[hpsdr] Heterodyne to Metis Throughput via multi-hop wireless link

[Mark Leone]

As I mentioned in a previous post, I have Heterodyne connected to Metis 
via a two-hop wireless link. In this configuration, Heterodyne seems to 
work well at 48,000 sps. At either of the two higher rates, the audio is 
mostly unintelligible. This surprised me, as the theoretical bandwidth 
of the link (27 Mbps) is far above what should be required to handle a 
single Metis stream even at the highest sample rate. (The routers are 
each rated at 54 MBps, but one of them is splitting its bandwidth as it 
receives Metis packets and forwards them to the MacBook laptop).

I took a look at the Metis protocol description, and came up with a 
rough estimate of max throughput for each sample rate. I assumed one I&Q 
stream at the selected sample rate, with 24 bits/sample. I also factored 
in a band scope stream at 4096 sps with 16 bits per sample. I added 1.5% 
overhead for HPSDR protocol (based on header sizes, but ignoring 
padding), and 6% overhead for Ethernet/IP/TCP|UDP, which is a standard 
benchmark estimate (although perhaps it should be a bit higher for wifi 
Ethernet). Based on these assumptions, I get the throughput estimates 
shown below. I've also added a column indicating the actual bandwidth 
utilization measured by  my router for each sample rate.

Sample Rate 
                                                                       
Estimated Throughput 
rate                                                                         
                 Measured bandwidth (Mbps)

48,000 
                                                                                   1.3 Mbps                                                                                                                             ~3 Mbps

96,000 
                                                                                   2.5 Mbps                                                                                                                             ~4 Mbps

192,000                                                                                 
5.0 
Mbps                                                                                                                             
~6 Mbps

So the utilized bandwidth was twice the value expected at 48,000 sps, 
and significantly higher (but at a lower ratio) for the other sample 
rates. I'd like to know why the measured values are so much higher than 
the predicted values? Is there something  wrong in my assumptions and/or 
calculations? I'd also like to know why the audio was unusable at the 
higher sample rates. Clearly the router is capable of higher bandwidth, 
so perhaps the buffer sizes need to be adjusted. However that doesn't 
appear to be possible with Heterodyne; at least there is no preference 
setting for it. I also looked inside the app bundle, and I see only 
plist files for setting defaults of the properties that can be 
configured in the UI.

Another curious thing is that the router showed a packet stream at 
~2.8Mbps from the MacBook to Metis whenever Heterodyne was operating. 
Sounds like audio incoming to Metis, but I never had Heterodyne in 
transmit mode because there doesn't appear to be a transmit mode. I see 
some preferences related to TX, but no controls to actually transmit.

Any light that can be shed on any of this would be appreciated.

Mark -- K4XML
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