LTE Identifiers: Types, Structure, and Purpose

Have you ever wondered how a mobile network knows who you are, which tower you’re using, or how your data is routed? In LTE networks, there are many special codes and numbers called identifiers that help the system keep track of everything: users, cells, devices, data sessions, and even locations. These identifiers make sure that calls, texts, and internet sessions reach the right people at the right time without any mix-ups.

Classification of LTE Identifiers

LTE identifiers can be broadly categorized into:

• UE Identifiers
• Cell Identifiers
• Bearer and Session Identifiers
• Mobility Identifiers
• Network Identifiers

UE Identifiers

IMSI (International Mobile Subscriber Identity)
A globally unique identifier assigned to each mobile subscriber, used for authentication and mobility management.

Structure:

• MCC (Mobile Country Code): 3 digits (identifies country, e.g., 404 for India)
• MNC (Mobile Network Code): 2 or 3 digits (identifies operator, e.g., 93 for Vodafone Idea)
• MSIN (Mobile Subscriber Identification Number): Remaining digits, up to 10 (identifies subscriber within the PLMN)
• Total length: Typically, 15 digits

Example: 404930123456789

Use:

• Permanently stored in SIM (USIM)
• Sent to network only if no valid temporary identity is available
• Used for initial attach, authentication, and lawful interception

Reference: Netmanias

GUTI (Globally Unique Temporary Identifier)
A temporary identifier allocated by the MME to a UE, providing subscriber anonymity by avoiding transmission of the IMSI over the air.

Structure:

• PLMN ID: MCC + MNC
• MMEGI (MME Group ID): Identifies a group of MMEs within the PLMN
• MMEC (MME Code): Uniquely identifies an MME within the group
• M-TMSI (MME Temporary Mobile Subscriber Identity): Temporarily assigned to the UE by the MME

GUTI = MCC + MNC + MMEGI + MMEC + M-TMSI

Example: 40493-0021-12-0A1B2C3D

Use:

• Assigned during attach or TAU procedures
• Used for NAS signaling and paging to minimize IMSI exposure
• Enables fast and secure UE identification during mobility

Reference: Netmanias

TMSI (Temporary Mobile Subscriber Identity)
A locally unique, temporary identifier for the UE within a serving area (Tracking Area), assigned by the MME as part of the GUTI.

Structure:

• TMSI: 32 bits, part of the M-TMSI field in GUTI

Use:

• Assigned during initial attach or TAU procedure
• Used in place of IMSI for subsequent signaling within the same tracking area
• Enhances user privacy and reduces risk of IMSI interception

S-TMSI (SAE Temporary Mobile Subscriber Identity)
A shortened version of the GUTI, used for efficient identification during paging and tracking area update (TAU) procedures.

Structure:

• S-TMSI = MMEC + M-TMSI

• MMEC (MME Code): 8 bits
• M-TMSI: 32 bits

Use:

• Carried in Paging messages and TAU requests
• Enables quick, temporary UE identification by the network without needing full GUTI or IMSI
• Used for identifying UE during emergency call procedures as well

Cell Identifiers

Cell identifiers are used to uniquely identify each radio cell in the LTE network, which is essential for mobility, handover, and efficient use of network resources.

ECGI (E-UTRAN Cell Global Identifier)

ECGI uniquely identifies each LTE cell across the globe. It ensures that every cell, even in different countries or operators, can be distinctly referenced.

Structure:

• PLMN ID: Combination of MCC (Mobile Country Code) and MNC (Mobile Network Code) uniquely identifies the operator.
• ECI: E-UTRAN Cell Identifier, a unique value for each cell within the operator’s network.

Example: 40493_00001ABC
(Here, “404” is MCC for India, “93” is MNC for Vodafone Idea, “00001ABC” is the unique cell code.)

Use:

• Used in signaling for mobility events like handovers (when a device moves from one cell to another).
• Maintains neighbor cell lists for reselection and handover decisions.
• Used in tracking which cell a UE is attached to.

