[TangerineSDR] Update on testing RTLSDR and KA9Q-radio

Tom McDermott tom.n5eg at gmail.com
Tue Sep 12 18:49:31 EDT 2023

Hi Phil - unfortunately there's no standard for Jumbo frame size amongst
the manufacturers.
While they can go up to 16k in size, support is usually widespread for 9000
bytes, fair at 9600 bytes, and
degrades from there. They are especially helpful starting at 10 GbE.
Routers benefit the most from
Jumbo frames as it reduces the number of packets per second that have to
lookup next-hop egress
ports, and that is mostly a software function.

The KA9Q software right now is sending multiple lower bit rate streams and
I think (without looking
through the code) doesn't benefit from the larger frames. I don't know what
Phil Karm might be thinking about there,
he would be the expert to comment.

The Tangerine modules are on hold right now due to various issues.  The
Clementine modules are intended to
feed into the KA9Q software.

-- Tom, N5EG

On Tue, Sep 12, 2023 at 2:08 PM Phil Erickson <phil.erickson at gmail.com>

> Hi Tom,
>   That's a lot of excellent work.  Curiosity: at Haystack, we often find
> we have to fiddle with network switch settings to handle 'jumbo frames'
> (MTU > 1500) right.  This comes up as you increase the sampling rate from a
> particular software radio and is of course totally dependent on the wire
> frame protocol between the SDR and driver.  Do you expect any of the
> Tangerine radios to produce jumbo packets, and in any case, does the RPi4
> handle that case OK without dropping things?
>   Or perhaps you never get to this use case?
> 73
> Phil W1PJE
> On Tue, Sep 12, 2023 at 3:36 PM Tom McDermott via TangerineSDR <
> tangerinesdr at lists.tapr.org> wrote:
>> WIth awesome support from Franco, K4VZ, there has been a lot of progress
>> in testing
>> KA9Q software over a wired home network.
>> The test setup uses an RTLSDR dongle to send 192 ksps IQ data from a
>> Raspberry Pi to
>> an Ubuntu 22.04 core i7 host running gnuradio with Franco's gr-rtp
>> module. This test
>> setup may be one where folks have available hardware available from the
>> junkbox and
>> thus able to validate their system.
>> The FFTW wisdom file was computed on each system before starting the
>> tests. All
>> connections are using 1 GbE wired connections, no WiFi links in the
>> system. The GbE
>> switch is configured with IGMP snooping turned on to avoid multicasting
>> traffic to
>> unsubscribed ports. IGMP was verified as working.
>> Initially a RPi 3B+ with GbE interface was used. At 192 ksps It had a
>> packet drop rate of over 10 %
>> resulting in unusable performance. The gnuradio flowgraph was not able to
>> demodulate
>> audio from the IQ stream.  It did successfully demodulate audio when the
>> test was run
>> at 48 ksps, with lower packet loss rate.
>> Changing to a Raspberry Pi 4 at 192 ksps improved the drop rate to 0.4%.
>> This allowed gnuradio
>> to demodulate audio, but with some artifacts.
>> Last night the fft-threads=4  line was commented out of the radiod conf
>> file, allowing
>> ka9q-radio to use its default of 2 threads for the fft.  This further
>> dropped to packet loss
>> rate to 0.1%.  The packet losses are now infrequent (perhaps once every
>> 15 seconds).
>> Separately all the computers and ethernet connections have been tested
>> with iperf3.
>> This is a multi-platform tool (originally Unix / Linux) that sends bursts
>> of data between the endpoints
>> measuring the throughput and the TCP retry rate.
>> The Ubiquiti switch does have retries, sometimes a low number, sometimes
>> a bit larger number.
>> A separate small HW switch was substituted and it has zero retries with
>> iperf3.  All the
>> tests show a throughput of about 940 Mbit/s regardless, so the retry
>> rates seem pretty small.
>> The ka9q-radio packet drop rate is the same (0.1%) using either the test
>> with the Ubiquiti switch
>> or the small HW switch, so it doesn't seem to make much difference.
>> The drop rate was tested with gnuradio running, and with gnuradio not
>> running. The latter uses
>> pcmcat {addr/ssrc} > /dev/null to keep the IGMP path open , and the loss
>> rate is measured with a
>> tshark script provided by Franco.
>> The test was repeated with ka9q-radio source and gnuradio both on a
>> single core i7 computer
>> (using the loopback data connection). That system runs without drops for
>> a few minutes
>> (which was the limit of the test duration).
>> Franco has tested RPi4 to Linux host with a direct connection without
>> drops. However he is
>> using a different SDR receiver than RTLSDR for that test.
>> A separate test was run with a direct hard wired connection from the RPI4
>> to the Linux host.
>> One has to set static IP addresses, and inject static IP routes for the
>> LAN and the multicast
>> range on both ends.   That setup was unusable with about 50%
>> packet loss rate,
>> but this was prior to the fft-threads change (above). It has not been
>> rerun since the fft-threads change.
>> Some speculations and observations:
>> * The different SDR drivers appear to have some significant performance
>> variations.
>> * Different switches and links have different iperf3 retry rates, but
>> they don't seem to affect the
>> drop rates much.
>> * The RPI3 B+seems a bit too slow for 192 ksps, but was marginally OK for
>> 48 ksps.
>> * A faster SBC might be beneficial in place of the RPI4. That would be a
>> good test to try.
>> Again, much thanks to Franco, who has provided software instrumentation
>> to measure the loss rates
>> both inside of gnuradio and outside of gnuradio (i.e. with gnuradio not
>> running).
>> -- Tom, N5EG
>> --
>> TangerineSDR mailing list
>> TangerineSDR at lists.tapr.org
>> http://lists.tapr.org/mailman/listinfo/tangerinesdr_lists.tapr.org
> --
> ----
> Phil Erickson
> phil.erickson at gmail.com
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