I am looking to use serial PPP for dial-up customers however I want to use full speed dialup (i.e V.92 with V.44 compression) which allows dial-up to be faster than 115000 which is the limitation of a standard com port. A good way to achieve this is to use USB modems as they are not restricted to this slow speed. Does anyone know if Mikrotik could be used with USB modems?
By the year 2000, most new serial ports supported higher speeds of 230.4k and 460.8k. Some also support 921.6k, so you need not to worry about serial port speed.
I do if I already have a bank of usb V.92 modems 
Can you tell the model name and manufacturer of your USB modem?
We have 2 types. Type 1 is the V.92 Zoom (model 3090- http://www.zoom.com/graphics/datasheets/dial_up/06443090.pdf) and the other is the creative Modem Blaster V.92 USB (model 7000000002184 I think)
We may add support for USB modems later.
What about USB serial ports?