The document discusses asynchronous and synchronous transmission. Asynchronous transmission is characterized by the sender and receiver not being synchronized in terms of information flow, but being synchronized for character timing. It can be used for low and high speed transmission such as modems and ATM. Synchronous transmission synchronizes the sender and receiver for both information flow and timing, requiring more resources. It is used for high speed links but asynchronous transmission is more widely used in practice due to advances allowing high speeds asynchronously.