[ldm-users] NOAAport DVB-S2 primary packet drop issue SOLVED!

Hello everyone,

The issue which is causing the 1 and 2 frame packet drops
from NOAAport for Linux kernels is/was this...


It's a dynamic kernel tuning parameter for the network stack.  Here's the

:  *"Redhat noticed in our latest test that we had a number of fragments
:  dropped after timeout.  The number lost matched almost exactly with the
:  number of frames reported lost by acq_stats.  They gave us a new kernel
:  parameter to try modifying: net.ipv4.ipfrag_max_dist.  The default value
:  is 64.  I increased it to 4096 per Redhat's suggestions and the lost
:  frames reported by acq_stats immediately stopped."*

But wait...I learned this morning that there's a way to get even better
results tuning this parameter!

Put this in your Linux system /etc/sysctl.conf file (or wherever it is
located in your system):

net.ipv4.ipfrag_max_dist = 0

And then type this as user root:

sysctl -w net.ipv4.ipfrag_max_dist=0

THAT solves everything. Big or small, the packets get through!
I am not losing ANY packets now. ZERO! Yippee!

Credit for this fix goes to Raytheon, RedHat, and NOAA for their collaborative effort. Now, if you are using another OS, you
may need to fine-tune this parameter. Set it to zero, or in some
cases, you may need to set it to 8192 if zero doesn't work for
you. 4096 wasn't good enough for my CentOS/RHEL 2.6.18 kernel.
I still dropped a few of the largest data chunks being sent across
when it was set at 4096.

The DVB-S2 issue isn't completely over yet. They still need to raise the power back up another 5-6 dB from the bird (SES-1) to get it to where it was before. For testing purposes, it was OK with a 3.7 meter dish. Here, have a look at what happened when a 55 dBZ cell producing 3/4" hail passed over and south of our dish, directly in the path of the satellite, just before 8Z (3 AM CDT) this morning (the data for today will disappear at 0Z this evening):


Also...for those of you running npstats, which makes those neat graphs and charts, author Jose Nieves is coming out with a fix shortly so that the
signal strength will be seen if it drops below 60%.

What is good about all of this happening is that the signal from NOAA is
clean as a whistle. Interference, and any uplink issues which you could
get away with under a DVB-S broadcast, is gone. I lost about 30 packets of
data in 12 hours after changing the parameter above to 4096, but again,
now it's zero. And I anticipate, barring thunderstorms, sun outages,
etc...that many days I will have perfect reception, as will many of you.
And with 5 days until the end of the original dual illumination was
scheduled...if they really had to or wanted to...NOAA could dump the
DVB-S broadcast and switch over to the new one.

Finally, do NOT forget to update the firmware on your Novra S-300!
I repeat...that's a must-do. Your reception, and most importantly,
having the unit crash with subpar or no reception depends on you
upgrading that immediately. Click here, and go to the bottom of the
page to get the file...


Also, for you Unix junkies, download and install CMCS on that same page,
so you can administer it remotely. You'll be glad you did...


Gilbert Sebenste                                                     ********
(My opinions only!)                                                  ******
Staff Meteorologist, Northern Illinois University                      ****
E-mail: sebenste@xxxxxxxxxxxxxxxxxxxxx                                  ***
web: http://weather.admin.niu.edu                                      **

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