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4G LTE Identifiers:IMSI,NAI,IMEI,GUTI,APN,ECGI,ECI,MSISDN

By RF Wireless Expert Team

This page explains different identifiers used in 4G LTE and their purposes and structures or formats. It covers IMSI, NAI, IMEI, GUTI, APN, ECGI, ECI and MSISDN.

IMSI

  • Full form : International Mobile Subscriber Identity
  • Purpose : Unique identity of a subscriber in the mobile network. Used for authentication and mobility management.
  • Structure : (Up to 15 digits : MCC (Mobile Country Code)-3 digits, MNC(Mobile Network Code)-2 to 3 digits, MSIN-remaining digits)
  • Example : 310150123456789 (US operator)

NAI

  • Full form : Network Access Identifier
  • Purpose : Used in IP-based authentication (e.g. EAP-AKA/EAP-SIM) for connecting to packet data networks or Wi-Fi offload.
  • Structure : (username@realm)
    • It is derived from IMSI or GUTI
  • Example : 123456789012345@operator.com

IMEI

  • Full form : International Mobile Equipment Identity
  • Purpose : Identifies the mobile device (not the subscriber). Helps track stolen phones and block devices.
  • Structure (15 digits): TAC-Type Allocation Code (8 digits), SNR-Serial Number (6 digits), CD-Check Digit (1 digit)

GUTI

  • Full form : Globally Unique Temporary Identifier
  • Purpose : Temporary subscriber identity assigned by MME to protect the permanent IMSI. Used during mobility.
  • Structure : GUTI = GUMMEI + M-TMSI

APN

  • Full form : Access Point Name
  • Purpose : Identifies the packet data network (PDN) a UE wants to connect to.
  • Structure :
    • Network identifier: service name (e.g. internet, ims)
    • Operator Identifier : operator domain (e.g. mnc001.mcc310.gprs)

ECGI

  • Full form : E-UTRAN Cell Global Identifier
  • Purpose : Unique identifier of an LTE cell. Used in mobility management and handovers.
  • Structure : ECGI = PLMN-ID + ECI
    • PLMN-ID: MCC (3 digits) + MNC (2–3 digits)
    • ECI (E-UTRAN Cell Identifier): 28 bits

ECI

  • Full form : E-UTRAN Cell Identifier
  • Purpose : Identifies a specific cell within an eNodeB.
  • Structure (28 bits):
    • eNB ID: 20 bits
    • Cell ID (within eNB): 8 bits

MSISDN

  • Full form : Mobile Station International Subscriber Directory Number
  • Purpose : The subscriber’s phone number, used for routing calls/SMS.
  • Structure (up to 15 digits):
    • CC (Country Code): 1–3 digits
    • NDC (National Destination Code): identifies operator
    • SN (Subscriber Number): unique within operator

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