Installing and Administering PPP

Appendix A 151
Modem Connections
Dial Up Modems
Dial Up Modems
PPP works well with any number of brand-name, non-proprietary,
dial-up modems. Modems for dial up protocols like PPP should conform
to non-proprietary standards because the local user will probably have
little knowledge of the equipment at the remote end of the connection. A
non-proprietary modem's ability to match carrier speed will make a
usable connection with whatever type of modem is operating at the other
end of a connection.
This section discusses a few features of modem performance and
configuration, including speed of transmission, error correction, data
compression, flow control, command mode, and S registers.
Speed
The higher the speed associated with the modem, the faster the data
transmits. There are two ways to gauge modem speed. One figure,
usually between 9600 and 28800 bps, measures the amount of data that
can be transmitted or received between modems. The other, a much
higher figure, perhaps 38400 to 115200 bps, describes data throughput
between a system and its modem. When you configure PPP, choose the
highest modem throughput that can be supported by the system.
Typically, this speed will be 38400. The figure can be found in the files
/usr/include/sys/ttydev.h or /usr/include/sys/termio.h.
In general, the supported baud rate is limited by the system hardware.
For example, some serial interface cards support a maximum speed of
19200 baud whereas series 700 built-in serial ports support a maximum
speed of 57600 baud. pppd supports up to 57600 baud in the software if
the hardware can support this speed.
Data Compression
Enable data compression if your modem supports it, unless your
application performs better without it. Data compression maximizes the
amount of information crossing a PPP connection. Generally, executable
(binary) files do not compress well and files that have been compressed
before transmission are difficult to compress further, but most protocols
provide at least 2:1 compression ratios for other types of data. Microcom,
Inc.'s Microcom Networking Protocol 5 (MNP 5) protocol can double the