V.FC definition communications, protocol
A serial line protocol
supported by some modems
. Uses symbol rates of 2400, 2800, 3000, >3200 and 3429 and up to 28800 baud
modems will also support V.FC if the manufacturer currently supports V.FC.
The first V.FC modems were shipped in November 1993 and there have been many thousands sold. There will probably be in excess of a million V.FC modems installed by the end of 1994.
V.FC was intended to take some of the techniques being proposed for V.34 and put them into a real modem that people could use. This also gave a lot of people the opportunity to try out 28.8 kilobit per second operation for the first time. There was never any intention from Hayes
(who worked together for two years on V.FC) that V.FC would be compatible with V.34 - even if they had wanted it, others would have made sure it didn't happen! In fact, they made the start-up deliberately different from V.34 so that it would be easy to distinguish between the two and easier to make dual-mode V.FC/V.34 modems.
V.FC is quite different from V.34. Most of the signal-processing algorithms
, whilst based on the same theory, are implemented in different ways. V.34 has some extra things like a secondary channel and a special mode for 28.8 kilobit per second fax.
The Rockwell V.FC implementation uses a single-chip mask-programmed DSP
for all the signal processing functions. You can also buy a modem controller chip from Rockwell to go with it which implements AT commands, error-control and compression
. Hayes made their own controller using the Motorola
68302 processor. When it comes to an upgrade from V.FC to V.34 you have to have a new, masked DSP chip and new controller firmware
to implement all the V.34-specific features. This means that Rockwell-DSP based modems must be returned to the manufacturer for upgrade. Upgraded modems will talk to either V.FC or V.34 modems.