The Linux Serial HOWTO: How Do I Set Up My Serial Ports For Higher Speeds? What Speed Should I Use With My Modem? 10. How Do I Set Up My Serial Ports For Higher Speeds? What Speed Should I Use With My Modem?This section should help you figure out what speed to use when using your modem with a communications program, or with a getty program. Normally you just set the speed by a parameter given to getty or by a menu in your communications program (or in a configuration file). If for some reason that doesn't work for high speeds, there is a workaround by giving special options to setserial: With the spd_hi or spd_vhi options given to setserial, setting 38400 will actually set the speed to 57600 (spd_hi) or 115200 (spd_vhi).Make sure you have at least 16550A UARTs for higher speeds. Here are some suggested speeds:If you have a 28.8K (V.34), 33.6K (V.34), or 56K (V.90) modem use 115200 bps.If you have a 14400 bps (V.32bis) modem, with V.42bis data compression, use 57600 bps.If you have a 9600 bps (V.32) modem, with V.42bis data compression, use 38400 bps.If you have something slower than a 9600 bps (V.32) modem, set your speed to the highest speed your modem supports. For example 300 bps (V.21/Bell 103), 1200 bps (V.22/Bell 212A), or 2400 bps (V.22bis).If you use setserial test them on the command line first, and then when you have them working, put them into /etc/rc.d/rc.serial or /etc/rc.d/rc.local or /etc/rc.boot/0setserial so that they are run at startup. Make sure that you are using a valid path for setserial, and a valid device name. You can check the settings of a serial port by running: setserial -a /dev/ttyS3