Re: Serial (rs232 etc.) to IP

Paul Keinanen wrote: 
On 26 Oct 2005 10:57:55 -0700, cs_posting@xxxxxxxxxxx wrote:


..and by doing this also ruin the serial line throughput when a half
duplex protocol is used with short message frames. Some converters
wait 10-100 ms after the last serial character received, before the
(TCP or UDP) IP-frame is sent. For instance at 115200 bit/s, this
corresponds to 100-1000 character times.


This is a valid concern, however there's a bit of a problem with single
character packets when many common embedded TCP devices try to talk to
common desktop operating systems.

TCP requires that you hang onto all outgoing data until it has been
acknowledged, because if it's not acknowledged you will have to resend
it.  Most embedded implementations handle this by sending a packet, and
then being unable to send another until the first has been
acknowledged.  The Windows TCP stack often takes as much as 200 ms to
acknowledge a packet when only one is outstanding, so single character
packets can be slowed down to the rate of only five per second!



If you are using a protocol that was initially written for serial line
communication with normal CRC checks and timeout controls, why bother
with TCP, just use simple UDP. If the UDP frame is lost, let the
original serial line protocol timeout mechanism handle any missing
data.


This has at least one problem: these serial line protocols usually assume that packets arrive in order. This makes sense for these protocols (I cannot imagine a message jumping over the previous one and reaching its destiny sooner than the other), but not for IP packets.
.

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