Abstract: An improved modem is disclosed. In a preferred embodiment, the improved modem detects a loss of host synchronism, indicated by an under flow event such as a lack of, or impending lack of data from the host. Normally, the under flow event would result in a lack of data to be sent to a receiving modem, causing a communication link between the two modems to be broken. The improved modem notifies the host of the under flow event and supplies alternate data to the data encoding unit of the modem in order to keep the communication link established. The alternate data supplied is typically an instruction or other action that requires no data from the host and keeps the communication link active. When the host regains synchronism, the host sends data to the modem and normal processing continues.