For normal dial-up data communications, modems enter data mode only once - starting when the session connects, and ending when the session disconnects. However, when modems are used for fax and voice (audio) communication, they rapidly switch between command and data modes several times during a call. This is because the role of the modem changes more frequently - rather than simply handing bytes from point A to point B, it is either negotiating parameters and pages with a fax machine, or switching between recording, pausing, and playback audio modes.