Friday, November 20, 2009

Knowing GUTI - Globally Unique Temporary ID

In LTE the GUTI is allocated to the UE by the MME and has two components. These are the GUMMEI (Globally Unique MME ID) and the M-TMSI (MME-TMSI). While the GUMMEI identifies the MME, the M-TMSI identifies the UE within the MME.

In GPRS and UMTS the mobile's temporary id was the (P-TMSI) Packet Temporary Mobile Identity. This id is changed on a frequent basis and used instead of the IMSI (The International Mobile Subscriber Identification) in most air interface messages for security reasons.

In LTE, the P-TMSI is now called the Globally Unique Temporary ID, or the GUTI. Some of the digits in the GUTI identify the Mobility Management Entity the mobile was last registered with and they are referred to as the Globally Unique MME Identifier, or the GUMMEI.

When contacting the network, the mobile sends the GUTI to the base station which then uses the parameter to identify the MME to which it will send the request to re-establish the communication session.It's also possible to roam between different radio technologies. If the mobile has reselected from a UMTS cell to an LTE cell, a TAU is made and since the mobile does not have a GUTI, the P-TMSI is sent instead.This way, the newly assigned MME can contact the 3G SGSN to request the subscribers current profile (IP address, PDP contexts, etc.).

The same mechanisms apply when the mobile reselects from an LTE cell to a UMTS or GPRS cell. In this case the GUTI is sent in the P-TMSI parameter and the procedure is reffered to as Routeing Area Update (RAU) instead of TAU.

The GUTI has two main components:
  • one that uniquely identifies the MME which allocated the GUTI; and
  • one that uniquely identifies the UE within the MME that allocated the GUTI.
The Globally Unique MME Identifier (GUMMEI) is constructed from the MCC, MNC and MME Identifier (MMEI). Within the MME, the mobile is identified by the M-TMSI.

Finally the GUTI is constructed from the GUMMEI and the M-TMSI.


18 comments:

kobu said...

good reference...

Vishwanth said...

Hi.. Interesting article.. Thanks for that info!!

Have a doubt.. Does the GUTI gets stored in the UE as well?

2nd question.. Whenever the UE attempts to make a call on the same MME again it uses GUTI to make the call right?

Vikram said...

Hi,

GUTI is stored in UE.
UE just forwards GUTI & GUMMEI to RRC layer to make a call.
eNB from GUMMEI finds the MME where this UE is attached. If eNB is not associated with MME, eNB will chose new MME. and MME may get context information from old MME using same GUTI.

fermin said...

hi there vikram,

I am trying to undertand at what point is the DNS query request sent is it from EnodeB or MME where does DNS comes into the call flow either userPlane or back plane I am bit confused..
please advice.

Vikram said...

Hi,

I dont understand why DNS should be involved, eNBs will have static routes to MME, and eNB should be in advance configured with MMEs which he is supposed to communicate

Muhammad said...

thanks for these info

could you please explain what is the difference between GUTI and S-TIMSI, since you're saying that in the NAS procedures GUTI will be used so what the use of S-TMSI

Vikram said...

There will be some UE's capable of handling circuit and packet services simultaneously. So during CSFB services S-TMSI will be used

Nagaraj said...

For paging purposes, the mobile is paged with the S-TMSI. The S-TMSI shall be constructed from the MMEC and the
M-TMSI

Tomek said...

S-TMSI = SAE TMSI is little shorten then GUTI that is used for iidentify. S-TMSI is included in the RRC connection req. and paging messages.

S-TMSI = MMEC = MMEI [ S-TMSI
does not include the MMEGI]

TRomek

Anbu said...

Hi Vikram,

Where do we use NAS mac and sequence number,

vikram korde said...

Hi Anbu,

They are used by NAS security and authentication algorithms.

Javed $k !! said...

Hi,

Can S-TMSI be used while performing a Tracking Area Update (Inter eNB-Inter MME) ?
I have seen it S-TMSI in Service request(Inter eNB-Inter MME) also.
How does the UE decides whether to send M-TMSI or S-TMSI during TAU or Service Request ?

Thanks !!

kmnaveen007 said...

Thanks a lot for all info...tis really is a very gud website... i had to do project related to LTE nd tis site really helped me :D

Ankur Z said...

@ TOMEC CAN U PLS EXPLAIN UR COMMENTS
S-TMSI = MMEC = MMEI [ S-TMSI
does not include the MMEGI]

AS PER MY UNDERSTANDING

Globally Unique MME Identifier (GUMMEI)
The format and size of the GUTI is:
GUTI = GUMMEI + M-TMSI, where
GUMMEI = MCC + MNC + MME Identifier and
MME Identifier = MME Group ID + MME Code
MCC and MNC shall have the same field size as in earlier 3GPP systems.
M-TMSI shall be of 32 bits length.
MME Group ID shall be of 16 bits length.
MME Code shall be of 8 bits length.
S-TIMSI=32 BIT

Unknown said...

Hi Vikram,

I wonder in which condition, the MME may update the GUTI for the UE.
Thanks

syed Saqlain said...

Hi,

I dont quite understand how voice fall back to 2G/3G works on LTE. Is there a seperate interface between MME and MSS? Which node maintains the VLR information. MSS or MME?

Sun said...

hi,

if the MCC+MNC is different from the imsi ,is there any problem ?

Roberto H. said...

Just one question to anyone, is the GUTI number actually store in the Ue phone or in the SIM?