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4G to 2G Handover (HSS-MME)

+3 votes
931 views

When subscriber moves from 4G to 2G network, will HSS send CLR to MME? If it doesn't, in that case how does it know that the UE is no longer attached to MME?

posted May 15, 2014 by Swarnima Singh

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1 Answer

+3 votes

Cancel-Location-Request can be triggered by HSS in respective scenarios:
1. Subscription is withdrawal for a user.
2. To inform previous MME/SGSN about change in location.
3. To inform MME/SGSN about initial attach.

So in this case user has changed his location so diameter message CLR/CLA would be exchanged.

answer May 15, 2014 by Hiteshwar Thakur
That may not be correct . As when it says, "user has changed the location", it means Tracking area update should  happen. And a new MME will send ULR in that case. Only when new MME sends ULR, CLR will be triggered to old MME by HSS. After getting CLA from old MME. HSS will respond as ULA to new MME.

In my question. There is no new MME since the user has moved from 4G to 2G network. Hence fresh ULR from new MME has not been received.
Thats the doubt.
@Swarnima:
Specification 29.272
http://www.arib.or.jp/IMT-2000/V720Mar09/5_Appendix/Rel8/29/29272-811.pdf
Section 5.2.1.2.3    Detailed behaviour of the HSS
In first paragraph second line it has written that:
The HSS shall make use of this procedure when the HSS detects that the UE has moved to a new MME or SGSN area.
@Hiteshwar                   
           
What you have said is right :  HSS shall make use of this procedure when the HSS detects that the UE has moved to a new MME or SGSN area .i.e HSS shall trigger CLR in the given scenario.

But elaborating this given scenario: it says that "it SHOULD move to new MME."
So in that case :

Old MME/SGSN                HSS                   New MME/SGSN


ULR/ULA----------------------->
                                       subsc moved
                                                                 <--------------------ULR
     <-----------------------------CLR
CLA  ----------------------------->
                                           ULA ----------------------------------->



So as per this call flow , the statement you have given is right.
Now coming to my question : subscriber moves from 4G to 2G network, will HSS send CLR to MME?

-- Yes it will send "IF" it would have received ULR from SGSN.

But I raised this question because I have noticed that in some scenarios Diameter interfaces between SGSN and HSS is not up. In that case HSS will not get this new ULR from SGSN and hence it will not trigger CLR.
(sgsn would inform HLR then).

Coming to the second part of my question, it will HSS not get to know that subscriber has moved to 2G. It will have stale data of MME ( Rat type, IMS Voice over PS Session and Last UE activity time).

During the call, (TADS Retrieval), it will try to go to PS leg 1st because of this, and will fail, and will automatically fall to CS leg then and call will go through.
Hence, there is no issue in terms of functionality of HSS, even in that case.
Though this may differ implementation to implementation.
@Swarnima: I am convinced by your reply. It may differ by implementation. Thank you.
(But I raised this question because I have noticed that in some scenarios Diameter interfaces between SGSN and HSS is not up) Here you meant S6d interface doesn't implemented/exist ?

(During the call, (TADS Retrieval), it will try to go to PS leg 1st because of this, and will fail, and will automatically fall to CS leg then and call will go through.)
The example is only for voice call what if it is only a packet call. In that case would there be a drop ?
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