In order that the data streams from

In order that the data streams from different exchanges are not allowed to drift too far apart in terms of their timing, the exchange clocks are synchronised using one of two possible synchronisation strategies. In mutual synchronisation each exchange clock is allowed to drift within a certain tolerance, but the clocks are interconnected and the mean frequency of all of the clocks provides a benchmark to which all of the clocks periodically adjust themselves. The mean frequency itself is kept within specified tolerances with reference to a primary reference clock (PRC) that conforms to the ITU-T G.811 specification. The second strategy that may be used is called master-slave synchronisation, in which each exchange clock at any given level of synchronisation is synchronised to the exchange clock at the next highest level. The highest level exchange clock is synchronised to a master clock that provides timing information for the entire network.