Tuesday, February 9, 2010

LTE Tidbits-II

Few more things are LTE below. Find part 1 here.

UE network attach:

We all know how important IMSI is and we wouldn't want to spill it out anytime. This requirement brings in a second type of attach procedure in LTE. Usually attach request from UE contains IMSI so that network can validate the same. Once network knows the UE's IMSI, it will generate a Globally Unique Temporary Identifier (Find details here) and assign it to UE. So for further communication UE can use GUTI instead of IMSI.

Why this? The Attach Request sent to the network is not encrypted. Security procedures are done only after network receives Attach Request. This means if IMSI is sent in Attach Request its susceptible to be known to outsider. To avoid this UE may send Old GUTI to network in Attach Request(The term "Old" is used here because it is assumed that UE is moving from one MME to other MME). Once the network receives that GUTI it will map the IMSI from it. Problem solved. What if the network doesn't know the GUTI, then it will have to ask the UE to send its IMSI. Network sends out an Identity Request to UE asking for its IMSI, to which UE will respond with Identity Response with its IMSI. Note that this message is not encrypted.

GUTI is assigned by the network. Which means UE should send its IMSI to network at-least once. If UE moves from one MME to other then its responsibility of MME to get the UE's IMSI from other MME based on the GUTI which UE has sent. Identity Request (for IMSI) should be sent only if MME fails to retrieve the IMSI.

Inter Rat Handovers:

There is special section in 3GPP TS 23.401 which we tend to ignore. Appendix. :-) If we look at inter handover requirements it is quite clear that SGSN's should be upgrade with new version of GTP protocol. But this Appendix has a different information altogether. Refer to Appendix 5 in the spec. Checkout the Routing and Tracking Area update procedures. They both speak of GTPv1 rather than GTPv2. This means MME has to support GTPv1 too. This sucks. Anyway, I still have my fingers crossed about inter RAT handovers. Hope they will be seems less.

More later. Comments/Corrections are welcome.

11 comments:

Anonymous said...

How will MME retrieve GUTI from previous MME?

Santosh said...

MME will not retrieve GUTI from previous MME, MME will retrieve previous MME information from the GUTI.

Vikas said...

The explanation provided is little confusing here.
When UE sends OLD GUTI to network (assuming to new MME):
a. if MME doesnt know GUTI, What will it do using old MME Id info?
b. what if it gets Old MME Id. The purpose is to get IMSI. How is it going to get that? A mapping is still maintained at MME with old MME vs IMSI you mean?

c. How long will MME store this mapping information? Dont you think Inactive MobileCtxt's cud be purged?

Is this not costly when compared to Identity Request?

Santosh said...

This is what happens:

- If the new MME knows the GUTI to IMSI mapping then there is no confusion.
- If new MME doesnt have the GUTI to IMSI mapping, then the new MME will retrieve the old MME information from the GUTI. Then it will send an Identity request to old MME asking for the IMSI. Old MME will respond with the UE's IMSI.
- If old MME doesnt respond with IMSI, then new MME will directly send a identity request to UE.
- As far as I know GUTI to IMSI mapping is present in the MME unless and until its manually purged. I am ok to spend a bit more than to reveal my IMSI all the time. Hence I would have a database that stores the mappings permanently (until purged).

Vikas said...

Thanks!

Kostubh said...

Hi Santosh,

-> Isn't it like GUTI to IMSI mapping lies at HSS and MME gets it from HSS and then keeps a local copy about the same?

-> Second ques: If old MME does not respond with IMSI, then new MME should query HSS to get it. Isn't it?

- Kostubh

Santosh said...

Hi Koustub

- GUTI has to be stored in MME. Spec doesnt talk of any communication between MME and HSS for Imsi retrieval from GUTI.
- Again , it is very clearly specified in the spec that if old MME deosnt know the IMSI then MME should send Identity request to UE asking for IMSI.

3GPP TS 23.401 5.3.2.1 (Paras 3 and 4)

Santosh

Anonymous said...

When the UE makes attach request with the (old) GUTI, where the UE retrieves the GUTI from? Is the GUTI saved somewhere in the UE?

I know the GUTI is assigned by the MME when the UE registers itself in MME. How could the UE remember this GUTI for later attach request?

Santosh said...

GUTI will be stored in UE unless and untill we remove the battery. Since I havent seen any 4G mobiles yet, I cannot confirm the statement. Atleast in 3G mobile stores PTMSI unless and untill the battery is removed.

Steven said...

Santosh,

For what reasons, the GUTI will be erased if the battery is removed?

Santosh said...

Steven

I am still not very sure of this. We need to test it to see if mobile holds GUTI even after battery is removed or not.