A hierarchy consisting of an ordered repetition of tandem digital multiplexers that produce signals of successively higher data rates at each level of the hierarchy. Note 1: Digital multiplexing hierarchies may be implemented in many different configurations depending on (a) the number of channels desired, (b) the signalling system to be used, and (c) the bit rate allowed by the communications media. Note 2: Some currently available digital multiplexers have been designated as Dl-, DS-, or M-series, all of which operate at T-carrier rates. Note 3: In the design of digital multiplex hierarchies, care must be exercised to ensure interoperability of the multiplexers used in the hierarchy.