A device used in a digital carrier system to multiplex level one bit streams into a level three bit stream. In an E-carrier system, for example, an M13 multiplexes four E-1s into an E-2 bit stream, and then multiplexes four E-2s into an E-3 bit stream.The corresponding T1 version of an M13 combines 28 T1 bit streams into a T3 bit stream. See also bit stream, carrier, E-1, E-2, E-carrier, and multiplexer.