Installing and Administering PPP

Appendix A 153
Modem Connections
Dial Up Modems
damaged. The protocol used determines whether just the corrupted
data, or all data sent since the error was discovered, is transmitted
again. MNP 4 and later protocols can also respond to poor transmission
quality by causing the data to be shipped in smaller packets, increasing
the number of checks and decreasing the amount of data corrupted by
line bursts.
Flow Control
Flow control provides the modem with a buffer for storing received data
from the system. This is beneficial because data transmission between
the modem and system or between two systems connected by a
null-modem cable is much faster than transmission between modems.
Flow control keeps system-to-modem data from being lost during the
modem-to-modem transmission.
NOTE The computer and modem must use the same type of flow control.
Flow control can be provided by system software or modem hardware.
Although pppd's default status is no flow control, you should use
hardware or software flow control if your system supports flow control.
Hardware flow control is recommended.
Hardware flow control is managed by the Request To Send (RTS) and
Clear To Send (CTS) signals. If you use hardware control, make sure the
connecting cable carries both signals. Also make sure the modem is
configured for hardware flow control. Lastly, on a HP-UX system, you
must configure the device to use hardware flow control. Instructions on
how to do this are later in the section “Enabling Hardware Flow
Control.”
If both your serial interface card and your modem support hardware flow
control (RTS/CTS), HP recommends using it. The minor number of the
device file has a bit to indicate hardware flow control. All systems with
HP Precision Bus (HP-PB) architecture support hardware flow control.
Refer to termio(7) for specific models and references.
If you choose to use software flow control, then make sure the modem is
configured for software flow control. You should also instruct pppd to use
software flow control by using the option xonxoff either in the Devices file
or on the pppd command line.