Modem Installation and Cabling

PC modems can be located internally or externally. Internal modems are placed inside the PC itself, whereas external modems are external to the PC. Laptops might come with a modem built in or simply might use a convenient type of internal modem called a PCMCIA card, or simply PC card. PC cards are roughly the size of a credit card and easily can be inserted and removed from a PC.

Most PC hardware comes with either a serial communications port, called a COM port, or a Universal Serial Bus (USB) port. Both USB and COM ports are intended to support external devices that communicate using a serial bit stream. So, External modems can be connected to a PC using either a COM port or a USB port. Figure 15-7 depicts the typical topology.

Figure 15-7 Modem Installation Options and Concepts

Figure 15-7 Modem Installation Options and Concepts

PC Card

COM ports usually consist of either a female RS-232 connector, which is a D-shell connector with 25 pins, or a DB-9 connector, which uses 9 pins. USB ports are rectangular female connectors about one quarter inch by 1 inch long. In either case, the computer sends a serial bit stream to the external modem over the cable, expecting the modem to send the data. At the same time, the modem forwards bits received from the phone line back to the PC.

Internal modems do not require the use of an RS-232, DB-9, or USB cable—they simply connect directly to the phone line. In the United States, that means using the same type of cable that is used to connect to an analog phone, with an RJ-11 connector. However, the installation of an internal modem still uses the logical concept of a COM port. That is true of internal modems that are installed in an expansion card slot and of PCMCIA modems that simply can be inserted into the convenient PC card slot in the side of a laptop computer. The operating system in the computer still uses the concept of sending data serially, but instead of it physically being sent over a cable to an external modem, it simply goes to the internal modem card.

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