[TangerineSDR] Packet drop problem fixed

[Dr. Nathaniel A. Frissell Ph.D.]

Thank you, Tom, Phil, and Franco. This really great news.

73 Nathaniel W2NAF

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________________________________
From: TangerineSDR <tangerinesdr-bounces at lists.tapr.org> on behalf of Tom McDermott via TangerineSDR <tangerinesdr at lists.tapr.org>
Sent: Saturday, September 30, 2023 11:12:39 AM
To: TAPR TangerineSDR Modular Software Defined Radio <tangerinesdr at lists.tapr.org>
Cc: Tom McDermott <tom.n5eg at gmail.com>
Subject: [TangerineSDR] Packet drop problem fixed

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 switches.

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 packet,
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 burst.

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 more
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 frames.
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 problem.
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 both
for all the expert help in working through this.

-- Tom, N5EG



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