The development of the VLSI technique has led to smaller transmission systems that require less power and which also contain more functions than previous systems. Ericsson has exploited this technique to develop a family of digital multiplexers which all comprise a single printed board assembly. The authors concentrate on digital multiplexers for 2/8, 8/34 and 30/140 Mbit/s and describe their applications and system structure and also the measures taken to ensure high reliability.