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MNP and V.42
Flow control
55
The modem card has two kinds of flow control:
❖ Serial port flow control
❖ Modem port flow control (only in reliable mode)
Flow control
Serial port flow control
Serial port data flow is controlled by the modem in its
communication with a personal computer.
If the serial port speed is higher than the modem port speed, this
control function sends a transmission halt request to the personal
computer before the buffers in the modem are about to become
full. When the buffers have room to receive data again, the
transmission halt request is canceled and data transmission from
the personal computer is resumed.
The internal modem provides three kinds of serial port flow
control: one determined by software and two determined by
hardware.
Computer
DTE
Receive
Buffer
Buffer
Modem
DCE
Phone
Circuit
Modem port
flow control
flow control
Serial port
Transmit
Phone line