V.FC: Rockwell's V.Fast Class¶

V.FC ("V.Fast Class") was Rockwell's 28800 bit/s modem protocol of 1993 to 1995, sold in modems with Rockwell chips and in USRobotics' Couriers while the ITU was still writing V.34. It was the first to reach 28800 bit/s over a phone line, by changing the symbol rate to suit the line (2400 to 3429 baud) instead of only adding points. V.34 took that idea and much else from it, but the two do not understand each other, and V.FC disappeared once V.34 modems were common. Rockwell never published it.

D-Modem's V.FC is built from the USRobotics Courier V.Everything's DSP firmware (see the technical details). Its data mode, which is how the bits become points on the line, is the Courier's: the same frames, the same constellations and the same trellis codes, checked point for point against the Courier's own code at every speed. Its start-up is not V.FC's yet. D-Modem uses V.34's start-up and then switches to V.FC's data mode, so two D-Modems talk V.FC to each other, but a real V.FC modem does not understand the call.

Using it¶

AT+MS=VFC        (or AT+MS=128)

1. The call starts like a V.34 call: V.8, then V.34's line probing and training, which choose the symbol rate.
2. In V.34's last step (the MP messages) each D-Modem says it wants V.FC.
3. If both do, the data goes in V.FC's data mode at 14400 to 28800 bit/s, the fastest V.34 chose that V.FC has at that symbol rate.
4. If the other modem does not ask for V.FC (any ordinary V.34 modem), the call goes on as plain V.34.

+MS limits the speeds as usual, for example AT+MS=VFC,0,14400,19200.

What is missing¶

• V.FC's own start-up, which a real V.FC modem needs. The Courier's firmware has the answering modem's script (V.32-like tones and training at 2400 baud, then a rate signal), but the line probing and rate negotiation after it are not read yet.
• The speeds below 14400 bit/s.
• Precoding and V.FC's non-linear encoder, which the Courier has and D-Modem does not use.