Janus (minutes)

Janus and Hydra are serial, asynchronous network protocols for the mostly automatic transfer of files between computer systems. These protocols were mainly used in the mailbox scene until the end of the 1990s . In today's standard IP-based networks, these protocols are practically no longer relevant.

As an essential extension of comparable file transfer protocols such as XModem or ZModem , this protocol group also uses the return channel for file transfer. These so-called bidirectional protocols such as Janus and Hydra transfer files in both directions at the same time and have fields in the header for the acknowledgments in the opposite direction.

These protocols are primarily used in the area of ​​automatic mailbox systems such as FidoNet or Z-Netz . These are message systems that are connected to one another via telephone lines using modems and exchange their messages periodically by dialing neighboring stations. The messages to be sent are first combined in a data packet from both communication sides, the so-called mailer . When transferring using bidirectional protocols such as Janus, the file transfer of these data packets can then take place in both directions at the same time. In extreme cases, with data packets of the same length on both sides, half the transmission time can be saved. This expansion is essential because the usual telephone connections are usually billed using a time cycle and these protocols thus enable shorter connection times.