On Dec 6, 18:21, Jean-Pierre Hofer wrote: It's a CCITT modem standard for half-duplex asynchronous communication at 600 or 1200 baud (one direction, on the forward channel) with a 75 baud back channel. Because the back channel is not just used for signalling, it somtimes used to be called "asymmetric duplex". It's an FSK system, the forward channel uses 1300Hz/1700Hz tones for 600 baud, or 1300Hz/2100Hz tones for 1200 baud; the back channel uses 390Hz/450Hz. The lower frequency is used for MARK and the higher for SPACE. The standard also specifies the answer tone frequency (2100Hz) and some timing but I can't remember the details. -- Pete Peter Turnbull Dept. of Computer Science University of York