Path: utzoo!attcan!uunet!husc6!bloom-beacon!gatech!hubcap!ncrcae!ncr-sd!ncrlnk!ncrcce!cavanaug
From: cavanaug@ncrcce.StPaul.NCR.COM (John David Cavanaugh)
Newsgroups: comp.protocols.misc
Subject: Re: About Protocols for File Transfer
Message-ID: <692@ncrcce.StPaul.NCR.COM>
Date: 23 May 88 20:31:47 GMT
References: <303@cfcl.UUCP>
Reply-To: John.Cavanaug@StPaul.NCR.COM
Organization: NCR Comten, Inc.
Lines: 31

In article <303@cfcl.UUCP> dwh@cfcl.UUCP (Dave Hamaker) writes:
>Using ordinary asynchronous RS-232 full-duplex serial communications, is an
>error-detecting/correcting file-transfer protocol possible which is as fast
>as or faster than non-protocol transfer?  If not, why not?  If so, how?

It depends on the error rate of your transmission medium.  A reliable
protocol will require the transmission of more characters than just sending
the bare data stream.  However, the correcting protocol will generally do
retransmission of fairly small chunks of data.  To figure what is more
efficient, you have to consider the error rate and the amount of data that
would have to be retransmitted when you have an error.  For example, if
you want to send a 1 meg file and you average 1 error every 5 million
characters, you expect to send 1.2 characters for every character that
ends up in the destination file (the first four times, the file made it OK,
but the fifth, you had an error and had to retransmit the whole thing).  A
protocol that used 1k blocks with 10 bytes of ack/checksum/etc. would expect
to get the file across at a rate of 1.01 characters (roughly) per character
in the file.  On the other hand, if your files are usually so small that
they fit into a single block, you wouldn't gain much.  And if your error
rate is zero, you only lose.

Note that one major gain in using a reliable protocol is that you _know_
your file is intact at the destination.  It's frustrating to send a big
file somewhere and have a single-character error make the whole thing
worthless.

John Cavanaugh                    John.Cavanaugh@StPaul.NCR.COM
NCR Comten, Inc.
2700 Snelling Ave N.              The opinions expressed ...
St. Paul, MN  55113
   (612) 638-2822