While "part 1" of this series says "FTP" (implying plain-text/unencrypted data), "part 2" [0] and "part 3" [1] both say "SFTP". This is "more correct", in my experience, as encryption is pretty much always used nowadays.

[0]: http://engineering.gusto.com/how-ach-works-a-developer-persp...

[1]: http://engineering.gusto.com/how-ach-works-a-developer-persp...