GUTI | Area and Subscriber Identities

The GUTI is assigned only by the MME during initial attach of a UE to the E-UTRAN.

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 E-UTRAN. It also allows identification of the MME and network to which the UE attaches. The GUTI can be used by the network to identify each UE unambiguously during signaling connections.

The GUTI has two main components: the Globally Unique Mobility Management Entity Identifier (GUMMEI) that uniquely identifies the MME which allocated the GUTI; and the M-TMSI that uniquely identifies the UE within the MME that allocated the GUTI. The GUMMEI is constructed from the MCC, MNC, and Mobility Management Entity Identifier (MMEI).

The MMEI should be constructed from a Mobility Management Entity Group ID (MMEGI) and a MMEC. The GUTI should be constructed from the GUMMEI and the M-TMSI as shown in Figure 1.

 

Figure 1: Format of GUTI and S-TMSI

For paging purposes, the mobile is paged with the S-TMSI. The S-TMSI is constructed from the MMEC and the M-TMSI. It is correct to say that the S-TMSI is a shorter format of GUTI that can be used because, after successful registration of a UE, the serving network as well as the serving MME group are known and stored in the core network databases.

The operator needs 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 should be used to support subscriber identity confidentiality and, in the shortened S-TMSI form, to enable more efficient radio signaling procedures (e.g., paging and service request).

MCC and MNC should have the same field size as described for the IMSI.

The M-TMSI has a length of 32 bits, MMEGI is 16 bits in length, and MMEC 8 bits in length.

It is important to understand that on the S1 interface the IMSI is typically not seen, just like the GUTI. Exceptions are initial attach to the network when no old GUTI is stored on the USIM card or the true subscriber's identity is checked using NAS signaling, which regularly happens when roaming subscribers attach. Also, in the case of the paging procedure the IMSI might be seen.

For monitoring and network performance measurement the IMSI on S1 can only be revealed if the changing temporary identities are tracked with a quite sophisticated architecture. Full IMSI tracking can only be ensured by monitoring all S1 interfaces of an operator's E-UTRAN and ideally all S6a interfaces and storing the current GUTI/IMSI relations in a central point as stored in the HSS.

IMSI | Area and Subscriber Identities

The IMSI allows unambiguous identification of a particular SIM or USIM card. The IMSI is composed of three parts (Figure 1):

• The Mobile Country Code (MCC), consisting of three digits. The MCC uniquely identifies the country of domicile of the mobile subscriber. MCC values are administrated and allocated by an international numbering plan.
• The Mobile Network Code (MNC), consisting of two or three digits for GSM/UMTS applications. The MNC identifies the home PLMN of the mobile subscriber. The length of the MNC (two or three digits) depends on the value of the MCC. A mixture of two- and three-digit MNC codes within a single MCC area is not recommended and is beyond the scope of this specification.
• The Mobile Subscriber Identification Number (MSIN), identifying the mobile subscriber within a PLMN. As a rule the first two or three digits of the MSIN reveal the identity of the Home Location Register (HLR) or HSS that is used for Signaling Connection Control Part (SCCP) Global Title translation procedures when roaming subscribers register in foreign networks.
  
  

 

Figure 1: Structure of IMSI (according to 3GPP 23.303). Reproduced with permission from © 3GPP™

The National Mobile Subscriber Identity (NMSI) consists of the MNC and the MSIN.

A combination of MCC and MNC can be used to aggregate call-specific performance measurement data (such as cumulative counters) on IMSI groups. This will help to highlight problems of roaming subscribers such as network failures during registration procedures, as described later in this book. Table 1 shows some samples from an IMSI group mapping table with MCC/MNC combinations in "IMSINumber" fields and operator names in the "IMSIGroupName" field. Note the three-digit MNC used for the American operator.

Table 1: IMSI group mapping table from Tektronix Communications NSA software

<IMSI IMSINumber= ‘26201’ IMSIGroupName= ‘T-MOBILE DEUTSCHLAND GMBH (GERMANY)’ />

<IMSI IMSINumber= ‘26202’ IMSIGroupName= ‘VODAFONE D2 GMBH (GERMANY)’ />

<IMSI IMSINumber= ‘26801’ IMSIGroupName= ‘VODAFONE TELECEL (PORTUGAL)’ />

<IMSI IMSINumber= ‘27201’ IMSIGroupName= ‘VODAFONE IRELAND PLC (IRELAND)’ />

<IMSI IMSINumber= ‘310560’ IMSIGroupName= ‘T-MOBILE USA, INC. (UNITED STATES)’ />

It is possible that one-use equipment will work with more than just one (U)SIM. A good example is a mobile phone that has both business and private SIM cards as one device. Depending on the nature of the call (private or business), the owner of the handset can choose which (U)SIM should be used to make the call. Such a procedure might be required in case private phone calls need to be charged separately due to national income tax laws (as found, for example, in Germany).

Domains and Strati | Area and Subscriber Identities

For the EPC a complete new NAS was designed including a new NAS protocol layer described in 3GPP 24.301.

In contrast to the core network of 3GPP Release 99 to Release 6 where a CS and PS domain were defined as subdomains of the serving network domain, the EPC will not host any CS domain due to its all-IP character. However, it still distinguishes between AS and NAS signaling and functions as shown in Figure 1.

 

Figure 1: Domains and strati in E-UTRAN and EPC

The AS comprises the radio chipset of the UE including the RRC protocol entity and all underlying transport layer entities. Here all parameters that more or less frequently change during radio access can be found, including transport formats and radio-specific identities of serving cell and possible handover candidates (neighbor cells).

The NAS covers all signaling exchanged between the USIM (UMTS Subscriber Identity Module) and the core network node, in case of LTE radio access: the MME. This is the home of all parameters that allow unambiguous identification of a subscriber or the handset hardware such as International Mobile Subscriber Identity (IMSI) and International Mobile Equipment Identity (IMEI). There are also temporary identities stored on the USIM card like Temporary Mobile Subscriber Identity (TMSI) and Globally Unique Temporary UE Identity (GUTI). From a protocol point of view the NAS is the home of network access, initial subscriber registration, and mobility management procedures. Due to the all-IP concept of LTE/EPC, a new NAS protocol was defined, namely 3GPP 24.301, while similar functions for 2G/3G networks are defined in the standard 3GPP 24.008. The E-UTRAN NAS protocol 3GPP 24.301 does not contain any functions for CS call control and SMS. In the early planning stages of the E-UTRAN it was assumed that all speech services via the E-UTRAN would use VoIP and the IMS architecture. As an alternative the CS fallback option (implemented in the S1AP protocol) was designed, but obviously this did not satisfy the need for reliable and cost-efficient CS services in the E-UTRAN. Hence, an initiative formed of operators and Network Equipment Manufacturers (NEMs) started to work on the Voice over LTE via Generic Access standards (VoLGA). VoLGA is beyond the scope of 3GPP. Its principle is to establish an IP connection between the UE and E-UTRAN and use the radio bearer for transparent forwarding of 3GPP 24.008 NAS signaling message and AMR (Adaptive Multirate) voice packets across the logical Z1 interface. Instead, in the S-GW the RAB used for VoLGA is terminated in a special protocol converter and media gateway device, the VoLGA Access Network Controller (VANC), that is, the interconnecting point between the E-UTRAN/EPC and UTRAN/GERAN/Legacy Core Network.