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Identities of a Dual-Mode Phone

This post is not meant to be comprehensive but I shall attempt to cover the most important identities used to identify the UE/MS. It has been a difficult task the reason being that information is spread across many specifications. If there are errors, do let me know. I also appreciate assistance on making this list complete.

Identities are summarized in Table 1. Shaded cells refer to group identities. Many of these identities can exist at the same time. Some identities are mutually exclusive. For example, TLLI cannot exist at the same time as C-RNTI. The former’s context is with the GPRS CN and the latter’s is within UTRAN.

An RNTI (Radio Network Temporary Identifier) exists only when the UE has an RRC connection. Depending upon the context and allocator, different RNTIs exist as listed in Table 1. RNTIs are deallocated when RRC connection is released.

Table 1: UE/MS Identities

Identity

Description

Length

(bits/

digits)

Context

C-RNTI

Cell Radio Network Temporary Identifier

  • Allocated by the CRNC.
  • Unique within the cell controlled by the CRNC.
  • Can be reallocated when UE changes cell with a Cell Update procedure.
  • Deallocated when RRC connection is released
  • Used by MAC to address a UE-UTRAN connection when DCCH/DTCH are mapped to common transport channels.
  • Can be used by MAC on UL and DL transport channels.

16

UE, UTRAN

DSCH-RNTI

Downlink Shared Channel Radio Network Temporary Identifier

  • Used only for TDD.
  • Used on the SHCCH.

16

UE, UTRAN

E-RNTI

E-DCH Radio Network Temporary Identifier

  • For E-DCH (HSUPA) only.
  • Two variants are possible: primary and secondary: both can be active at the same time.
  • RRC signalling configures the UE to use either primary or secondary.
  • Identity is used by PHY to address the UE on E-AGCH. Identity is used as a mask on the 16-bit CRC.

16

UE, UTRAN

G-RNTI

GERAN Radio Network Temporary Identifier

  • Applicable from Release 5 onwards for GERAN Iu Mode.
  • Allocated by RRC in the Serving BSC and unique within the BSC.
  • Reallocated when the SBSC changes
  • Used at RLC/MAC during contention resolution. If there is no allocation, a random G-RNTI is used; otherwise the allocated identity is used.
  • Deallocated when RRC connection is released.
  • Derived from TLLI codespace. Used on Iu interface and quite similar in purpose to TLLI used on A/Gb.

UE, GERAN, CN

H-RNTI

HS-DSCH Ratio Network Temporary Identifier

  • For HSDPA only.
  • Identity is used by PHY to address the UE on HS-SCCH. Identity is used as a mask on the 16-bit CRC.

16

UE, UTRAN

IMEI

International Mobile Station Equipment Identity

  • Unique number allocated to each Mobile Equipment (ME) in the PLMN.
  • Unconditionally implemented by the MS manufacturer.
  • Generally used to block stolen mobiles. IMEI can be changed but it is illegal in some countries (UK for example).
  • Composed of Type Allocation Code (TAC) + Serial Number (SNR) + spare digit = 8 + 6 + 1= 15 digits.
  • IMEI will be used for emergency call establishment and re-establishment only when SIM/USIM and IMSI are not available.

15

UE, CN

IMEISV

International Mobile Station Equipment Identity with Software Version Number

  • Similar to IMEI; composed of Type Allocation Code (TAC) + Serial Number (SNR) + Software Version Number (SVN) = 8 + 6 + 2= 16 digits.
  • Optionally included by the mobile in Authentication and Ciphering Response.

16

UE, CN

IMGI

International mobile group identity

??

??

??

IMPI

IP Multimedia Private Identity

  • Used in IMS.
  • Could be based on IMSI if not allocated explicitly.
  • Of the form “user@domain”.

Textual

UE, IMS

IMPU

IP Multimedia Public identity

  • Used in IMS.
  • Of the form “sip:user@domain”.

