Question

Why does UpdateLocation happen before CSR in Attach procedure, whereas the otherway round in TAU procedure?

Comments

A very nice catch Chandra, I dont know the answer but my guess -
Tracking Area Update takes place (in connected or Idle mode) during an X2 handover, which is directly negotiated between two base stations, the Mobility Management Entity (MME) in core network is **only informed of the handover** after it has taken place. Also, there's no direct communication between the MME and the mobile device during the handover procedure so that may be the reason why ULR is not required before CSR.  This is my guess only, if you find the answer please answer to your question.

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Probably TAU is done in the registered state while Attach is a registration process itself though I am also not sure.

Answer 1

I think Kumar is right here TAU is done in the registered state while Attach is a registration process itself.