20020626: LDM setup at Howard (cont.)

>From: "Pavel Byles - Howard Univ. Engineering" <address@hidden>
>Organization: Howard University Engineering
>Keywords: 200206241943.g5OJhXa29444 LDM ldmd.conf

Pavel,

>ok there was an error in the ldmd.conf file. very minor error. Was an 
>accidental brace that wasn't supposed to be there.

That is all it takes for the LDM to not start when running
'ldmadmin start'.

>It still doesn't recieve any data in my ~ldm/data/gempak dir.

One thing you should do is verify that the upstream site you are attempting
to feed from has allowed you to request a feed.  You do this using
the LDM 'notifyme' command.  For instance, run the following:

notifyme -vxl- -f NNEXRAD -o 3600 -h snow.nrcc.cornell.edu

Execution of this command sequence will tell you a couple of things:

1) whether or not the upstream site is willing to feed you
2) what products in the NNEXRAD feed that site has received in the
   past hour (3600 seconds)

Next, if you are allowed by the upstream site and you see that they are
receiving the data that you want, you will receive data if your
ldmd.conf request line is correct.

You can see if your LDM has actually received any data by again using
notifyme:

notifyme -vxl- -f NNEXRAD -o 3600

In this invocation, the '-h' (host) flag was not used, so you are
asking the local host (your own machine) to list out the NNEXRAD
products it has received in the past hour.  If this list returns
nothing (you will have to wait a bit to see since notifyme will need to
look through your entire queue), and if the first notifyme was
successful, it should mean that you have a mistake in your ldmd.conf
request line.  If the notifyme listing shows products, then it means
that you are receiving products and the fact that you are not seeing
them in the directory you want to file them in would indicate that you
have some sort of a problem in your pqact.conf file.

>Are you saying that it would take a very long time to download.

I am saying that the request you sent is asking for a LOT of data.
If your network connection is slow (limited bandwidth), then you
will most likely not be able to keep up with the stream of products
that you want to get.

>Is there 
>any other package that I could use it to download and use Gempak/garp to 
>display the data.

The issue is not one of overloading the LDM; it can handle this load
and lots more.  The issue is one of overloading your network.  At one
point I was under the impression that the network bandwidth at Howard
was very limited, something like the entire campus only had a T1
connection to the internet.  A T1 connection allows for a maximum
transfer of 1.544 Mbps.  If each NEXRAD product is 20 Kb, then your
request was asking for as much as 160x17x12x20000 bytes every hour.
This works out to be 6.528 * 10**8 bytes per hour, or 1.813 * 10**5
bytes per second.  Since a byte is more-or-less 10 bits, you could be
asking for as much as 1.8 Mbps, a volume that exceeds the total
capacity of a T1 connection.  So my comment was intended to alert
you that you were asking for a LOT of data.  This would not make
your ldmd.conf request fail, but it could clog your network.

>If so could you also semd the pqact entry for it too.
>
>Thanks alot

What is the network bandwidth at Howard?

How many NEXRADs do you really need/want to get data from?

The typical Unidata site is getting products for a small subset of the
available NEXRADs, something like 3-5 radars that are proximate to
their area.  Sites with a lot of network capacity (and disk space to
store the data) ask for more NEXRADs.  The top level NNEXRAD relays
have to receive and resend all of the NEXRADs since they would not know
before hand what a downstream site might request.

Tom