TAU dilemmas – take 3

Posted: August 10, 2010 in technical
Tags: , , , , , , , ,

PGW initiated dedicated bearer creation versus Downlink Data Notification

There are 2 scenarios that involve the “awakening” of the UE from ECM-IDLE to ECM-CONNECTED:

A. The PCRF decides the creation of a new dedicated bearer, via the Create Bearer Request message. The Spec (TS 23.401) describes this procedure in 3 places:

1. TS23.401 – 5.4.1 Dedicated bearer activation

– in step 3 (corresponding to the sending of Create Bearer Request from SGW to MME), it states that, if the UE is in ECM-IDLE, then the MME with do the “network triggered service request” procedure ( first, buffering the Create Bearer Request message received from the SGW, until it awakens the UE

2. TS23.401 – Network Triggered Service Request

– this section starts with an introduction that indicates the procedure should begin from step 3 if this procedure is triggered by a control-plane signaling coming from the SGW (like in my case); and step 3 is Paging the UE

– after this, the procedure “calls” yet another procedure, namely the UE triggered service request procedure

3. TS23.401 – UE Triggered Service Request

– this procedure includes the NAS Service Request, the Authentication process and the setup of the bearer contexts

It’s like calling recursive procedures in programming. Very hard to follow, when trying to understand a scenario. Visually it should look something like this…


B. The “second” scenarios happens (or at least to my understanding happens) specifically when there is downlink data to be sent from the SGW to the UE, without (necessarily???) having to send control-plane signaling.

In this case, the hop-by-hoping through the sections of TS 23.401 starts with

1. TS 23.401 – Network Triggered Service Request

with Step 1

and, nevertheless, we get to

2. TS 23.401 – UE Triggered Service Request

which is executed fully, just as in the first scenario, then the execution flows goes back to

  1. Dumitru says:


    There’s nothing complicated here, it looks you didn’t have too much experience with GPRS.
    The point is that either there’s pending downlink traffic on SGW, or NAS signaling on MME, the MME will page the MS with a proper bits set to identify that this is either for signaling or for data, then the MS does Service Request…
    Network initiated Service request procedures are different to MS initiated only by existence of Paging triggered either by SGW(if there’s pending downlink data) or MME(NAS messaging).
    The same method is used in GPRS…
    Maybe it will make sense to read also some stuff from 23.060 R9, there the procedures are described bette…

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