 The limited available frequency range meant the symbol rate of 1,200 bit/s modems was still only 600 baud (symbols per second).The bit rate increases were achieved by defining four or eight distinct symbols, which allowed the encoding of two or three bits per symbol instead of only 1.The use of smaller shifts had the drawback of making each symbol more vulnerable to interference, but improvements in phone line quality at around the same time helped compensate for this.