MODEM7

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MODEM7, also known as MODEM7 batch or Batch XMODEM, is a protocol for file transfer capable of transferring a "batch" of several files in a row. MODEM7 was used only for a short time, replaced by more capable batching protocols such as YMODEM.

MODEM7 was the first known extension of the XMODEM protocol. An XMODEM file transfer starts with the receiver sending a single NAK character to the sender, which then starts sending packets of 128-bytes of data prefixed with a SOH. MODEM7 changed this behavior only slightly, by sending the filename, in 8.3 filename format, before the first data packet. For a non-aware XMODEM implementation this data would simply be ignored while it waited for the SOH to arrive.

MODEM7 was quickly supplanted by more formal systems that used a "zero packet" that could be error-corrected like any data being sent. In these systems a full 128-byte packet was sent containing the file's name, size, and timestamp in a regular XMODEM block, but whereas a normal XMODEM implementation would start with "block 1", the header was labeled "block 0". Again, to a normal XMODEM implementation this would simply be discarded, the assumption being that the packet number had been corrupted. First introduced in TeLink, this system was widely copied in newer protocols, including SEAlink and YMODEM.