Due to the low pin count of MHL versus HDMI, the functions that are carried on separate dedicated pins on HDMI, namely: the Display Data Channel (DDC) (pins 15 & 16) and CEC (pin 13) are instead carried on the bi-directional control bus (CBUS).
MHL uses the same Transition-minimized differential signaling (TMDS) as HDMI to carry video, audio, and auxiliary data.