[TangerineSDR] Packet drop problem fixed

Tom McDermott tom.n5eg at gmail.com
Sat Sep 30 11:12:39 EDT 2023

The packet drop problem that I have been having has been resolved.
Phil Karn, KA9Q recently made several additions to the ka9q-radio package
while going through this.

The problem did not occur when the two computers (server and client)
were directly wired together with a GbE cable.  It only occurred when
going through an Ethernet switch between the two. The problem was
confirmed on 3 different brands of widely available gigabit ethernet

This problem only occurred on higher-data-rate demodulators,
those 48k and faster. These are used commonly for sending wider
spectra in IQ format to another program such as gnuradio.

1. Jumbo frames did not resolve the issue. The packet drop rate was
the same with normal or jumbo frames. The signature of the dropped
packets was a little different due to the greater number of samples per
but the drop rate ended up being pretty much the same.

2. Phil then added 'packet pacing' to his code. This can be turned on with
an option   pacing = yes  in the particular demodulator.   With the option
turned off, each 20 milliseconds there can be a burst of packets sent
contiguously representing the previous 20 ms worth of samples. With it
turned on there is a delay inserted between each of the packets in the

Turning on the pacing option resolved the packet loss problem for me with
a Ubiquiti switch. The Netgear switch also runs well - 1 dropped packet in
than 256000 packets.  Unfortunately the Linksys burned up when I plugged it
into the wrong AC power adapter, so no retest there.

This was then tested with Franco's gr-rtp module in gnuradio, and that works
also without packet drops. This worked with 384k sample rate and normal
Jumbo frames work with Franco's module at lower sample rates but not at
384k due to buffer exhaustion.  I don't see Jumbo+384k being a real need
at this time.

Both Phil and Franco have been tremendously helpful in resolving this
They have each spent a lot of time instrumenting gr-rtp. tshark, and the
radio code
to narrow down what was happening. I would like to express my thanks to
for all the expert help in working through this.

-- Tom, N5EG
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