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  Flow Control

As DTE to DCE speed is a few times faster than DCE to DCE speed, the PC can send data to your modem at a rate of 115,200 BPS. In this connection sooner or later the data may be lost because of the buffers overflow, so the control of data flow should be realized. Data flow control has two main variants: Hardware or Software.

Software controls the data flow sometimes described as Xon/Xoff; it is connected with the fact that two characters Xon and Xoff are used. Xon is usually a 17 character, and Xoff is a 19 character . The modem will only have a small buffer as when the computer fills it, the modem sends a Xoff character to inform the computer about data transfer termination. As soon as the modem empties the most of the memory for data, it will send the Xon character to the computer and the latter starts the data transfer. This type of data flow control has the following advantages: it doesn't need any other wires as the characters are sent via TD/RD lines. But if the connection is slow, every character needs 10 bits, which can reduce the connection speed.

Hardware flow control is also known as RTS/CTS flow control. To realize this control two additional wires in the sequential cable are used. This results in increasing the data transmisssion rate, as no time is spent for Xon-Xoff characters transmission. The transmission mechanism is reduced to activation of the Request to Send line when the computer is ready to send data .If the modem has a free bucket for this data, it activates the Clear to Send line in respponse and the computer starts sending data. If the modem lacks free memory, it won't activate the Clear to Send line.

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