Halliday & Matthiessen (2014: 42):
Channel determines the ‘bandwidth’ of the flow of meanings in a situation. For most of human history, the channel was only phonic, but typically with visual contact (thus also allowing for accompanying gestures, facial expressions and other forms of visual ‘paralanguage’); but with the gradual emergence of writing, initially in certain city-based civilisations around five thousand years ago, graphic channels were added, and archival uses of language became possible.