Bug #6469


Weird behaviour with phone calls in 3G

Added by dobby about 2 months ago.

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Hello, I'm trying to setup a 3G network following your "Network In The Box" tutorial/documentation.

I use an ipaccess femtocell and the components required to have a 3G network :
  • HLR
  • MSC
  • MGW
  • STP
  • SGSN
  • GGSN
It is working well for SMS and Data/Internet, however I have some issues with phone calls.
More precisely, I have different phones (different models) and here is what I noticed :
  • Calls initiated by phone with model X (phone with number 004, in reference to PCAP and logs) towards another phone with model Y (with number 002) works great : X(004) -> Y(002) = OK
  • Same in the other direction : Y(002) -> X(004) = OK
So for now, everything is working great. However I have another phone, let's call it model Z (with number 003) that behaves as follow :
  • X(004) -> Z(003) = KO : this direction never works, phone Z won't even ring.
  • Z(003) -> X(004) = OK-only-once : In this direction, only the first call is working properly. If I try a second call in this direction, phone X rings, but when the call is answered phone Z doesn't notice it is answered, but then if I hang up from phone X, phone Z detects the hangup.
Here are the things I tried and didn't work :
  • Swapping SIM cards / Re-register SIM in HLR : Doesn't change anything, look like its tied to phone model
  • Turn on and off airplaine mode : It does not give us back the first working call, it stays in a non-working state

The only thing that make it "work" again (in a sense that it just gives us another oneshot call, but doesn't work fully) is to restart MSC and reconnect the phone (airplane mode on then off). Restarting the entier network also works but only restarting the MSC works (restaring only any other component does not work).

I put a PCAP with the following filter : sctp || gsm_ipa || rtp || mgcp and logs of MSC,HLR,HNBGW,MGW of the following 3 scenarios :
  • first_ko : It corresponds to a call in the direction X(004) -> Z(003) which does not work at all, the phone doesn't even ring.
  • first_ok_second_ko : It corresponds to 2 phone calls, the first is working and in the direction Z(003) -> X(004), and the second is not working and is in the same direction. (in log files, there are a lot of blank lines between the 2 phone calls to distinguish them better)
  • first_ok_second_ok : It correspond to 2 phone calls, both working and in the direction X(004) -> Y(002) and Y(002) -> X(004). (in log files, there are a lot of blank lines between the 2 phone calls to distinguish them better)

I first thought it was a CODEC issue/mismatch, but I guess the first call wouldn't work as well if it was a codec issue. Also, the fact that restarting the MSC makes it "work" back again (again, just one phone call is working after this restart) makes me feel like there is maybe a state issue in the MSC at some point.

As a side note, I also have a 2G setup working, but in this case there is no issues with phone calls (at first I had issues, but setting the config value codec-support fr hr in BSC fixed the issue (efr and amr codec was causing some issues)). (However I still have issue when I create a 2G+3G network and I try phone calls between a 2G connected phone and a 3G connected phone, but it will have its own opened issue).

Do you have an idea of what is happening ? If you need more logs or another PCAP filter please tell me.

Thanks in advance

Files 550 KB PCAPs and logs of 3 different call scenarios dobby, 05/27/2024 03:47 PM

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