LTE: Why does the MME execute Update Location Procedure towards HSS ?

Question

In what all scenarios, this procedure gets executed ? And why this procedure need to be executed ?

Answer 1

Hi Vikram,
Perhaps I am not an expert in this interface,from my experience and basic understanding of S6a interface I have replied.

Prior to answer your question we need to know the historical background of MME Evolution.
As we know in 2G/3G network the SGSN used to does the same work what MME does in LTE.
But in LTE network SGSN splits into two parts one to control Data Session (MME) and other to control User Packets(S-GW).

In order to allow a GPRS flow the MME needs to have few information, such as :-
1. Keeps track of UE that is registered to the network.
2.Acts as GateKeeper,Handling user Request for network access,setting up and tear down data sessions.
3.Handles the authentication of users on the network as well as authorization.

Reason:-
1.MME keep updating the UE location with the HSS by providing the Identity details(the MME-Identity is received within the Origin-Host AVP).
2. IMSI detail is mandatory in order to check the registered SIM card details.
3. Terminal Information (IMEI)
4. Visited PLMN Id

If all the above parameters are fine then HSS would reply with Subscription Data
All the above parameters are necessary to keep track of UE and UE Subscrption ,so that HSS needs to be aware of.

Comments

Thank you Chinmoy to answer my query.

Answer 2

MME can initiate an Update Location Procedure towards the HSS in any of the the following scenarios -

1. In case of an Emergency Attach where a UE is not successfully authenticated.

2. If there is no valid UE context in the MME.

3. If the GUMMEI of an MME has changed.

4. If the UE attaches to a new MME.

5. As part of initial attach, if the UE provides an IMSI / GUTI which does not map to a valid UE context in the MME.

6. In case of network sharing (GWCN).

7. if the PLMN Identifier of the Tracking Area Identity shared by the eNB is different from the PLMN Id present in the GUTI of the UE's context.

Hope this helps!

Comments

Thanks Ankush. I have a doubt related to attach procedure. I went through 3GPP 23.401 attach call flow. Why does MME completes Update Location Procedure even before the Create Session Procedure ? Should it not execute Update Location Procedure once the attach procedure gets complete ?

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Hi Vikram.
Imagine a scenario where there is no valid UE context in the MME. Now how will MME carry out the attach procedure if there is no UE valid context. For sending a create session request to the SGW for a particular UE, MME has to download the UE Subscription profile (AMBR, MSISDN, APN, etc) from the HSS.