Abstract: Packet based telecommunications networks are being developed to carry Constant Bit Rate (CBR) data streams between source and destination exchanges in the form of multiplexed packets, which are recombined at the destination exchange into a CBR stream. In such a network it is desirable to synchronise the frequency and maintain the phase relation of the recombined CBR stream with those of a source CBR stream arriving at the source exchange. The described synchronising circuit for use in such a network comprises a local oscillator (46), a local Residual Time Stamp (RTS) generator (50) arranged to determine a local RTS value (51) from the local oscillator frequency, a comparator (52) to determine the difference between the local RTS value (51) and a source RTS value (41) extracted from packets received at a destination exchange (10), and an oscillator controller (54) responsive to the difference to adjust the frequency and phase of the local oscillator to maintain the difference constant.