 In the Bell 103 system, the originating modem sends 0s by playing a 1,070 Hz tone, and 1s at 1,270 Hz, with the "answering" modem transmitting its 0s on 2,025 Hz and 1s on 2,225 Hz.These frequencies were chosen carefully; they are in the range that suffers minimum distortion on the phone system and are not harmonics of each other.In the 1,200 bit/s and faster systems, phase-shift keying was used.