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SmartSDR (Win) via Hybrid DSL (LTE/DSL) or other Lines with small MTUs

Hi all,

after reading many post regarding MTU Sizes especially when operating "M" Series TRX that do not have the "Advanced Network" System setting Tab to conveniently set the VITA-49 MTU here's a short summary of what to do to get things up and running via "non standard MTU Lines" for example some VPN Tunnels or Telekom Hybrid DSL (Germany).

MTU Sizes for the "Control" connection (TCP 4994)

Since SSDR 3.1.12 does not implement Path MTU discovery, in some cases its possible to use TCP MSS-Clamping on the Router next to the FlexRadio, do that the connection can be established. The MSS needs to be adjusted to match the MTU of the respective line.

MTU Sizes for the VITA-49 connection Waterfall & Spectrum (UDP 4993).

When the connection can be established after adjusting the TCP-MSS as stated above and no Waterfall & Spectrum shows up in SSDR (Win) the MTU of the VITA-49 traffic needs to be matched to the respective Line also.

Since SSDR (Win) 3.1.12 does not allow to conveniently set the MTU via the "Network -> Advanced" Tab when running "M" Series Radios (6400M for example) you need to Tweak the SSDR.settings File located in "User/Username/AppData/Roaming/FlexRadio.../
Please adjust the MTU Value <MTU>VALUE</MTU>
For Telekom Hybrid DSL in Germany try to set the Value to 1360 and save the changes (While SSDR is NOT running)

You now should see Spectrum and Waterfall Data since now the VITA-49 UDP packets can pass the line without being discarded or fragmented.


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