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Re: Chuck Forsberg has died - author of Zmodem

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In article <n2mdri$bfr$1@solani.org>, RS Wood  <randy@therandymon.com> wrote:
>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.

It is not the phone system that is the main culprit regarding this
latency. Even a half-rotten PDH system will only make a few score of
milliseconds delay. (it is 125 us per repeater hop, and once you get
to the p3/ds3 level the distances tend to be long, like several hundred
kilometers between each repeater hop.)

It is the physical distance and the modem protocols that really make
delays. 75 ms across Europe, a little more across the USA, 80 across
the pond. And easily 300 for all the V.3x protocols with error correction, 
compression and whatnot.

>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.

Modems got to be _very_ intelligent, and started to participate in the
zmodem, xmodem, kermit and uucp protocols, acting as repeaters. The
pinnacle of this was the Trailblazer, a truly weird modem. It violated
all the standards by using lots of 7Hz (Yep, Hz, not kHz) channels
in parallell, and using the ones that worked, ignoring the ones where
there was noise. This worked amazingly well with old, decrepit phone
lines. Since it participated in uucp, it was a mainstay with uucp
dialups with a lot of isp's (or, rather, back then, usp's, since it
wasn't IP, but uucp). They had a _lot_ of delay, something like 700 ms, 
which is very noticable if you run something inteactive over it.

I remember we had do scavenge old modems for parts to have as many
of these in service as possible as the year 2000 approached. In mid
2000 we had to cancel the service, since these were in practice
unobtainable; and other modems had gotten a lot better.

-- mrr

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Thread

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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