Enables a computer or device that does not have built-in networking capability to access a network

A communications device is any type of hardware capable of transmitting data, instructions, and information between a sending device and a receiving device. One type of communications device that connects a communications channel to a sending or receiving device such as a com- puter is a modem. Computers process data as digital signals. Data, instructions, and information travel along a communications channel in either analog or digital form, depending on the com- munications channel. An analog signal consists of a continuous electrical wave. A digital signal consists of individual electrical pulses that represent bits grouped together into bytes. 

For communications channels that use digital signals (such as cable television lines), the modem transfers the digital signals between the computer and the communications channel. If a communications channel uses analog signals (such as some telephone lines), however, the modem first converts between analog and digital signals.

The following pages describe the following types of communications devices: dial-up modems, ISDN and DSL modems, cable modems, wireless modems, network cards, wireless access points, and routers.    

As previously discussed, a computer’s digital signals must be converted to analog signals before they are transmitted over standard telephone lines. The communications device that performs this conversion is a modem, sometimes called a dial-up modem. The word, modem, is derived from the combination of the words, modulate, to change into an analog signal, and demodulate, to convert an analog signal into a digital signal.

A modem usually is in the form of an adapter card that you insert in an expansion slot on a com- puter’s motherboard. One end of a standard telephone cord attaches to a port on the modem card and the other end plugs into a telephone outlet.

ISDN and DSL Modems

If you access the Internet using ISDN or DSL, you need a communications device to send and receive the digital ISDN or DSL signals. An ISDN modem sends digital data and information from a computer to an ISDN line and receives digital data and information from an ISDN line. A DSL modem sends digital data and information from a computer to a DSL line and receives digital data and information from a DSL line. ISDN and DSL modems usually are external devices, in which one end connects to the telephone line and the other end connects to a port on the system unit.

Cable Modems

A cable modem is a digital modem that sends and receives digital data over the cable television (CATV) network (Figure 8-18). With more than 110 million homes wired for cable television, cable modems provide a faster Internet access alternative to dial-up for the home user and have speeds similar to DSL. Cable modems currently can transmit data at speeds that are much faster than either a dial-up modem or ISDN.

Some mobile users have a wireless modem that uses the cell phone network to connect to the Internet wirelessly from  a notebook computer, a smart phone, or other mobile device (Figure 8-19). Wireless modems, which have an external or built-in antenna, are available as PC Cards, ExpressCard modules, and flash cards. 

Network Cards

A network card is an adapter card, PC Card, ExpressCard module, USB network adapter, or flash card that enables a computer or device that does not have networking capability to access a network. The network card coordinates the transmission and receipt of data, instructions, and information to and from the computer or device containing the network card.

Network cards are available in a variety of styles (Figure 8-20). A network card for a desktop computer is an adapter card that has a port to which a cable connects. A network card for mobile computers and devices is in the form of a PC Card, ExpressCard module, USB network adapter, or a flash card. Network cards that provide wireless data transmission also are available. This type of card, sometimes called a wireless network card, often has an antenna.

A network card follows the guidelines of a particular network communications standard, such as Ethernet or token ring. An Ethernet card is the most common type of network card.

A wireless access point is a central communications device that allows com- puters and devices to transfer data wirelessly among themselves or to transfer data wire- lessly to a wired network (Figure 8-7 on page 303). Wireless access points have high-quality antennas for optimal signals.

Routers

A router is a communications device that connects multiple computers or other routers together and transmits data to its correct desti- nation on the network. A router can be used on any size of network. On the largest scale, routers along the Internet backbone forward data packets to their destination using the fastest available path. For smaller business and home networks, a router allows multiple computers to share a single high-speed Internet connection such as a cable modem or DSL modem (Figure 8-21). These routers connect from 2 to 250 computers. 

To prevent unauthorized users from accessing files and computers, many routers are protected by a built-in firewall, called a hardware firewall. Some also have built-in antivirus protection. Today’s routers or combination wireless access point/routers are easy to configure and secure against unauthorized access.