So in full-duplex communications, two frequencies(the originate pair) will be used by the originator of the conversation, and another two (the answer pair) by the receiver- so four audio frequencies will be in use in total.

• FSK technique was easy to design into a chip, and they worked very well in modems over public-switched telephone networks for rates between 200bps and1200 bps. The US Bell modems use different frequencies CCITT standards-so these modems can't communicate.
• The general expression for a binary FSK

V(t) = Vc cos [(wC + (Vm(t) Dw)/2) t ]

Where: V(t) = binary FSK waveform
              Vc = peak unmodulated carrier amplitude
              WC = radian carrier frequency
              Vm(t) = binary digital modulating signal
              Dw = change in radian output frequency

Originate : 1070 Hz & 1270 Hz
Answer : 2025Hz & 2225 Hz

Note:

Bit rate

• rate of change at the input to the modulator
• expression of data transmission speed
  

Baud rate

• rate of change at the output to the modulator.
• A measure of the rate of change in line condition.

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