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Re: Chuck Forsberg has died - author of Zmodem

Cross-posted to 2 groups.

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On 2015-11-19, Rich <rich@example.invalid> wrote:
> In comp.misc Charles Richmond <numerist@aquaporin4.com> wrote:
>> "RS Wood" <rsw@therandymon.com> wrote in message 
>> news:db5uqqFeeamU1@mid.individual.net...
>> > On 2015-11-19, Rich <rich@example.invalid> wrote:
>> >
>> > Rest in peace, Mr. Forsberg.  I have many fond memories of zmodem -
>> > an early introduction into the magic of being able to find things
>> > located elsewhere and acquire them.  Talk about a new world opening
>> > up - get to a terminal, gopher/wais your way around, zmodem them
>> > down.  Life didn't get much better than that.  And in a way, it
>> > /didn't/.
>
>> And using CRCs instead of checksums was *way* better!!!
>
> Very true, but my favorite Zmodem feature was the restart capability.
> Back in my dialup days, the local phone line was 'ok' but not 100%
> reliable.  So one never knew if one would be online for an hour, or if
> 10 minutes in, one would get the dreaded "NO CARRIER" message.
>
> Being able to restart an aborted download from the point where it
> aborted was a godsend when the specter of a dropped connection was
> always hanging out there.
>
> Second favorite feature, the fact that the modem receive light lit up
> solid, and remained solid, for the whole transfer.  All but one modem
> that I had were externals, so they had at least receive/transmit
> lights.
>

Yah, all hail modem lights.  Looking up at the modem, you could
immediately figure out what's going on.  I rely more on gkrellm these
days for my ethernet connection.  Modem lights: when those two in the
middle were lit, you knew the magic was flowing ...

I went to wikipedia to remind myself what else zmodem did.  Here's a
clip, and yes restartable transfers are on the list:

//--clip
The key improvement in ZMODEM was the introduction of sliding window
support for improved performance. Generally file transfer protocols
break down a file into a series of packets, and then send them
one-at-a-time to the receiver. If the packet is received correctly an
ACK message is sent and the sender then starts sending the next packet.

However, the telephone system introduces a small delay known as latency
that interferes with this process. Even if the receiver sends the ACK
immediately, the delay in the phone lines means there will always be
some time before the sender receives it and sends the next packet. As
modem speeds increase, this delay represents a larger and larger number
of packets that could be sent during the delay, decreasing the overall
performance.

Sliding window protocols avoid this problem by allowing the sending
machine to move on to the next packet without waiting for an ACK.
Instead, the receiver sends both an ACK (or NAK if there was an error)
along with the packet number it is confirming. The sender can process
these at its leisure, re-sending packets as required when need be. This
effectively reduces the latency to zero at the cost of the very small
overhead data. This is similar to TCP's approach to sending ACKs.

ZMODEM's performance was so improved over previous common protocols that
it generally replaced even special protocols such as YMODEM-g, which
included no error correction at all and instead relied on error-free
links maintained by the modems. Although YMODEM-g was noticeably faster,
the lack of other features such as restartable transfers made it less
appealing.
//--clip

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Chuck Forsberg has died - author of Zmodem Rich <rich@example.invalid> - 2015-11-19 12:04 +0000
  Re: Chuck Forsberg has died - author of Zmodem RS Wood <rsw@therandymon.com> - 2015-11-19 12:49 +0000
    Re: Chuck Forsberg has died - author of Zmodem Michael Black <et472@ncf.ca> - 2015-11-19 12:14 -0500
    Re: Chuck Forsberg has died - author of Zmodem "Charles Richmond" <numerist@aquaporin4.com> - 2015-11-19 14:53 -0600
      Re: Chuck Forsberg has died - author of Zmodem Rich <rich@example.invalid> - 2015-11-19 23:07 +0000
        Re: Chuck Forsberg has died - author of Zmodem RS Wood <rsw@therandymon.com> - 2015-11-20 06:16 +0000
          Re: Chuck Forsberg has died - author of Zmodem Morten Reistad <first@last.name.invalid> - 2015-11-20 07:42 +0100
    Re: Chuck Forsberg has died - author of Zmodem Jan van den Broek <fortytwo@xs4all.nl> - 2015-12-07 21:21 +0100

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