The present invention relates to a serial interface transmission system with more than one data line, in which the transmitted data has in-band and out-of-band characters. More particularly, the present invention relates to methods and systems for sending side channel data over a high-speed digital communications link. One embodiment of the invention provides a high-speed digital transmitter (20) capable of sending side channel data. The transmitter (20) includes a channel zero encoder (50a), a multiplexer (90), data enable out control logic (92), and a channel one encoder (52a). Another embodiment of the invention provides a high-speed digital receiver (60) capable of receiving side channel data. The receiver (60) includes a channel zero decoder (50b), a channel one decoder (52b), DEI signal and FIFO control signal recovery logic (199), and a channel one de-multiplexer (140).