Textual

UE, IMS

IMSI

International Mobile Subscriber Identity

  • Variants exist based on the type of CN: IMSI-DS-41, IMSI-GSM-MAP, IMSI-and-ESN-DS-41. Here we are refer to IMSI-GSM-MAP when we say IMSI.
  • Allocated to each mobile subscriber in the GSM/UMTS system.
  • ITU-T refers to this as International Mobile Station Identity.
  • This is the common identity across both CN domains.
  • RAN uses it for coordination with UE. CN transfers identity to RAN using RANAP. RAN deletes it when RRC connection is released.
  • Consists of MCC (3 digits) + MNC (2 or 3 digits) + MSIN.
  • Used by RRC for CN originated paging.
  • Generally avoided on the air interface in lieu of TMSI or P-TMSI unless the latter is not recognized by the CN or not available at the mobile.
<= 15

UE, CN

IMUN

International Mobile User Number

  • A diallable number allocated to a 3GPP System user.
  • Not sure how this is used.

??

??

IUI

International USIM Identifier

??

??

??

LMSI

Local Mobile Station Identity

  • Allocated by the VLR and mapped to IMSI.
  • Used for communication with HLR although HLR makes no use of it. HLR merely sends it back in its responses.
  • Facilitates easy search of database at the VLR.
  • Advantage is that TMSI could be reallocated while LMSI is persistent.

32

VLR

MSID

Mobile Station Identifier

??

??

??

MSIN

Mobile Station Identification Number

  • Part of IMSI.
  • Identifies the mobile within the PLMN.
  • The leading digits may in some cases used by the PLMN to identify quickly the HLR.
  • If MNC has just 2 digits, this identity has a leading zero.

<= 10

UE, CN

MSISDN

Mobile Station/Subscriber ISDN Number

  • Number by which the mobile can be called.
  • Composed of Country Code (CC) + National Destination Code (NDC) + Subscriber Number (SN).
  • An NDC is required for each PLMN. Some PLMNs may require more than one NDC.
  • Parts of the identity are used by SCCP for routing messages to the HLR.

Variable??

UE, CN, PSTN/ISDN

MSRN

Mobile Station Roaming Number

  • Allocated by the VLR.
  • Used to route calls to the mobile.
  • Passed by the HLR to GMSC for routing purpose.
  • Identity addresses the MSC/VLR where the mobile currently has a context.
  • Has a structure similar to MSISDN.
  • A mobile may have more than one MSRN.

Variable??

UE, CN

MUI

Mobile User Identifier

??

??

??

NMSI

National Mobile Station Identifier

  • Part of IMSI.
  • Composed of MNC + MSIN.
  • Managed within relevant national bodies.

<= 12

UE, CN

NUI

National User / USIM Identifier

Network User Identification

??

??

??

P-TMSI

Packet Temporary Mobile Subscriber Identity

  • Variants exist based on the type of CN: P-TMSI-GSM-MAP, P-TMSI-and-RAI-GSM-MAP. Here we refer to P-TMSI-GSM-MAP when we say P-TMSI.
  • Used instead of IMSI to implement subscriber confidentiality in the PS domain.
  • Allocated by SGSN which maps it to the IMSI.
  • Extensively used by GMM.
  • Generally reallocated when Routing Area changes. Has local context within a RA, outside which it has to be combined with RAI.
  • Used by RRC for CN originated paging. If it exists, preferred over IMSI for packet paging.
  • A P-TMSI signature exists to enable GMM establish the context of a mobile.

32

UE, CN

S-RNTI

SRNC Radio Network Temporary Identifier

  • Allocated by the SRNC and unique within the SRNC.
  • Reallocated during SRNC Relocation.
  • Deallocated when RRC connection is released.
  • A shorted version (10 bits) exists for signalling purpose only during a handover to UTRAN.

20

UE, UTRAN

TLLI

Temporary Logical Link Identity

  • Used on the GPRS air interface (RLC/MAC) to address the mobile.
  • Allocated by the MS based on P-TMSI (local or foreign TLLI) or directly (random TLLI).
  • Auxiliary TLLI is allocated by the SGSN but this is no longer required from R99 onwards.
  • Part of the TLLI codespace is reserved for G-RNTI.
  • Used by LLC in SGSN to uniquely identify the MS.
  • TLLI along with NSAPI is used by SGSN to identify the PDP context to which a packet belongs.

