6 Ways To Connect Linux to the Outside World That Are Not Wireless, Bluetooth, or Ethernet
Linux -- No Failure to Communicate
Communicating between computers is one of those capabilities we just seem to take for granted these days. Options abound in the wireless and wired world of Bluetooth, Ethernet and 3G. It hasn't always been so. Modems were all the rage for many years and still find some usage particularly in remote locations.
If you have a high-speed Internet connection using either DSL or Cable, you still use a modem (modulator / demodulator) to convert the standard Ethernet traffic into something capable of transmission over moderate distances. The same goes for the latest EV-DO devices for connecting your laptop to the Internet. Many laptop vendors now offer EV-DO or HSDPA modems built right in. Check out this recent Linux Planet article with hints and tips on getting a USB EV-DO modem from Novatel working with a Lenovo S10e netbook.
Communicating with another computer or something like a network switch or hub over a serial line is one of the things you might find yourself in need of one day. In the world of Linux devices this would be a TTY device. Many older desktop systems came with at least one serial port and a parallel port for connecting to a printer (see this Wikipedia entry for pictures).
The most common connector for a serial port has 25-pins and is designated as the DB-25. A DB-9 connector is another smaller common size with only 9-pins. One of the things you have to know when connecting serial cables between computers is the wiring of the pins. Each of the pins carry specific signals with a minimum of 3 required including Transmit Data, Receive Data and Signal Ground. To connect two computers you'll need something called a null-modem cable with pin 2 (Transmit Data) on one end wired to pin 3 (Receive Data) on the other.
If you're connecting your computer to an external modem using a serial cable, you'll need a normal straight through cable. Other lines included in a typical RS-232 serial cable provide signaling to the modem to allow the data flow to be controlled by the hardware. This makes it possible to send data as fast as the modem will accept it. Standard Ethernet cables to connect your computer to a hub or switch are of the pass-thru variety, but you can use a cross-over cable to connect two computers in much the same way as a serial null-modem cable.