Multiple synchronous data stream format for an optical data link

The present invention relates to fibre optic data links and in particular to the merging of multiple separate input data streams into a single data stream for transmission. More particularly, the invention relates to a data format using a single word which includes multiple synchronous data streams of different clock rates and multiple isochronous data streams of different clock rates and one or more asynchronous data stream or streams. This single word is split into multiple synchronous data streams for transmission. An isochronous output data stream format in which a single n bit word is transmitted as m multiple synchronous data streams where the length of the word transmitted in each data stream is n/m, the output data stream containing multiple isochronous different data rate data streams and one or more asynchronous data stream or streams. One bit from each of the m multiple synchronous data streams are combined and transmitted as a single signal at one of 2analog levels. Data which is it desired to have a lower error rate than other data is transmitted in the synchronous data stream corresponding to the most significant bit associated with the 2analog levels. Calibration data is transmitted in the synchronous data stream corresponding to the most significant bit associated with the 2analog levels, the calibration data also being transmitted distributed over each of the m synchronous data streams.