Network clock synchronization

DETAILED DESCRIPTION According to the present invention, there is provided a network comprising an exchange having a masterclock and a number of subscriber stations connected to the exchange and arranged to communicate data cells with the exchange, the subscriber stations being arranged to recover the masterclock frequency from data received from the exchange and to transmit data to the exchange at that masterclock frequency, the exchange including a delay line arranged to receive incoming data cells, means to sample data from different taps of the delay line and means to determine from the sampled data which tap carries data optimally in phase with the masterclock and to select that for outputting a data cell accordingly.
The present invention provides a system with a combination of features which together overcome the disadvantages of the above described systems.
The present invention uses a single clock in the head-end station but instead of attempting to recover an appropriate clock for the incoming data uses a delay line to provide a progressive delay for the data, and then identifies the tap from the delay line which carries data having a phase appropriate to the masterclock.
In this manner the exchange is capable of handling data transmitted from subscriber stations with a phase which varies arbitrarily with respect to the system clock.
It is therefore sufficient for the subscriber stations to time the retransmission of data to an accuracy of no more than one clock cycle.
At the same time, because the entire system is locked to a single masterclock frequency there is not the continual phase drift encountered with systems such as that described in the above cited U.
S.
patent.
It is therefore sufficient for a single phase determination to be made for an entire cell transmitted by a given termination.
The choice of a tap to output a current cell may be made on the basis of determinations made for one or more preceding cells from the same subscriber station.
Preferably the data cells transmitted by the subscriber stations include a header portion having a predetermined bit pattern and the means to determine includes means to identify the position of the predetermined bit pattern within the sampled data and to select the appropriate tap accordingly