identifying tetra

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Sim IJskes sim at ijskes.org
Tue Mar 1 10:10:30 UTC 2011


On 01-03-11 10:54, Holger Hans Peter Freyther wrote:
>> I still don't have the right architectual image of GSMTAP i think. Do i need
>> to direct wireshark to sniff on the same interface that tetra-rx opens its udp
>> socket on, with a port filter, or is there a pseudo interface that listens for
>> raw packets on the same UDP port?
>
> There is no pseudo interface. The tetra decoding program sends UDP packets to
> the IANA assigned port for GSMTAP on the specified ip address. E.g we like to
> use multicast addresses as this avoid seeing ICMP messages as the destination
> is not reachable.
>
> The easiest would be to use wireshark's pseudo interface to scan every
> interface and then filter for udp in the trace and select gsmtap in the
> filterbar of wireshark.

Ok. I've straced tetra-rx and it opens on 127.0.0.1. So i wiresharked it 
on lo with a port filter. That should work. But tetra-rx doesn't do a 
write on the udp-socket. That could of course mean there is no tetra 
data in the capture.

I've rebuild the usrp1-tetra_demod.py in the latest grc from git trunk, 
and the 25Khz lowpass gives me a nasty spike on -22Khz. A 20Khz gives me 
a much sharper idle constellation. Do you see any problem with 20Khz 
lowpass?

Gr. Sim




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