ECM, EMM, TAU – take 3

Posted: July 23, 2010 in technical
Tags: , , , , , , , , ,

This one should be shorter, because I only have the questions, not the answers for all 😛 . Fortunately, there is one guy that strives to make me understand stuff. Hopefully, tonight his wife won’t beat his ass and let him write a post that (hopefully) would clarify some of my dilemmas:

(10:01:13 AM) Cristina: a few days 🙂 I still have the week-end
(10:01:28 AM) Cristina: and..more importantly..I want to understand
(10:01:42 AM) SD: how about I write a nice article describing TAU tonight
(10:01:49 AM) SD: or by early tomorrow and send it to u?

So, till then:

Q1: When does the TAU relocate MME/SGW?

A1: see table


Q2: Why doesn’t the ECM-CONNECTED relocate MME/SGW?

A2: Because, if the UE is ECM-CONNECTED and it moves to an eNB that may require the relocation of both MME and SGW, there will be a Handover taking place. This Handover will relocate the MME and the SGW (as necessary). Then, the TAU’s place comes. But at this point, the UE is already behind the new eNB and the Handover took care of the “relocation” dilemmas. So, the TAU will only update the Network with the new UE’s location, without changing MME, nor SGW.

Q3: [Timing dilemmas] Scenario:

– UE is in ECM-IDLE

– UE moves to a new TAL, so it has to do TAU with MME relocation and SGW relocation (or just one of them, but at least one is relocated via TAU)

– then UE needs to do some traffic, so it becomes ECM-CONNECTED

What happens with the Handover in this case? Does it take place anymore? And what type of Handover would be – if it takes place?

Q4: Generic dilemma: I know that TAU and Handover are “independent”. Handover is always network-triggered, while TAU can be triggered by eNB or MME. Still, it seems to me they are very connected together right now.


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