Data link with an imbedded channel

A method and apparatus for arranging an HDLC data link to transport a special class of data such as, for example, synchronization or other signalling information, in addition to the normal data transported by the data link is provided. According to the invention, at the sending end of the link the all-1's sequence corresponding to the idle sequence between individual HDLC packets is detected. Once eight (8) consecutive 1's are detected, a desired number of 1's is removed and a like number of synchronization bits are inserted in their place. At the receiving end of the link the inserted synchronization bits are detected, removed, and a like number of 1's inserted in their place.