Re: Ethernet Modems

[Jeff Blakeney <CUTjblakeney@sympatico.ca>]

On Sat, 11 Dec 2004 20:37:39 -0700, "Terry Olsen"
<tolsen64@hotmail.com> wrote:

>Well, color this project as a bust.  The device I was talking about was the 
>MOXA DE-311.  Picked up one on ebay for $38.00.  Though it looked promising, 
>it's completely useless for these reasons:
>
>1. It works great as a "serial - ethernet" server.  I can hook my GS up to 
>its serial port, and telnet into the GS.  However, file transfers do not 
>work.  Don't know what the problem is, it's supposed to pass "RAW DATA" 
>according to the manual.  I know about what some folks have said about the 
>non-sequential aspect of TCP packets, but if that's the case, why do file 
>transfers work for Land of Confusion BBS?  And wouldn't you get messed up 
>looking text also (say if the word "Terry" was transmitted, but received as 
>"Teryr".

It has to do with timeouts and possibly packet sizes as well.

The way file transfer protocols work is the server sends a block of
data (128 bytes on up to 4 KB or more) and waits for a response from
the client before continuing (the response can be as small as a single
byte).  If the server doesn't receive a response within a certain
amount of time, it times out and resends the block.  Most servers will
abort a transfer after receiving a certain number of errors.
Streaming protocols don't wait for responses after each block but keep
sending blocks of data until they receive a response saying something
went wrong and then they will resend the bad block.  I believe there
is a response to let the server know that all blocks made it through
okay as well.

TCP/IP sends information in packets which are much like the blocks
used in the transfer protocols above except that each packet contains
information about when it was sent so that the proper sequence can be
regenerated.  Packets can range in size although I'm not certain what
determines packet size.  Each packet can, and often does, take a
different path through the internet to get from the server to the
client.

So if the servers sends a block and then starts waiting for a response
and the client doesn't receive all the packets that make up that block
in the right order, it has to wait until all the packets have arrived
before it can act on them.  Once it has received them and sent a
response, the response could possibly get split up over multiple
packets as well making the server wait even longer for the response.
If all this doesn't happen within the allotted time that the server is
giving, then there will be an error.

Another thing that could possibly delay the response is that the
client's TCP/IP implementation may not send a packet until it has been
completely filled with data (or a timeout waiting for more data to
send has occurred).  This type of behaviour hit me using Spectrum
(possibly while I was beta testing v2.2).  When I hit carriage return
on my IIgs while telnetting to a sight using Marinetti, the carriage
return wouldn't be sent immediately as I believe Marinetti was waiting
for more data to fill the current packet it was building.  I'm pretty
sure Ewen fixed this by explicitly telling Marinnetti to send the
single character immediately.  Because the BBS software you are using
doesn't even see the TCP/IP side of things, it can't do this and must
work with the TCP/IP timeouts that are set on the Ethernet Modem.

This is, again, a simplification of what is happening and is only a
few possible reasons why there are problems.  I may not be 100%
accurate on all this.

>2. The device evidently uses unicode characters for its "Ethernet Modem" 
>mode and "Console" control.  As I said before, I can telnet into it and talk 
>back & forth with my GS just fine.  But sending commands to the device from 
>the GS (like AT&V) gets no response, and I also get no response from it in 
>"Console" mode either.  However, both of these features work great when the 
>device is hooked up to the serial port of my PC.

Are you sure your serial cable on the IIgs is wired properly for the
settings on both the IIgs and the Ethernet Modem?  If you want to use
hardware handshaking between the IIgs and the Ethernet Modem, which
would probably be the best way to hook it up, you have to make sure
that both the IIgs and Ethernet Modem are set up to use the same
hardware handshaking as well as making sure your serial cable is wired
correctly.

-- 
 Jeff Blakeney - Dean of the Apple II University in the
                 Apple II Community on Syndicomm.com
 CUT the obvious from my address if you want to e-mail me