A telecommunications transmission system carries multiple tributary data streams with SDH compatible multiplexing and demultiplexing arrangements that include a tributary justification algorithm utilising both byte and bit justification means as well as a double-ended payload protocol. The byte justification means employs an offset clock with a nominal constant frequency offset from the transmission system clock so that the offset clock can be used to force a regular rate of byte justifications of constant sign. The bit justification means employs the difference between the tributary clock and the offset clock to determine the bit justifications necessary to multiplex the tributary data stream and the bitjustifications are normally of complementary sign to the forced regular byte justifications except where the frequency of the offset clock lies between the frequency of the transmission clock.