 The advent of metal–oxide–semiconductor field-effect transistor (MOSFET) technology, integrated circuit chips and shift register MOS memory led to the application of solid-state microelectronic semiconductor technology and digital circuitry to the implementation of digital multi-channel modems in the 1960s.This enabled advantages such as a very small size, light weight, stability, high reliability, low cost, and freedom from adjustment.The "201A" and "201B Data-Phones" were synchronous modems using two-bit-per-baud phase-shift keying (PSK).