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[aprssig] Weird characters from station VKA

Steve Dimse steve at dimse.com
Mon Dec 1 02:37:49 UTC 2008


Warning: this reply uses the term packet to mean TCP/IP packet, while  
line refers to a single piece of APRS data. This could be confusing  
because that data is sent is a single AX-25 packet!

First, if you didn't know, findU gets its data from the APRS IS. In  
other words, findU's parser is seeing the same data as whatever you  
are using to see the data.

How do you determine that the packet is passed along "with the next  
line"? There is a way, by looking at the actual packets sent by the  
APRS IS, which places each line of APRS data in a single TCP/IP  
packet. That packetization is transparent to the end user application  
though, you would need to capture the raw TCP packets to see if this  
is really the case. The user's OS joins these packets into a serial  
stream of data, and it is up to the program (or development  
environment) to turn the stream into lines of data. Whether two (or a  
hundred) consecutive lines of data are passed in a single TCP/IP  
packet makes no difference to the user application.

findU gets the data one line (there may be one or many lines in a  
single TCP/IP packet, or one line may be split between multiple  
packets) at a time using the Perl IO:Socket module. What I think is  
happening is you are using some other method to access the data, and  
this is the reason appears different to you and findU.

NPC at the end of lines have caused problems with APRS data before.  
findU and many other clients strip away such characters.

Steve K4HG

On Nov 30, 2008, at 8:03 PM, Scott Miller wrote:

> While monitoring a station in Canada on the APRS-IS stream to help a
> user debug a problem there, I saw a packet come through with a report
> from VKA in Australia that had one of the Canadian packets tacked onto
> the end.
>
> Checking on findu, I see that there's a non-printable character at the
> end of every packet from VKA.  Presumably something out there is  
> getting
> confused by this.  Findu itself doesn't show the concatenated packets,
> but it's there on the APRS-IS.  Perhaps it's a null character and  
> findu
> thinks it's the end of the string, while the IS servers get confused  
> and
> pass it along with the next line?
>
> I'll monitor VKA and see if I can figure out what exactly it's  
> sending.
>
> Scott
> N1VG
>
>
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