Abstract: In accordance with the present invention a plurality of input control data bits are added to a plurality of input telecommunications transmission channels in the form of an input control word wherein any of the individual data bits can be changed without adversely effecting the ability of the system to synchronize and recover the added control word from the multiplexed data stream at the receiving end. A service channel is thus provided. The synchronization circuit described by the present invention is particularly useful in telecommunications transmission of a large plurality of communication channels as is the case in fiber optic transmission wherein a single control word may be useful, for example, as an alarm with respect to one or more of the multiplex telecommunications channels. In a preferred embodiment of the invention, an 8 Kbit/sec control channel is described.