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Why do use IMSI for calculating the Paging Frame(PF) and not S-TMSI?

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Why do use IMSI for calculating the Paging Frame(PF) and not S-TMSI?

PF= SFN mod T = (T div N ) * (UE_ID mod N)

Where UE_ID= IMSI mod 1024.

posted Aug 12, 2014 by anonymous

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1 Answer

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IMSI is more predictable number for the MME all the cases and in certain cases S-TMSI may not be there (allocated at the time of LTE-ATTACH)

answer Aug 13, 2014 by Salil Agrawal
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0 votes

In TS 24.301 5.5.1.3.4.2, if combined attach is success(i.e Combined attach successful for both EPS services and non-EPS), why core network contains IMSI in MS identity IE of combined attach accept, and why not just allocate a new TMSI?
In what situation will core network use IMSI in MS identity?

+1 vote

Because your identity will be registered in Home HSS and while roaming how other operator will get these information

0 votes

There are two things related to paging 1) paging frame 2) paging occasion.
To get the paging frame UE_ID is used provided by the MME to eNodeB.
Now I have question why 1024 is used to get UE_ID. UE_ID = IMSI % 1024

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