN, S‑GW, P‑GW or a GGSN supports IPv6 in the backbone network, then it shall also support IPv4. The IP addresses of GSNs and other backbone nodes of all PLMNs build a private address space that is not accessible from the public Internet. For the GGSN, S‑GW, P‑GW and SGSN, each of these IP addresses may also correspond to one or more DNS-type logical GSN names.

14.11.2 GSN Number

Each SGSN shall have an SGSN number for communication with e.g. HLR and EIR.

Each GGSN that supports the optional SS7-based Gc interface shall have a GGSN number for communication with HLRs.

14.12 RNC/BSC Addresses (Iu mode)

14.12.1 RNC/BSC Address

Each RNC or BSC shall have one or more IP addresses for inter-communication over the Iu interface. When the ATM transport option is applied on the Iu interface, each RNC or BSC shall be able to support addresses of type IPv4 and optionally of type IPv6. When the IP transport option is applied on the Iu interface, each RNC or BSC shall be able to support both IPv6 addresses and IPv4 addresses.

NOTE: These statements refer to RNC and BSC implementation requirements. When both IP versions are required to be supported in the RNC or BSC, it is still an operational choice whether to configure and use both or only one of the address types in a particular network set-up (i.e. in a network where it is known that all interconnected RNCs and SGSNs support the same IP version, it is legitimate to operate IPv4 only or IPv6 only).

14.12.2 RNC/BSC Number

Each RNC or BSC shall have an RNC/BSC number for inter-communication over the Iu interface.

14.13 Access Point Name

In the backbone, Access Point Name is a reference to the GGSN or P‑GW to be used. In addition, Access Point Name may, in the GGSN or P‑GW, identify the packet data network and optionally a service to be offered. Access Point Name is composed of two parts as defined in TS 23.003 [4].

The APN stored in the HLR shall not contain the APN Operator Identifier. A wild card may be stored in the HLR instead of the APN. This wild card indicates that the user may select an APN that is not stored in the HLR. The use of the wild card is described in Annex A.

14.14 Closed Subscriber Group ID

A CSG ID is a unique identifier within the scope of PLMN defined in TS 23.003 [4] which identifies a Closed Subscriber Group (CSG) in the PLMN associated with a CSG cell or group of CSG cells.