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Mobile goes to SOS (no signal)

Viewing 5 posts - 1 through 5 (of 5 total)
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  • #68179

    Hi all,
    I am facing a problem in a 2G/3G network. The RNC is Huawei and the MSC is Nortel.
    What happens is that when the mobile is turned on it immediatly tries to LU on the 3G network with a normal 2G sim. The LU gets rejected because the 2G sim is not provisioned for 3G service in the HLR.
    A lot of 2G customers are also complaining about their phone going to SOS mode or no signal (when they enter the 3G coverage area).
    Has anyone faced this problem before? Does anyone know the fix for this issue?
    All suggestion are welcome at this moment and I appreciate every participation.
    Thanks in advance.


    hello elfa,

    were you able to reproduce this huge problem during a drive-test, with a test phone?

    for how long did the problem occur?

    are you certain the MS is not selecting the 2G network first and then doing a LU towards the 3G ?

    in other words, are you certain the MS is immediately looking up for a 3G network, without stopping on the 2G network first ?

    Best regards,


    Hi Pix,
    Thanks for replying.We did not perform any drivetest for this problem. As soon as it presented itself we reserved all the 3G sites for operator use. So only sims with access class 11 configured can access the 3G network at the moment.
    We have setup a test cell inside a building on which we are testing the 2G sims.The problem is still there.
    You are right, I did not explain the situation correctly. The MS first goes to the 2G network first, but because the 3G network is available inside the building, the MS then tries to LU on the 3G network and gets rejected. The phone then stays in SOS mode.The test cell is not reserved for operator use. So all phones with 2G sims that are on dualmode(2G/3G)are trying to LU on 3G. The only way to avoid it is to set the phone on 2G only.
    So I am looking for a solution to this issue.



    ok, so the problem lies in the reselection algorithm, and more specifically in the Non Access Stratum part, in my opinion.
    I’m far from an expert on this topic, but I’ll just entertain you until an expert comes around 🙂

    The solution is probably in this document :

    I think that when your MSC sends the LU Reject to the MS, this message contains a list of forbidden “things”.
    Ie. Forbidden PLMN & Forbidden LAC.

    If the forbidden PLMN or forbidden LAC is the same as the 2G LAC or 2G PLMN, then the MS cannot fall back on the 2G network. As a result, it ends up in “SOS” mode.

    Even if my explanation is not fully correct, it should give you some investigation ideas. I think you should focus on the MSC parameters, in order to better handle LU req from 2G-only SIM.

    At first I wanted to check the radio reselection inter-RAT algorithms, but the solution is not there. Indeed, the radio side cannot distinguish 2G sim and 2G/3G sim. So if you prevent 2G to 3G reselection, then 3G MS will not be able to camp on 3G cell anymore… it could be a temporary workaround though. To do that, just remove all your 2G to 3G adjacency relationships. The 3G MS might still select 3G network, without going through 2G.

    Ok, that’s enough for tonight 🙂



    I am also facing this issue. the interesting thing is that in SOS mode I can use the 3G data services only.
    It might be something related to the APN settings of ur device.

Viewing 5 posts - 1 through 5 (of 5 total)
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