S-TMSI is sent by UE when It was earlier attached to Core network. It is not necessary that Ue will send S-TMSI in rrcConnectionRequest message. S-TMSI is derived from GUTI {PLMN + MMEGI + MMEC + TMSI). S-TMSI is combination of two parts {MMEC + TMSI}. GUTI is an optional assignment from MME to UE.It's used only to avoid UE identity enquiry messages like (Ue identity request and response) messages. when it is send by Ue in rrcConnectionRequest message. eNodeB fetch MMEC and forward InitialUeMessage to appropriate MME.
It may be an easy question but still I have some doubt. How RRC Connection setup procedure gets impacted with the presence of "S-TMSI" as an UE-Identity within the RRC Connection Request message ? Note: RRC Connection request message can carry either 5 bytes long (S-TMSI) or 5 bytes long random value.
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.
ue send s-tmsi in place of ue id in rrc con req, will s-TMSI remain same in case of HO also ?
When UE sends GUTI/STMSI to MME and what all massages are involved?
