5.5. Tuning Linux dialup

If your connection to the rest of the world is through a dialup link, don't worry. Section 5.4 covers many of the issues that you will see for dialup. The MTU for PPP is recommended at 1500, but slower links may want an MTU of 296 to have improved latency.

Modems in Linux should be full fledged modems, not ones labeled WinModem or Soft Modems. Each of these styles of modem pass much of the processing work off to the CPU. This makes the cards very inexpensive, but increases the load on the CPU. Modems can be internal or external, but internal modems include their own port settings that may be easier to use than with external modems, since the modem and serial port are in the same card. If you use external modems, set the data link between the serial port and the modem to be the highest the modem supports, which is usually 115,200bps or 230,400bps. This ensures that the modem can talk as fast as it needs to with the machine. You should also ensure that you are using RTS/CTS handshaking, also known as hardware, or out-of-band flow control. This allows a modem to immediately tell Linux to stop sending data to the modem, preventing loss of data.

Controlling the Linux serial port is used with the setserial command. You can find the available serial ports at bootup, or by looking at /proc/tty/driver/serial. Entries in that file that have a UART listed exist in Linux. Remember that COM1 or Serial 1 listed on your box will be listed as /dev/ttyS0, and COM2 is /dev/ttyS1. Most modern applications can comprehend speeds greater than 38,400bps, but some older ones do not. To compensate for this, Linux has made 38,400bps (or 38.4kbps) a "magic" speed, and if an application asks for 38.4kbps, Linux will translate this to another speed. Currently, this can be as high as 460kbps. To use this, the setserial command is used.

# setserial /dev/ttyS0
/dev/ttyS0, UART: 16550A, Port: 0x03f8, IRQ: 4
# setserial /dev/ttyS0 spd_vhi
# setserial /dev/ttyS0
/dev/ttyS0, UART: 16550A, Port: 0x03f8, IRQ: 4, Flags: spd_vhi

Speeds are listed as codes for setserial, and these codes are listed in Table 5-2. These values can be set by non-root users.

There is also a spd_cust entry that can use a custom speed instead of 38.4kbps. The resulting speed becomes the value of baud_base divided by the value of divisor.

If both sides have it available, compression using Deflate or BSD is available and can increse throughput. Even though many newer modem protocols provide compression, it isn't very strong. By using Deflate or BSD, greater compression at little loss of CPU or memory space can be attained. The down side is that both sides need to have either Deflate or BSD compression available built into the PPP software. BSD compression can be activated by using the bsdcomp option passed to pppd followed by a compression level between 9 and 15. Higher numbers indicate higher compression. Deflate compression can be used with the deflate option followed by a number in the range of 8 to 15. Deflate is preferred by the pppd used by Linux.