32

UE, GERAN, CN

TMGI

Temporary Mobile Group Identity

  • Used to identify an MBMS Bearer Service.
  • Allocated by BM-SC.
  • Used for either broadcast or multicast service.
  • This is the radio resource equivalent of MBMS Bearer Service Identification consisting of IP multicast address and APN.
  • Contains 24 bits of MBMS Service Id and optionally MCC (12 bits) MNC (12 bits) if the session is from a non-local PLMN.
  • Identity is used by RR/GRR for packet paging procedures for MBMS (pre-)notification. An optional MBMS Session Identity is used along with TMGI if notification is done on A/Gb mode.
  • Identity used by Layer 3.

24, 48

UE, CN

TMSI

Temporary Mobile Subscriber Identity

  • Variants exist based on the type of CN: TMSI-DS-41, TMSI-GSM-MAP, TMSI-and-LAI-GSM-MAP. Here we are refer to TMSI-GSM-MAP when we say TMSI.
  • Used instead of IMSI to implement subscriber confidentiality in the CS domain.
  • Allocated by VLR which maps it to the IMSI.
  • Extensively used by MM.
  • Generally reallocated when Location Area changes. Has local context within an LA, outside which it has to be combined with LAI.
  • Used by RRC for CN originated paging.
  • Format of TMSI is not standardized.

32

UE, CN

U-RNTI

UTRAN Radio Network Temporary Identifier

  • Identifies the UE uniquely within UTRAN.
  • Made of two parts: SRNC identifier (12 bits) + S-RNTI (20 bits).
  • Used to identify UE in many RRC procedures in CELL_FACH (Cell Update, URA Update, UTRAN Originated Paging).
  • Used by MAC to address a UE-UTRAN connection when DCCH/DTCH are mapped to common transport channels.
  • Not used by MAC in the UL but can be used by RRC. Used by MAC only in the DL when DCCH is mapped to FACH for SRB1 (alternative is the C-RNTI).
  • During a handover to UTRAN, RRC signalling uses U-RNTI-Short in which the S-RNTI component has only 10 bits. The shortened S-RNTI is expanded to 20 bits to (MSBs set to zeros) and the final U-RNTI is generated.
  • Rel5 RRC signalling uses this in a modified form to address a group of UEs for RRC connection release.

32

UE, UTRAN

Some identities need explanation. Since C-RNTI and U-RNTI are usually meant for use in CELL_FACH states, it may be puzzling to note that these can be reallocated in CELL_DCH state. Such a reallocation will generally happen in the case of handovers when the CRNC or SRNC change. The UE will merely store the identities until a later time when they may be required. For example, after the handover the channel conditions may worsen triggering a Cell Update procedure on CCCH. This procedure will then make use of the U-RNTI within RRC Cell Update message. The Cell Update Confirm may be on CCCH or on DCCH. In the former case, the U-RNTI will be returned within the message. In the latter case, U-RNTI is used at MAC. Either way, Cell Update Confirm may in its turn allocate new identities.

Specifications [TS 24.008] define the conditions under which the mobile shall use IMEI, IMSI, TMSI or P-TMSI. Some of these have been mentioned in Table 1. Specification titled “Numbering, addressing and identification” [TS 23.003] is a good source of information. The use of some of these identities by RR/GRR/RRC can be found in relevant specifications [TS 44.018, TS 44.060, TS 44.160, TS 25.331]. The use of E-RNTI and H-RNTI is mentioned in a PHY layer specification [TS 25.212]. Some details on TMGI is in an MBMS specification [TS 23.246].

Other important identities for the mobile are DLCI, XID, NSAPI and TI. Because these pertain more to connections at a specific layer rather than the MS/UE as a whole, they have been left out of this discussion.

 

 

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