External modem is positioned outside the chassis and connects to the host via a serial port. This modem is handy, easy to install, flashing lights to monitor the working condition of modems. But external modems need to use additional power and cables.
Built-in modems need to disassemble the boot box at installation and to set up terminals and COM ports, and install more cumbersome. This modem takes up the expansion slot on the motherboard, but does not require additional power and cables, and the price is cheaper than the external modem.
PCMCIA Plug-in Card
Plug-in modems are mainly used for laptops, and are slim. With the mobile phone, easy to achieve mobile office.