A zero reference where everything is measured from. Step 1 and 2 are essential to ensure that the modem is on-line and connected to another modem. Software will have to be present to determine if a byte received is either a data byte or a control byte. Transmit data, but watch receive data line closely. Transmit the data as long as an Xoff has not been received.
Communication takes the form of a hand-shake between the sender and receiver.
Communication of a message from sender to receiver takes the format of this diagram. Long messages are broken up into a series of data packets, and transmitted one at a time across the link. Each packet is acknowledged before the next packet is transmitted. If a packet is not acknowledged, the sender will time out and then retransmit the packet. If the packet is acknowledged by the receiver, the sender sends the next packet and so on until the entire message has been sent.