Network+ Exam Cram 2

2017-07-07 02:10:07

A modem, short for modulator/demodulator, is a device that converts the digital signals generated by a computer into analog signals that can travel over conventional phone lines. The modem at the receiving end converts the signal back into a format the computer can understand. Modems can be used as a means to connect to an ISP or as a mechanism for dialing up to a LAN.

Modems can be internal add-in expansion cards, external devices that connect to the serial or USB port of a system, PCMCIA cards designed for use in laptops, or proprietary devices designed for use on other devices such as portables and handhelds.

The configuration of a modem depends on whether it is an internal or external device. For internal devices, the modem must be configured with an interrupt request (IRQ) and a memory I/O address. It is common practice, when installing an internal modem, to disable the built-in serial interfaces and assign the modem the resources of one of those (typically COM2). Table 3.2 shows the resources associated with serial (COM) port assignments.

Table 3.2. Common Serial (COM) Port Resource Assignments
Port ID	IRQ	I/O Address	Associated Serial I/F Number
COM1	4	03F8	1
COM2	3	02F8	2
COM3	4	03E8	1
COM4	3	02E8	2

For external modems, you need not concern yourself directly with these port assignments, as the modem connects to the serial port and uses the resources assigned to it. This is a much more straightforward approach and one favored by those who work with modems on a regular basis. For PCMCIA and USB modems, the plug-and-play nature of these devices makes them simple to configure, and no manual resource assignment is required. Once the modem is installed and recognized by the system, drivers must be configured to enable use of the device.

Two factors directly affect the speed of the modem connectionthe speed of the modem itself and the speed of the Universal Asynchronous Receiver/Transmitter (UART) chip in the computer that is connected to the modem. The UART chip controls the serial communication of a computer, and although modern systems have UART chips that can accommodate far greater speeds than the modem is capable of, older systems should be checked to make sure that the UART chip is of sufficient speed to support the modem speed. The UART chip installed in the system can normally be determined by looking at the documentation that comes with the system. Table 3.3 shows the maximum speed of the commonly used UART chip types.

Table 3.3. UART Chip Speeds
UART Chip	Speed (Kbps)
8250	9600
16450	9600
16550	115,200
16650	430,800
16750	921,600
16950	921,600

Keep in mind that Internal modems have their own UARTs, but External modems use the UART that works with the Com Port.

If you have installed an internal modem and are experiencing problems with other devices such as a mouse, there might be a resource conflict between the mouse and the modem. Also, legacy ISA NICs often use IRQ3 and might conflict with the modems.

Table 3.2. Common Serial (COM) Port Resource Assignments

Table 3.3. UART Chip Speeds

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