Handshaking
Did you ever wonder what all that noise means that occurs when analog modems or fax machines begin to communicate? They are handshaking-or negotiating the rules (protocols) of communication. Because not every modem and computer is exactly the same, there must be some way for the two machines to determine how to communicate. That is what happens in that short burst of information between the two modems: decisions are made about what transmission speed to use (the fastest speed of the slowest device), how the data will be packaged, and who will control the transfer. If any of the parameters cannot be satisfied by both machines, the negotiations will fail and both parties will disconnect.
TIP
If you experience communication difficulty between two modems, be sure that you have not limited one of them to parameters that the other is unable to meet. For example, if one modem has a minimum speed restriction imposed by the software, it might need to be changed before it can communicate with other modems.
Connections between a sending device (sometimes referred to as Data Communications equipment or DCE) and a receiving device (Data Terminal Equipment or DTE ) are called handshaking signals. They ensure that each sending and receiving device is in sync with the other. The flow control of data between modems is handled by the modems themselves. However, the local flow control between modem and COM port can be set by the user. There are two types of flow control:
- Hardware flow control: This takes advantage of some of the extra wires in the serial connection between the modem and COM port. These wires are used to let the other device know that the DCE is ready to send or receive data. The wires are named RTS (Request to Send) and CTS (Clear to Send). Hardware handshaking is sometimes referred to as RTS/CTS.
- Software flow control: This uses special characters known as XON and XOFF to let the other device know that the DCE is starting to send data or that the data transmission is finished. Software handshaking is slower and not as dependable as hardware handshaking. Only some very old modems use software handshaking. If given a choice, always use hardware flow control.