mux27010 capture

[poeschel () lemonage de]

Hello!

I have to debug a problem with the multiplex protocol of a gsm modem. I 
came across wireshark being able to dissect mux27010 protocol which 
would be of big value to me.
I did manage to capture some mux data from the uart but that does not 
seem to fit to that what wireshark expects. Here is my setup:
I have a gsm modem connected to the uart of an arm processor running 
linux. In linux the n_gsm mux driver is attached to the uart and does 
the muxing. I now modified the n_gsm driver to hand me out a copy the 
data it sends to the uart right before it leaves the mux driver. Okay, I 
now have captured data and what I capture this way looks valid to me 
according to the mux spec in 3GPP TS 07.10 V7.2.0. I then convert this 
data to a hexdump with od -Ax -tx1 -v as stated in wireshark 
documentation and this is what I import to wireshark using the Import 
from hex dump... dialog. There I select my file and MUX27010 as 
encapsulation type. The dissection wireshark then does is garbage. In 
the MUX27010 Protocol wireshark expects an extended header which I do 
not have in my capture and which I can not find in the specification. If 
I remove this extended header part from the dissector and compile 
wireshark, it correctly dissects the first (and only the first) mux 
packet to me.
So my questions are: Where does this extended header come from and what 
does it contain ? As it does not seem to be part of the mux 
specification (and it is very unlikely to be seen on the uart line) I 
suspect some capturing tool injecting this data. What is the preferred 
way of capturing this mux data ?

Thanks in advance,
Lars
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