14 Identities

03.603GPPGeneral Packet Radio Service (GPRS)Release 1998Service descriptionStage 2TS

14.1 IMSI

A unique International Mobile Subscriber Identity (IMSI) shall be allocated to each mobile subscriber in GSM. This is also the case for GPRS-only mobile subscribers, except for anonymous-only access subscribers. IMSI is defined in GSM 03.03 [4].

14.2 Packet TMSI

A Packet Temporary Mobile Subscriber Identity shall be allocated to each GPRS-attached MS. P‑TMSI is defined in GSM 03.03.

14.3 NSAPI and TLLI

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 SGSN, TLLI unambiguously identifies the logical link.

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

NSAPI is a part of the tunnel identifier (TID).

For example (shown figuratively below), an X.25 packet is received by the MS from a connected TE at the X.121 address SAP. The X.25 PDU is encapsulated and NSAPI is initialised to NSAPI-1. TLLI is set to the MS’s TLLI before the encapsulated X.25 packet is passed to the SNDC function.

Figure 52: 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 subclause "User Identity Confidentiality".

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, or when the MS originates an anonymous access. An Auxiliary TLLI is selected by the SGSN and is used by the SGSN and MS to unambiguously identify an Anonymous Access MM and PDP Context.

If the MS has a valid P‑TMSI associated with the RA where the MS is currently located, then 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, then 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, then 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 transmission plane, and within the GMM/SM layer in the signalling plane. NSAPI is represented by a transaction identifier (TI) in some SM signalling messages. The TI is dynamically allocated by the MS for MS-requested (AA) 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 GSM 04.07 and GSM 04.08.

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 PDP Address

A GPRS 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;

– an IP version 6 address; or

– an X.121 address.

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

14.5 TID

A Tunnel Identifier (TID) is used by the GPRS Tunnelling protocol between GSNs to identify a PDP context. A TID consists of an IMSI and an NSAPI. The combination of IMSI and NSAPI uniquely identifies a single PDP context.

The TID is forwarded to the GGSN upon PDP Context Activation and it is used in subsequent tunnelling of user data between the GGSN and the SGSN to identify the MS’s PDP contexts in the SGSN and GGSN. The TID 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 the anonymous access case, AA‑TID is allocated locally by the SGSN. An AA‑TID consists of an A‑TLLI and an NSAPI similar to TID. Since the IMSI is longer than A‑TLLI, the unused digits shall be used to create a unique identity within one PLMN. The allocated AA‑TID shall not collide with the TID address space.

14.6 Routeing Area Identity

Routeing Area Identity (RAI), defined by an operator, identifies one or several cells. RAI is broadcast as system information and is used by the MS to determine, when changing cell, if an RA border was crossed. If that was the case, the MS initiates the RA update procedure.

The location of an MS in STANDBY state is known in the SGSN on an RA level. Cells that do not support GPRS within an LA are grouped by the SGSN and BSS 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;

– LAI = MCC + MNC + LAC;

– RAI = MCC + MNC + LAC + RAC;

– CGI = LAI + CI.

14.7 Cell Identity

Cell Identity (CI) identifies one cell. CI is defined in GSM 03.03.

14.8 GSN Addresses

14.8.1 GSN Address

Each SGSN and GGSN shall have an IP address, either of type IPv4 or IPv6, for inter-communication over the GPRS backbone network. The IP addresses of GSNs and other GPRS backbone nodes of all PLMNs build a private address space that is not accessible from the public Internet. For the GGSN and the SGSN, this IP address may also correspond to one or more DNS-type logical GSN names.

14.8.2 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.

14.9 Access Point Name

In the GPRS backbone, Access Point Name is a reference to the GGSN to be used. In addition, Access Point Name may, in the GGSN, identify the external network. Access Point Name is composed of two parts as defined in GSM 03.03:

– the APN Network Identifier is mandatory and is a label (for example "corporation") or a set of labels separated by dots which is a fully qualified domain name according to the DNS naming conventions (for example "company.com"). In order to guarantee the uniqueness of the APN, the GPRS PLMN should allocate, to an ISP or corporation, an APN Network Identifier identical to their domain name in the public Internet. The APN Network Identifier shall not end with ".gprs";

– the APN Operator Identifier is optional. It is a fully qualified domain name according to the DNS naming conventions, and consists of three labels. The APN Operator Identifier shall end in ".gprs". For example, it may be "MNCyyyy.MCCzzzz.gprs". The exact format is defined in GSM 09.60.

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.