Editing XMODEM (section)

===TeLink===
MODEM7 sent the filename as normal text, which meant it could be corrupted by the same problems that XMODEM was attempting to avoid. This led to the introduction of '''TeLink''' by [[Tom Jennings]], author of the original [[FidoNet]] mailers.

TeLink avoided MODEM7's problems by standardizing a new "zero packet" containing information about the original file. This included the file's name, size, and [[timestamp]], which were placed in a regular 128 byte XMODEM block. Whereas a normal XMODEM transfer would start with the sender sending "block 1" with a <code><SOH></code> header, the TeLink header packet was labeled "block 0" and began with a <code><SYN></code>. The packet contained the file creation date and time, filename up to 16 characters, the file size as a 4-byte value, and the name of the program sending the file.{{sfn|Bush|1995|p=G.1}}

A normal XMODEM implementation would simply discard the packet, the assumption being that the packet number had been corrupted. But this led to a potential time delay if the packet were discarded, as the sender could not tell whether the receiver had responded with a <code><NAK></code> because it did not understand the zero packet or because there was a transmission error. As TeLink was normally used only by [[FidoNet]] software, which demanded it as part of the FidoNet standards, this did not present a real-world problem as both ends would always support this standard.{{sfn|Bush|1995|p=G.1}}

The basic "block 0" system became a standard in the FidoNet community, and was re-used by a number of future protocols like [[SEAlink]] and [[YMODEM]].