Back in the day, when I was running a bbs, blinkyterm was just coming out with the new janus bidirectional file transfer protocol which was hoped to displace zmodem et al for bbs to bbs file transfer (ie mail), but its adoption was slowed by the limited availability of fast truly bidirectional modems (most high speed modems of the time had performance issues with high bandwidth bidirectional data that did not show under asymmetric loads like zmodem).

I have searched off and on the last couple years, but I cannot find any information about janus. Maybe it is just my bad spelling and janus inc.'s ftp server (no relation) but I can't find anything relevant. I would like to find a protocol specification and/or reference implementation, but any details about its history and demise (probably killed by tcp) would be welcome.

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    It's not your spelling: the German Wikipedia remembers it as Janus too. There's no information there that you don't already know though — it pretty much just repeats that it's a full duplex improvement to protocols like ZModem. See de.m.wikipedia.org/wiki/Janus_(Protokoll)
    – Tommy
    Aug 12, 2017 at 19:12
  • I wish I could say my german is rusty, but I learned half a dozen words this year which brings me up to maybe a dozen.
    – hildred
    Aug 12, 2017 at 19:34
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    A bit of googling finds documentation, specification and implemention here as JANUS.ZIP. (I added "-underwater" as Google keyword to avoid the underwater communications protocol with the same name).
    – dirkt
    Aug 12, 2017 at 19:52
  • Such as it might pertain to searching strategies, I got to the same page as @dirkt by Googling janus zmodem, assuming the two would be mentioned together only when addressing the transmission standards. With two distinct searches reaching only the same piece of documentation, that also suggests it really might be the best thing still available. Surprisingly, an exact text search for "Introduction to the Opus Janus full-duplex file transfer protocol" as per the contained file suggests zero unzipped copies hosted on the web.
    – Tommy
    Aug 13, 2017 at 13:06

1 Answer 1


Short answer:

Because it was useless to begin with.

For a more detailed answer the whole picture is needed:

Back in the real old days modems were symmetrical. A V.21 modem split the line into two 300 bps channel, one for each direction. So single direction transfer would use only a half the available bandwidth. This stayed true including the 2400 bps standard (V.22bis). Janus was developed (IIRC) in the 1985-87 time frame, including adaptions for FIDO and so on. For 2400 bps Modems it did improve transfer times. So far a great idea.

Around the same time USR did introduce their proprietary HST modems. HST was based on an asymmetric channel distribution. One direction did get about 9600 bps bandwidth, while the other got only 300 bps. So perfect for single direction transfers. USR also got a huge foothold in the US by applying an aggressive marketing toward BBS operators with substantial discounts, sometimes up to 60% off regular retail. Now, while running Janus on a symmetric line is a great idea, it totally sucks on asymmetric connections. Thus, BBS running a high speed modem were better off utilizing single directional protocols.

Since V.32 was already standardized years before (1984?), using a V.32 modem would have been a better idea, just they were prohibitive expensive (I remember Hayes asking for tier fist V.32 Modem in like 1988 more than 2000 USD, compared to a HST sold below 1000 USD) - and as said before, USR already had a great hold among BBS operators. With the advent of lower priced standard (V.32) modems USR did crank up the HST (16.8 and 19.2) to stay ahead, but ultimately lost. Still, this wasn't until like 1992/3, and many BBS were still on a locking to the HST.

The same time standard 14.4/28.8 modems became available at reasonable prices, the mid 90s, is also when BBS lost it's appeal and major dial in networks or at last the internet took over.

So, yeah, Janus was a good idea, but market forces meant different.

On a more professional side it was never really something to be considered. People connecting to Unix systems or interconnecting Unix systems were mainly using Telebits PEP modems. For one, they not only offered a dynamic channel adjustment, but also protocol spoofing. PEP split the bandwidth into 512 channels each could be assigned to either direction, so the bandwidth was dynamically allocated. Protocol spoofing meant that the modem recognized a transfer using uucp-g or ZModem and handling all protocol issues locally, thus reducing turn around times to zero.

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    Just adding a couple of details: the earlier HST's used a 300 baud back-channel but the later ones could run at 300 or 450. The use of compression and other optimizations could theoretically take the main channel to an effective rate of 19.2k, but in practice it was much lower. Also - the specific deal for sysops at the time was specifically $500.
    – rnxrx
    Aug 13, 2017 at 3:55
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    Glad to know I was a good idea, but ouch. Useless? That’s harsh, man! Aug 13, 2017 at 10:16
  • I appologize. Think of it like the Zeppelin. In theory lighter than air plus slow running propeller engines and comfortable spaced cabins are the way better solution (and I'd pay a premium for such a trip across the world), but in reality we settled for fixed wing design, small man storage tubes and fuel guzzling jet engines :)
    – Raffzahn
    Aug 13, 2017 at 10:52
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    "USR also got a huge foothold in the US by applying an aggressive marketing toward BBS operators with substantial discounts, sometimes up to 60% off regular retail." -- Darn it, I paid retail. I had no idea. Aug 14, 2017 at 21:24

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