Wednesday, May 5, 2010

Additional GUTI and additional P-TMSI

The release 8 specifications talk about sending additional GUTI / additional P-TMSI in attach reqeusts and Tracking Area Update (TAU) or Routing Area Update (RAU) requests. So what exactly is the purpose of them?
As mentioned in the previous post, an EPS capable UE can have itself registered both in an MME and an S4-SGSN. In that case the UE's USIM will have a valid GUTI from the MME and P-TMSI and P-TMSI signature from the SGSN.
Whenever the UE does a new attach into an E-UTRAN network (or) UTRAN/GERAN network, it has two temporary identifiers. It informs one of them as an additional identifier to the MME / SGSN. Lets consider each of the cases

Attaching to MME
The UE has a valid GUTI and a valid P-TMSI. TIN = P-TMSI
In the attach request:
  • Old GUTI = GUTI mapped from P-TMSI.
  • Additional GUTI = GUTI. P-TMSI signature is also sent in this case
The UE has a valid GUTI and a valid P-TMSI. TIN = GUTI / RAT related TMSI
In the attach request:
  • Old GUTI = GUTI at UE. No additional GUTI is sent
So what is the purpose of sending the additional GUTI in the first case above?
Since the old GUTI received by the MME is a GUTI mapped from P-TMSI, ideally it has to query the UE's identity from an S4 SGSN. But why do it if is possible for the MME to locate the subscriber's context within itself? To locate the subscriber's context it needs a GUTI that it allocated to the UE. So the additional GUTI helps (provided that additional GUTI was allocated by this MME).

Attaching to S4-SGSN
The UE has a valid GUTI and a valid P-TMSI. TIN = GUTI
In attach request:
  • Old P-TMSI = P-TMSI mapped from GUTI.
  • Additional P-TMSI = P-TMSI
The UE has a valid GUTI and a valid P-TMSI. TIN = P-TMSI or RAT related TMSI
In attach request:
  • Old P-TMSI = P-TMSI.
  • Aditional P-TMSI is also = P-TMSI if TIN = P-TMSI. The P-TMSI signature is also sent.
The usage of additional P-TMSI in the first case to look up the subscriber's context locally at SGSN instead of going to MME.
The same logic applies to TAU and RAU procedures as well.

3 comments:

fluotone said...

great blog! was searching for RAU timer to learn about it and google brough me here and I learnt more than RAU timers =)

cheers!

Santanu said...

Hi,

Suppose a mobile is attaching to S4 SGSN and (TIN = GUTI) the UE has a valid GUTI, PTMSI and RAI. In Attach Request, it converts GUTI to PTMSI, PTMSI signature and RAI (according to spec 24.008) and populates the PTMSI in Additional Mobile Identity. What does the P-TMSI signature signify here? What will the SGSN do with the P-TMSI signature (Old P-TMSI signature IE in Attach Request)?

If the P-TMSI signature is derived from GUTI, what is its use? can u plz explain?

Cheers!

mah said...

hi I have a basic question that how UE will select an attach type from the EPS/IMSI/combined attach in the attach request message and on which criteria it is based.
This blog is good to clera the understanding.
looking for the clarification thanks