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Title: Listening to UDP port

Posted on 2006-06-24
Medium Priority
Last Modified: 2008-03-10

  I am doing a GPS application using Vb.net (Windows application).
The application is  listening to a UDP port where the GPS data is coming.
The test GPS unit is reporting to the server and I am able to identify the IP of GPS unit thru my  program  . But the actual data send from the gps unit is not available in my receiving application  . Instead, whenever some data comes , I see only 8 byte long junk characters in my application.
     (Strangely, if I run another application developed to recieve the GPS data exactly the same way from the same port - After that when I start my application I get all the GPS Data  and the 8 byte junk data.)
  Someone say something to be done with the socket layer.

Can you help me ?

Question by:sgs1970
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Expert Comment

ID: 16986094
What is the data format the GPS sends (NMEA or a proprietary format)? If it is NMEA you should receive a few packets every second.

NMEA sends sequences of alphanum data plus comma and CR/LF. Proprietary protocols usually use binary data. Did you try to hex dump incoming data?
What is the GPS model that you use?


Author Comment

ID: 16993259

Hi vilia ,
 I use intellitac X8 gps  units .  It may be a   proprietary format (my assumption) .
  I know only to hexdump a file .

    thanking you,

Expert Comment

ID: 17006139

the very frst thing you should look for is the technical documentation that describes GPRS UDP/TCP communication.

I have looked at:
pdf files:
Program Manual for InstallWizard-130        
Program Manual for Tracer-1.30

It looks X8 sends additional data in additon to GPS data on UDP/TCP.

If you do not find documentation, and you like hacking, you need to monitor comunication between the X8 and the Working program you have mentioned. It could be done by network/packet sniffing or puting a proxy between the X8 and the program.


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Author Comment

ID: 17067306

   Sorry for the delay as this is like a research  project  .

  I sniffed the port  . It seems like the GPS data is blocked to get into the port when my application runs .(Trying to receive but fails) . When the other application runs this blocking is someway removed and data is made available. This appears to me like people waiting for a free show. Until the guard opens the door, people need to wait.

    How do I get the key for this free show?


Expert Comment

ID: 17081258
Hi Sam,

The "blocking" you are tolking about is some kind of the communication protocol, probably.

In order to capture one you need a sniffer. You can use FramePad for example:
the one has a free trial period.

- Start FramePad and put IP filter both for Tx and Rx packets on the IP address of the GPS.
- Turn on GPS
- Start Application
(try different startup sequence as well)

You would see UDP packets. Look for the UDP port of the GPS (in the packets Application sends to the GPS). It coul be different then the Aplication UDP port. (UDP headers).

Analyze timing and data. Who sends what and when. (data portion of UDP packets).

If you can't decode portion or all of captured data. Try simply to send the same data from your application in the same sequence as the Working Appication does. (monitor correctness of the communication with sniffer).


Accepted Solution

vilia earned 750 total points
ID: 17090673
Hi Sam,

I have found the protocol document:

This should be the last nail.


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