ECI (E-UTRAN Cell Identifier)

ECI is a unique 28-bit identifier for a cell within a specific PLMN (operator network). No two cells under the same PLMN will share the same ECI.

Bit Length: 28 bits (allows more than 268 million unique cells per operator).

Use:

• Provides uniqueness to each LTE cell under the same network.
• Helps the core network, O&M, and management systems in cell tracking and statistics.

PCI (Physical Cell ID)

PCI is a local identifier used on the radio interface to distinguish different cells in the same area, mainly for UE cell search and physical-layer operations.

Range: 0 to 503 (504 unique values)

Use:

• Essential during cell search and synchronization by the UE when it powers on or moves.
• Supports efficient radio planning, minimizing confusion between adjacent cells.
• Used in broadcast channels and physical-layer reference signals.

Bearer and Session Identifiers

Bearers define logical paths for user data within LTE, with each having specific quality (QoS) requirements. Identifiers help in mapping, management, and forwarding of data.

EPS Bearer ID (EBI)

EBI uniquely identifies each EPS bearer (data tunnel) between the UE and the core network.

Range: 5 to 15 (IDs 1–4 is reserved for other purposes; 11 bearers possible per UE)

Use:

• Every IP flow (e.g., web, VoLTE call) gets a specific bearer for QoS management.
• Used by eNodeB, MME, and SGW/PGW for correct data forwarding.

TEID (Tunnel Endpoint Identifier)

TEID is a 32-bit identifier used in GTP-U protocol to distinguish user-plane tunnels on S1-U (eNodeB to SGW) and S5/S8 (SGW to PGW) interfaces.

Use:

• Ensures correct delivery of user data packets to the appropriate context or session.
• Uniquely identifies the data tunnel per user/session, avoiding mix-up between users’ traffic.

Mobility Identifiers

These identifiers track the UE’s location and facilitate mobility management as the UE moves across cells and areas.

TAI (Tracking Area Identity)

TAI uniquely identifies a Tracking Area, which is a group of cells within the LTE network.

Structure:

• MCC (3 digits): Identifies the country.
• MNC (2 or 3 digits): Identifies the operator.
• TAC (Tracking Area Code): 16-bit code for each tracking area within the operator’s network.

Use:

• Used for paging UEs, tracking location at a regional level.
• Key part of the Tracking Area Update (TAU) procedure to optimize signaling and UE reachability.

TAC (Tracking Area Code)

TAC is a 16-bit field within TAI that uniquely identifies each tracking area inside a PLMN.

Use:

• Helps network to group cells for efficient mobility, paging, and load balancing.

Network Identifiers

These uniquely identify networks and core network elements to route signaling and user data properly.

PLMN ID (Public Land Mobile Network Identity)

PLMN ID is a global identifier for every mobile operator’s network.

Structure:

• MCC (Mobile Country Code): 3 digits
• MNC (Mobile Network Code): 2 or 3 digits

Example: 404 93 (India, Vodafone Idea)

Use:

• Included in all identifiers above (ECGI, TAI, etc.).
• Allows UEs and other networks to distinguish between different operators.

MME ID

Identifies a specific Mobility Management Entity (MME), which manages mobility and session control for UEs.

Structure:

• MME Group ID: Groups MMEs for redundancy/load balancing.
• MME Code: Uniquely identifies each MME within the group.

Use:

• Used in signaling between UEs, eNodeBs, and the core network to direct messages and procedures to the correct MME.
• Important for handover, attach, TAU, and session management.

Reference:

• 3GPP TS 23.003:
  Defines numbering, addressing, and identification schemes used in LTE networks.
• 3GPP TS 36.331:
  Specifies the Radio Resource Control (RRC) protocol and associated identifiers for LTE.
• 3GPP TS 36.413:
  Details S1AP signaling procedures and cell/user identifiers for LTE core and RAN.
• 3GPP TS 29.281:
  Describes the structure and use of TEID in the GTP-U protocol for data tunnels.
• LTE – The UMTS Long Term Evolution (Sesia, Toufik, Baker):
  Provides an in-depth textbook explanation of LTE architecture and identifiers.
• Netmanias