Unable to send more than 255 bytes of data over PPP link

[harsh]

Hi All,

I am trying to upload a file from my target board.

I use tcp socket (My client code) to send data using HTTP POST to
the remote apache server using ppp link.

I have proper connectivity. My client code sends data to pppd over
ttyU0 (virtual serial port on ussp driver) which forwards the data to
cmux (gsm0710) driver which inurn gets transmitted to the Modem.

I observed that when ever I send a file size greater than 255 bytes
the data transfer takes a long time. The remote server times out and
send HTTP error code 500.

I enabled the debug level of CMUX and USSP driver according to it the
data reaches to them after a long delay from PPPd .

How do I fix this ? Can I print PPP packets ? I don't have pppStats
facility for pppd. Even, I don't have debug option support in my pppd.

[James Carlson]

Post by harsh
I am trying to upload a file from my target board.

You've left out a ton of crucial information. What is your "target
board?" What OS does it run? What sorts of development facilities do
you have? Do you have a remote debugger?

Post by harsh
I observed that when ever I send a file size greater than 255 bytes
the data transfer takes a long time. The remote server times out and
send HTTP error code 500.

That sounds like a pretty straightforward flaw somewhere in the code
you're using.

One possible problem (and a *really* wild guess) would be an incorrect
MTU/MRU setting. Another would be broken flow control handling with
the modem. Still another would be a buffer boundary problem.

Depending on what sorts of tools you have, you might want to try
inserting either debug messages ("printf") or enabling tracing for the
functions called.

Here are several general suggestions (nothing specific, as you've
provided few useful details):

- Simplify in order to isolate the problem. Try getting rid of PPP
and writing an application that opens the serial port and sends
and receives large buffers of data. Check if the serial port and
modem actually work. Try using the same modem on a different
system. Get rid of the modem and try a hard-wired serial
connection. Eliminate as many variables as possible to test each
part separately.

- Re-examine configuration options. Make sure you're really setting
the modem up properly according to its technical reference
manual. Modems are *hard* to use right. Make sure the serial
port parameters are correct. Check all of the PPP options
(particularly LCP) that are being negotiated. Get two or three
coworkers to help out with a design and/or code review.

- Get an analyzer. A line analyzer can help you determine whether
there's misbehavior on the wire and can provide an impartial look
at the way the software is working.

- Buy an off-the-shelf solution. If your time is worth anything to
you, consider just buying a product from someone that already
supports what you're trying to do. There are a lot of embedded
system vendors out there.