What is Modem?

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 2 shows the resources associated with serial (COM) port assignments.

Table 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 connection the 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 shows the maximum speed of the commonly used UART chip types.

Table 3 UART Chip Speeds
UART Chip	Speed (Kbps)
8250	9600
16450	9600
16550	115,200
16650	430,800
16750	921,600
16950	921,600

Transceivers (Media Converters)

The term transceiver does describe a separate network device, but it can also be technology built and embedded in devices such as network cards and modems. In a network environment, a transceiver gets its name from being both a transmitter and a receiver of signal sthus the name transceivers. Technically, on a LAN, the transceiver is responsible for placing signals onto the network media and also detecting incoming signals traveling through the same wire. Given the description of the function of a transceiver, it makes sense that that technology would be found with network cards.

Although transceivers are found in network cards, they can be external devices as well. As far as networking is concerned, transceivers can ship as a module or chip type. Chip transceivers are small and are inserted into a system board or wired directly on a circuit board. Module transceivers are external to the network and are installed and function similarly to other computer peripherals, or they can function as standalone devices.

There are many types of Tran carvers RF transceivers, fiber optic transceivers, Ethernet transceivers, wireless (WAP) transceivers, and more. Though each of these media types is different, the function of the transceiver remains the same. Each type of the transceiver used has different characteristics, such as the number of ports available to connect to the network and whether full-duplex communication is supported.

Listed with transceivers in the Comp TIA objectives are media converters. Media converters are a technology that allows administrators to interconnect different media types for example, twisted pair, fiber, and Thin or thick coax within an existing network. Using a media converter, it is possible to connect newer 100Mbps, Gigabit Ethernet, or ATM equipment to existing networks such as 10BASE-T or 100BASE-T. They can also be used in pairs to insert a fiber segment into copper networks to increase cabling distances and enhance immunity to electromagnetic interference (EMI).