Query_1: What is GUTI and why it is needed?
  • It stands for Globally Unique Temporary Identifier
  • Assigned by MME during the Initial Attach procedure of the UE
  • If the UE transmits it's IMSI every-time when it wants to talk to the network, then any intruder can hack the interface and determine the IMSI. With this information the intruder can play the role of the original subscriber of the UE and fulfill his/her vested interest. Therefore, there was a need of a different Identity through which the network will recognize the UE. This solves the purpose of the GUTI. Therefore, in other words, GUTI is used for the identification of the UE which doesn't reveal the UE's permanent identity.
  • GUTI allows identification of the MME to which the UE attaches

Let us see the ingredients and their resemblance to construct GUTI
  • MMEI: MME Identifier (Identify a MME uniquely; operator configures in eNB)
  • MMEGI: MME Group ID (Unique within a PLMN, used to identify different MME Groups)
  • MMEC: MME Code (Identify a MME uniquely within a MME group)
  • GUMMEI: Globally Unique MME Identifier globally
  • M-TMSI: MME-Temporary Mobile Subscriber Identity
Operators make sure that MMEC is unique within the MME pool. A collection of MMEC makes MMEGI as depicted in the following diagram:

Therefore, the unique MME-Code and the MME-Group Identifier makes the MME-Identifier. From the MMEI, a particular MME can be identified. Hence,

Now, MME-Identifer and PLMN makes the Globally Unique MME-Identifier. Hence,

Query_2: What is M-TMSI? 

  • Identifies the UE within the MME
  • MME allocates M-TMSI to UE, to make M-TMSI, MME uses the UE IMSI
  • Signalled to UE within the ATTACH ACCEPT or TAU ACCEPT message
  • M-TMSI changes for the UE, when the UE moves to another MME
  • is 32 bit
Hence, using GUMMEI and M-TMSI, GUTI is constructed.

Query_3: What is S-TMSI?
  • stands for SAE-TMSI
  • a shorter variant of GUTI
  • used to locally identify a UE within a MME group
  • used in Paging the UE
S-TMSI (40 bits) = MMEC (8 bits) + M-TMSI (32 bits)

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